From d10bf3f45a8abce883b3e6e98d2677f43b3029b9 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Micha=C5=82=20Dobranowski?= <mdbrnowski@gmail.com>
Date: Thu, 27 Nov 2025 11:39:37 +0100
Subject: [PATCH 01/24] Upgrade prometheus

---
 rebar.config |  4 ++--
 rebar.lock   | 30 +++++++++++++++---------------
 2 files changed, 17 insertions(+), 17 deletions(-)

diff --git a/rebar.config b/rebar.config
index 65a959a38a2..42b7861f38b 100644
--- a/rebar.config
+++ b/rebar.config
@@ -56,8 +56,8 @@
   {exometer_report_statsd, {git, "https://github.com/esl/exometer_report_statsd.git", {branch, "master"}}},
   {syslogger, "0.3.0"},
   {flatlog, "0.1.2"},
-  {prometheus, "5.0.0"},
-  {prometheus_cowboy, "0.1.9"},
+  {prometheus, "6.1.1"},
+  {prometheus_cowboy, "0.2.0"},
 
   %%% Stateless libraries
   {opuntia, "1.1.2"},
diff --git a/rebar.lock b/rebar.lock
index 12d8886864b..bcaed5cdb60 100644
--- a/rebar.lock
+++ b/rebar.lock
@@ -1,5 +1,5 @@
 {"1.2.0",
-[{<<"accept">>,{pkg,<<"accept">>,<<"0.3.6">>},2},
+[{<<"accept">>,{pkg,<<"accept">>,<<"0.3.7">>},2},
  {<<"amqp_client">>,{pkg,<<"amqp_client">>,<<"4.2.1">>},0},
  {<<"backoff">>,{pkg,<<"backoff">>,<<"1.1.6">>},1},
  {<<"base16">>,{pkg,<<"base16">>,<<"2.0.1">>},1},
@@ -19,6 +19,7 @@
  {<<"credentials_obfuscation">>,
   {pkg,<<"credentials_obfuscation">>,<<"3.5.0">>},
   1},
+ {<<"ddskerl">>,{pkg,<<"ddskerl">>,<<"0.4.2">>},1},
  {<<"eini">>,{pkg,<<"eini">>,<<"1.2.9">>},1},
  {<<"epgsql">>,{pkg,<<"epgsql">>,<<"4.7.1">>},0},
  {<<"eredis">>,{pkg,<<"eredis">>,<<"1.7.1">>},0},
@@ -86,10 +87,9 @@
   0},
  {<<"parse_trans">>,{pkg,<<"parse_trans">>,<<"3.4.0">>},1},
  {<<"pooler">>,{pkg,<<"pooler">>,<<"1.5.3">>},1},
- {<<"prometheus">>,{pkg,<<"prometheus">>,<<"5.0.0">>},0},
- {<<"prometheus_cowboy">>,{pkg,<<"prometheus_cowboy">>,<<"0.1.9">>},0},
- {<<"prometheus_httpd">>,{pkg,<<"prometheus_httpd">>,<<"2.1.13">>},1},
- {<<"quantile_estimator">>,{pkg,<<"quantile_estimator">>,<<"1.0.2">>},1},
+ {<<"prometheus">>,{pkg,<<"prometheus">>,<<"6.1.1">>},0},
+ {<<"prometheus_cowboy">>,{pkg,<<"prometheus_cowboy">>,<<"0.2.0">>},0},
+ {<<"prometheus_httpd">>,{pkg,<<"prometheus_httpd">>,<<"2.1.15">>},1},
  {<<"quickrand">>,{pkg,<<"quickrand">>,<<"2.0.7">>},1},
  {<<"rabbit_common">>,{pkg,<<"rabbit_common">>,<<"4.2.1">>},1},
  {<<"ranch">>,{pkg,<<"ranch">>,<<"2.2.0">>},0},
@@ -117,7 +117,7 @@
  {<<"worker_pool">>,{pkg,<<"worker_pool">>,<<"6.4.0">>},0}]}.
 [
 {pkg_hash,[
- {<<"accept">>, <<"AD44AC7D704BF70EF8FB2E313EF5B978F9D1330BDDAC64509E93AFDA13281215">>},
+ {<<"accept">>, <<"CD6E34A2D7E28CA38B2D3CB233734CA0C221EFBC1F171F91FEC5F162CC2D18DA">>},
  {<<"amqp_client">>, <<"CFF0CC13186E57457DC5745F1B3A4127C6857717CB8F5920DC457C84D0AD00A2">>},
  {<<"backoff">>, <<"83B72ED2108BA1EE8F7D1C22E0B4A00CFE3593A67DBC792799E8CCE9F42F796B">>},
  {<<"base16">>, <<"F0549F732E03BE8124ED0D19FD5EE52146CC8BE24C48CBC3F23AB44B157F11A2">>},
@@ -131,6 +131,7 @@
  {<<"cowlib">>, <<"623791C56C1CC9DF54A71A9C55147A401549917F00A2E48A6AE12B812C586CED">>},
  {<<"cpool">>, <<"76222AA1DAC0F8089707167BD69D221EDA63DB65B8BD67DBF6E133075392EEDC">>},
  {<<"credentials_obfuscation">>, <<"61E282ADFB4439486B3994FAAEC69543C7EE6CC7E70C6340E8853FD9DEAF8219">>},
+ {<<"ddskerl">>, <<"A51A90BE9AC9B36A94017670BED479C623B10CA9D4BDA1EDF3A0E48CAEEADA2A">>},
  {<<"eini">>, <<"FCC3CBD49BBDD9A1D9735C7365DAFFCD84481CCE81E6CB80537883AA44AC4895">>},
  {<<"epgsql">>, <<"D4E47CAE46C18C8AFA88E34D59A9B4BAE16368D7CE1EB3DA24FA755EB28393EB">>},
  {<<"eredis">>, <<"39E31AA02ADCD651C657F39AAFD4D31A9B2F63C6C700DC9CECE98D4BC3C897AB">>},
@@ -162,10 +163,9 @@
  {<<"p1_utils">>, <<"67B0C4AC9FA3BA3EF563B31AA111B0A004439A37FAC85E027F1C3617E1C7EC6C">>},
  {<<"parse_trans">>, <<"BB87AC362A03CA674EBB7D9D498F45C03256ADED7214C9101F7035EF44B798C7">>},
  {<<"pooler">>, <<"898CD1FA301FC42D4A8ED598CE139B71CA85B54C16AB161152B5CC5FBDCFA1A8">>},
- {<<"prometheus">>, <<"8A37A3216D8DB019D19068602669C9819C099120F8E39994DD1BD3A3F5553376">>},
- {<<"prometheus_cowboy">>, <<"D9D5B300516A61ED5AE31391F8EEEEB202230081D32A1813F2D78772B6F274E1">>},
- {<<"prometheus_httpd">>, <<"F086390B4E4E3F41112889B745BAC53D26437B6139496E6700C2508858F5985B">>},
- {<<"quantile_estimator">>, <<"ECD281D40110FDD9BA62685531E4435E0839A52FD1058DA5564F1763E4642EF7">>},
+ {<<"prometheus">>, <<"3C9D8E2D4FCF948450550693F6A82FAE013D3665BA10AA55955B64C2875FADB3">>},
+ {<<"prometheus_cowboy">>, <<"526F75D9850A9125496F78BCEECCA0F237BC7B403C976D44508543AE5967DAD9">>},
+ {<<"prometheus_httpd">>, <<"8F767D819A5D36275EAB9264AFF40D87279151646776069BF69FBDBBD562BD75">>},
  {<<"quickrand">>, <<"D2BD76676A446E6A058D678444B7FDA1387B813710D1AF6D6E29BB92186C8820">>},
  {<<"rabbit_common">>, <<"1D64E391E12116B76B1425EB96B7552DE51F0301093EBA669B5334F4759CC1E8">>},
  {<<"ranch">>, <<"25528F82BC8D7C6152C57666CA99EC716510FE0925CB188172F41CE93117B1B0">>},
@@ -184,7 +184,7 @@
  {<<"uuid">>, <<"B2078D2CC814F53AFA52D36C91E08962C7E7373585C623F4C0EA6DFB04B2AF94">>},
  {<<"worker_pool">>, <<"0347B805A8E5804B5676A9885FB3B9B6C1627099C449C3C67C0E8E6AF79E9AA6">>}]},
 {pkg_hash_ext,[
- {<<"accept">>, <<"A5167FA1AE90315C3F1DD189446312F8A55D00EFA357E9C569BDA47736B874C3">>},
+ {<<"accept">>, <<"CA69388943F5DAD2E7232A5478F16086E3C872F48E32B88B378E1885A59F5649">>},
  {<<"amqp_client">>, <<"8AE00B055A58500E0557F73D9C0FFE257487131E603F7F84FE72CBFAAF03838A">>},
  {<<"backoff">>, <<"CF0CFFF8995FB20562F822E5CC47D8CCF664C5ECDC26A684CBE85C225F9D7C39">>},
  {<<"base16">>, <<"06EA2D48343282E712160BA89F692B471DB8B36ABE8394F3445FF9032251D772">>},
@@ -198,6 +198,7 @@
  {<<"cowlib">>, <<"0AF652D1550C8411C3B58EED7A035A7FB088C0B86AFF6BC504B0BC3B7F791AA2">>},
  {<<"cpool">>, <<"430E18DF4A9D584EB1ED0D196A87CC02E878AF5B4888BFDC9B65F86A96480E30">>},
  {<<"credentials_obfuscation">>, <<"843ADBE3246861CE0F1A0FA3222F384834EB31DEFD8D6B9CBA7AFD2977C957BC">>},
+ {<<"ddskerl">>, <<"63F907373D7E548151D584D4DA8A38928FD26EC9477B94C0FFAAD87D7CB69FE7">>},
  {<<"eini">>, <<"DA64AE8DB7C2F502E6F20CDF44CD3D9BE364412B87FF49FEBF282540F673DFCB">>},
  {<<"epgsql">>, <<"B6D86B7DC42C8555B1D4E20880E5099D6D6D053148000E188E548F98E4E01836">>},
  {<<"eredis">>, <<"7C2B54C566FED55FEEF3341CA79B0100A6348FD3F162184B7ED5118D258C3CC1">>},
@@ -229,10 +230,9 @@
  {<<"p1_utils">>, <<"D0379E8C1156B98BD64F8129C1DE022FCCA4F2FDB7486CE73BF0ED2C3376B04C">>},
  {<<"parse_trans">>, <<"F99E368830BEA44552224E37E04943A54874F08B8590485DE8D13832B63A2DC3">>},
  {<<"pooler">>, <<"058D85C5081289B90E97E4DDDBC3BB5A3B4A19A728AB3BC88C689EFCC36A07C7">>},
- {<<"prometheus">>, <<"80D29564A5DC4490B53FD225D752B65FB0DBEBA41497F96D62223338127C5659">>},
- {<<"prometheus_cowboy">>, <<"5F71C039DEB9E9FF9DD6366BC74C907A463872B85286E619EFF0BDA15111695A">>},
- {<<"prometheus_httpd">>, <<"9B5A44D1F6FBB3C3FE6F85F06DAFE680AD9FFD591EC65A10BB51DFF0FBBE45D2">>},
- {<<"quantile_estimator">>, <<"DB404793D6384995A1AC6DD973E2CEE5BE9FCC128765BDBA53D87C564E296B64">>},
+ {<<"prometheus">>, <<"C3ECC7A35676948089E28FF4383CCB85DCEF447F0982C01E5630A084008C456A">>},
+ {<<"prometheus_cowboy">>, <<"2C7EB12F4B970D91E3B47BAAD0F138F6ADC34E53EEB0AE18068FF0AFAB441B24">>},
+ {<<"prometheus_httpd">>, <<"67736D000745184D5013C58A63E947821AB90CB9320BC2E6AE5D3061C6FFE039">>},
  {<<"quickrand">>, <<"B8ACBF89A224BC217C3070CA8BEBC6EB236DBE7F9767993B274084EA044D35F0">>},
  {<<"rabbit_common">>, <<"FF509B07E639B1784898C28031E5204FEA14260172E4FC339F94405586037E40">>},
  {<<"ranch">>, <<"FA0B99A1780C80218A4197A59EA8D3BDAE32FBFF7E88527D7D8A4787EFF4F8E7">>},

From 8f9296629eed2e5ade75d4d95478f4db25f0884c Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Micha=C5=82=20Dobranowski?= <mdbrnowski@gmail.com>
Date: Fri, 28 Nov 2025 15:47:15 +0100
Subject: [PATCH 02/24] Update prometheus histogram tests

---
 test/mongoose_instrument_metrics_SUITE.erl | 61 ++++++++++++++++++----
 1 file changed, 50 insertions(+), 11 deletions(-)

diff --git a/test/mongoose_instrument_metrics_SUITE.erl b/test/mongoose_instrument_metrics_SUITE.erl
index 69b7a7d39a1..eaa28c3a707 100644
--- a/test/mongoose_instrument_metrics_SUITE.erl
+++ b/test/mongoose_instrument_metrics_SUITE.erl
@@ -10,6 +10,24 @@
 -define(HOST_TYPE, <<"localhost">>).
 -define(HOST_TYPE2, <<"test type">>).
 
+-define(assertRelEqual(Expect, Actual, Tolerance),
+    (fun() -> 
+        __Exp = (Expect),
+        __Act = (Actual),
+        __Tol = (Tolerance),
+        case __Exp == 0 of
+            true -> ?assertEqual(__Exp, __Act);
+            false ->
+                __RelErr = abs(__Exp - __Act) / __Exp,
+                __Check = case __RelErr < __Tol of
+                    true -> ok;
+                    false -> {fail, [{expected, __Exp}, {actual, __Act}, 
+                                        {rel_error, __RelErr}, {tolerance, __Tol}]}
+                end,
+                ?assertEqual(ok, __Check)
+        end
+    end)()).
+
 -import(mongoose_instrument_exometer, [exometer_metric_name/3]).
 %% Setup and teardown
 
@@ -30,6 +48,7 @@ groups() ->
                                prometheus_counter_cannot_be_decreased,
                                prometheus_counter_is_updated_separately_for_different_labels,
                                prometheus_histogram_is_created_and_updated,
+                               prometheus_histogram_is_calculated_correctly,
                                prometheus_histogram_is_updated_separately_for_different_labels,
                                multiple_prometheus_metrics_are_updated]},
      {exometer, [parallel], [exometer_skips_non_metric_event,
@@ -184,13 +203,33 @@ prometheus_histogram_is_created_and_updated(Config) ->
     ok = mongoose_instrument:set_up(Event, ?LABELS, #{metrics => #{time => histogram}}),
 
     %% Prometheus histogram shows no value if there is no data
-    ?assertEqual(undefined, prometheus_histogram:value(Metric, [?HOST_TYPE])),
+    ?assertEqual(undefined, prometheus_quantile_summary:value(Metric, [?HOST_TYPE])),
     ok = mongoose_instrument:execute(Event, ?LABELS, #{time => 1}),
-    ?assertMatch({[1, 0|_], 1}, prometheus_histogram:value(Metric, [?HOST_TYPE])),
+    ?assertMatch({1, 1, _}, prometheus_quantile_summary:value(Metric, [?HOST_TYPE])),
     ok = mongoose_instrument:execute(Event, ?LABELS, #{time => 1}),
-    ?assertMatch({[2, 0|_], 2}, prometheus_histogram:value(Metric, [?HOST_TYPE])),
+    ?assertMatch({2, 2, _}, prometheus_quantile_summary:value(Metric, [?HOST_TYPE])),
     ok = mongoose_instrument:execute(Event, ?LABELS, #{time => 2}),
-    ?assertMatch({[2, 1|_], 4}, prometheus_histogram:value(Metric, [?HOST_TYPE])).
+    ?assertMatch({3, 4, _}, prometheus_quantile_summary:value(Metric, [?HOST_TYPE])).
+
+prometheus_histogram_is_calculated_correctly(Config) ->
+    Event = ?config(event, Config),
+    Metric = prom_name(Event, time),
+    ok = mongoose_instrument:set_up(Event, ?LABELS, #{metrics => #{time => histogram}}),
+
+    %% Prometheus histogram shows no value if there is no data
+    ?assertEqual(undefined, prometheus_quantile_summary:value(Metric, [?HOST_TYPE])),
+    Values = [1, 2, 2, 2, 2, 2, 2, 9, 9, 10],
+    lists:foreach(fun(V) -> ok = mongoose_instrument:execute(Event, ?LABELS, #{time => V}) end, Values),
+    Sum = lists:sum(Values),
+    Length = length(Values),
+    ?assertMatch({Length, Sum, _}, prometheus_quantile_summary:value(Metric, [?HOST_TYPE])),
+    %% Check some quantiles
+    {_, _, Quantiles} = prometheus_quantile_summary:value(Metric, [?HOST_TYPE]),
+    Tolerance = 0.01,
+    ?assertRelEqual(2, proplists:get_value(0.5, Quantiles), Tolerance),
+    ?assertRelEqual(9, proplists:get_value(0.9, Quantiles), Tolerance),
+    ?assertRelEqual(10, proplists:get_value(0.95, Quantiles), Tolerance),
+    ?assertRelEqual(10, proplists:get_value(0.99, Quantiles), Tolerance).
 
 prometheus_histogram_is_updated_separately_for_different_labels(Config) ->
     Event = ?config(event, Config),
@@ -199,8 +238,8 @@ prometheus_histogram_is_updated_separately_for_different_labels(Config) ->
     ok = mongoose_instrument:set_up(Event, ?LABELS2, #{metrics => #{time => histogram}}),
     ok = mongoose_instrument:execute(Event, ?LABELS, #{time => 1}),
     ok = mongoose_instrument:execute(Event, ?LABELS2, #{time => 2}),
-    ?assertMatch({[1, 0|_], 1}, prometheus_histogram:value(Metric, [?HOST_TYPE])),
-    ?assertMatch({[0, 1|_], 2}, prometheus_histogram:value(Metric, [?HOST_TYPE2])).
+    ?assertMatch({1, 1, _}, prometheus_quantile_summary:value(Metric, [?HOST_TYPE])),
+    ?assertMatch({1, 2, _}, prometheus_quantile_summary:value(Metric, [?HOST_TYPE2])).
 
 multiple_prometheus_metrics_are_updated(Config) ->
     Event = ?config(event, Config),
@@ -211,18 +250,18 @@ multiple_prometheus_metrics_are_updated(Config) ->
     %% Update both metrics
     ok = mongoose_instrument:execute(Event, ?LABELS, #{count => 1, time => 2}),
     ?assertEqual(1, prometheus_counter:value(Counter, [?HOST_TYPE])),
-    HistogramValue = prometheus_histogram:value(Histogram, [?HOST_TYPE]),
-    ?assertMatch({[0, 1|_], 2}, HistogramValue),
+    HistogramValue = prometheus_quantile_summary:value(Histogram, [?HOST_TYPE]),
+    ?assertMatch({1, 2, _}, HistogramValue),
 
     %% Update only one metric
     ok = mongoose_instrument:execute(Event, ?LABELS, #{count => 2}),
     ?assertEqual(3, prometheus_counter:value(Counter, [?HOST_TYPE])),
-    ?assertEqual(HistogramValue, prometheus_histogram:value(Histogram, [?HOST_TYPE])),
+    ?assertEqual(HistogramValue, prometheus_quantile_summary:value(Histogram, [?HOST_TYPE])),
 
     %% No update
     ok = mongoose_instrument:execute(Event, ?LABELS, #{something => irrelevant}),
     ?assertEqual(3, prometheus_counter:value(Counter, [?HOST_TYPE])),
-    ?assertEqual(HistogramValue, prometheus_histogram:value(Histogram, [?HOST_TYPE])).
+    ?assertEqual(HistogramValue, prometheus_quantile_summary:value(Histogram, [?HOST_TYPE])).
 
 exometer_skips_non_metric_event(Config) ->
     Event = ?config(event, Config),
@@ -386,7 +425,7 @@ prometheus_and_exometer_metrics_are_updated(Config) ->
     ?assertEqual({ok, [{count, 1}]}, exometer:get_value([?HOST_TYPE, Event, count], count)),
     ?assertEqual({ok, [{mean, 2}]}, exometer:get_value([?HOST_TYPE, Event, time], mean)),
     ?assertEqual(1, prometheus_counter:value(prom_name(Event, count), [?HOST_TYPE])),
-    ?assertMatch({[0, 1|_], 2}, prometheus_histogram:value(prom_name(Event, time), [?HOST_TYPE])).
+    ?assertMatch({1, 2, _}, prometheus_quantile_summary:value(prom_name(Event, time), [?HOST_TYPE])).
 
 %% Helpers
 

From fd16df3a6a3fc7fcadb9f1bd69688c0199658b9a Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Micha=C5=82=20Dobranowski?= <mdbrnowski@gmail.com>
Date: Wed, 3 Dec 2025 10:59:33 +0100
Subject: [PATCH 03/24] Use prometheus quantile summaries

---
 src/instrument/mongoose_instrument_exometer.erl   |  2 +-
 src/instrument/mongoose_instrument_prometheus.erl | 15 +++------------
 2 files changed, 4 insertions(+), 13 deletions(-)

diff --git a/src/instrument/mongoose_instrument_exometer.erl b/src/instrument/mongoose_instrument_exometer.erl
index eb2109fa151..567a74e59e1 100644
--- a/src/instrument/mongoose_instrument_exometer.erl
+++ b/src/instrument/mongoose_instrument_exometer.erl
@@ -153,7 +153,7 @@ handle_metric_event(EventName, Labels, MetricName, MetricType, Measurements) ->
             ok
     end.
 
--spec update_metric(exometer:name(), spiral | histogram, integer()) -> ok.
+-spec update_metric(exometer:name(), mongoose_instrument:metric_type(), integer()) -> ok.
 update_metric(Name, gauge, Value) when is_integer(Value) ->
     ok = exometer:update(Name, Value);
 update_metric(Name, counter, Value) when is_integer(Value) ->
diff --git a/src/instrument/mongoose_instrument_prometheus.erl b/src/instrument/mongoose_instrument_prometheus.erl
index f0843d37625..df73da95700 100644
--- a/src/instrument/mongoose_instrument_prometheus.erl
+++ b/src/instrument/mongoose_instrument_prometheus.erl
@@ -57,7 +57,7 @@ declare_metric(MetricSpec, counter) ->
 declare_metric(MetricSpec, spiral) ->
     prometheus_counter:declare(MetricSpec);
 declare_metric(MetricSpec, histogram) ->
-    prometheus_histogram:declare([{buckets, histogram_buckets()} | MetricSpec]).
+    prometheus_quantile_summary:declare([{quantiles, [0.1, 0.3, 0.5, 0.9, 0.95, 0.99]} | MetricSpec]).
 
 -spec reset_metric(name(), [mongoose_instrument:label_value()],
                    mongoose_instrument:metric_type()) -> boolean().
@@ -68,7 +68,7 @@ reset_metric(Name, LabelValues, counter) ->
 reset_metric(Name, LabelValues, spiral) ->
     prometheus_counter:remove(Name, LabelValues);
 reset_metric(Name, LabelValues, histogram) ->
-    prometheus_histogram:remove(Name, LabelValues).
+    prometheus_quantile_summary:remove(Name, LabelValues).
 
 -spec initialize_metric(name(), [mongoose_instrument:label_value()],
                         mongoose_instrument:metric_type()) -> ok.
@@ -92,15 +92,6 @@ metric_spec(EventName, LabelKeys, MetricName) ->
      {duration_unit, false} % prevent unwanted implicit conversions, e.g. seconds -> microseconds
     ].
 
--spec histogram_buckets() -> [integer()].
-histogram_buckets() ->
-    histogram_buckets([], 1 bsl 30). % ~1.07 * 10^9
-
-histogram_buckets(AccBuckets, Val) when Val > 0 ->
-    histogram_buckets([Val | AccBuckets], Val bsr 1);
-histogram_buckets(AccBuckets, _Val) ->
-    AccBuckets.
-
 -spec handle_metric_event(mongoose_instrument:event_name(), [mongoose_instrument:label_value()],
                           mongoose_instrument:metric_name(), mongoose_instrument:metric_type(),
                           mongoose_instrument:measurements()) -> ok.
@@ -139,4 +130,4 @@ update_metric(Name, Labels, counter, Value) when is_integer(Value) ->
 update_metric(Name, Labels, spiral, Value) when is_integer(Value), Value >= 0 ->
     ok = prometheus_counter:inc(Name, Labels, Value);
 update_metric(Name, Labels, histogram, Value) when is_integer(Value) ->
-    ok = prometheus_histogram:observe(Name, Labels, Value).
+    ok = prometheus_quantile_summary:observe(Name, Labels, Value).

From b05c8329f3650e5ae46976bddcc1b936ead4f986 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Micha=C5=82=20Dobranowski?= <mdbrnowski@gmail.com>
Date: Wed, 3 Dec 2025 14:08:42 +0100
Subject: [PATCH 04/24] Fix prometheus_endpoint test

---
 test/prometheus_endpoint_SUITE.erl | 34 +++++-------------------------
 1 file changed, 5 insertions(+), 29 deletions(-)

diff --git a/test/prometheus_endpoint_SUITE.erl b/test/prometheus_endpoint_SUITE.erl
index 20bbebcf996..b063481e1e7 100644
--- a/test/prometheus_endpoint_SUITE.erl
+++ b/test/prometheus_endpoint_SUITE.erl
@@ -54,7 +54,7 @@ test_metrics(Event, Labels) ->
     check_counter(Event, requests, Labels, 1, Scraped), % 'spiral' is a Prometheus counter
     check_gauge(Event, sessions, Labels, 5, Scraped), % 'counter' is a Prometheus gauge
     check_gauge(Event, seconds, Labels, 10, Scraped),
-    check_histogram(Event, time, Labels, #{count => 1, sum => 2, bucket_num => 32}, Scraped).
+    check_histogram(Event, time, Labels, #{count => 1, sum => 2}, Scraped).
 
 %% Checks for the parsed metrics
 
@@ -71,38 +71,14 @@ check_gauge(Event, Metric, Labels, ExpValue, Scraped) ->
     ?assertEqual({Labels, ExpValue}, Value).
 
 check_histogram(Event, Metric, Labels, ExpValues, Scraped) ->
-    #{count := ExpCount, sum := ExpSum, bucket_num := ExpBucketNum} = ExpValues,
-    [Type, Help] = get_metric([Event, Metric], Scraped),
-    ?assertEqual({<<"TYPE">>, <<"histogram">>}, Type),
+    #{count := ExpCount, sum := ExpSum} = ExpValues,
+    [Type, Help | _] = get_metric([Event, Metric], Scraped),
+    ?assertEqual({<<"TYPE">>, <<"summary">>}, Type),
     ?assertEqual({<<"HELP">>, help(Event, Metric)}, Help),
     [Count] = get_metric([Event, Metric, count], Scraped),
     ?assertEqual({Labels, ExpCount}, Count),
     [Sum] = get_metric([Event, Metric, sum], Scraped),
-    ?assertEqual({Labels, ExpSum}, Sum),
-    Buckets = get_metric([Event, Metric, bucket], Scraped),
-    ?assertEqual(ExpBucketNum, length(Buckets)),
-    check_buckets(ExpCount, Labels, Buckets).
-
-%% Check that the histogram buckets have growing thresholds and counts,
-%% and that the last bucket has the expected total count (because they are cumulative).
-check_buckets(ExpCount, Labels, Buckets) ->
-    InitState = #{labels => Labels, last_count => 0, last_threshold => 0},
-    #{final_count := LastCount} = lists:foldl(fun check_bucket/2, InitState, Buckets),
-    ?assertEqual(ExpCount, LastCount).
-
-check_bucket({Labels, Count}, State) ->
-    #{labels := BaseLabels, last_count := LastCount, last_threshold := LastThreshold} = State,
-    {ThresholdBin, Labels1} = maps:take(le, Labels),
-    ?assertEqual(Labels1, BaseLabels),
-    ?assert(Count >= LastCount),
-    case ThresholdBin of
-        <<"+Inf">> ->
-            #{final_count => Count};
-        _ ->
-            Threshold = binary_to_integer(ThresholdBin),
-            ?assert(Threshold > LastThreshold),
-            State#{last_count => Count, last_threshold => Threshold}
-    end.
+    ?assertEqual({Labels, ExpSum}, Sum).
 
 help(Event, Metric) ->
     <<"Event: ", (atom_to_binary(Event))/binary, ", Metric: ", (atom_to_binary(Metric))/binary>>.

From ef6ed93863f8e1bb8555673963933382bf1a51f9 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Micha=C5=82=20Dobranowski?= <mdbrnowski@gmail.com>
Date: Thu, 4 Dec 2025 12:37:11 +0100
Subject: [PATCH 05/24] Change predefined quantiles and update docs

---
 doc/operation-and-maintenance/MongooseIM-metrics.md | 11 ++++++-----
 src/instrument/mongoose_instrument_prometheus.erl   |  2 +-
 2 files changed, 7 insertions(+), 6 deletions(-)

diff --git a/doc/operation-and-maintenance/MongooseIM-metrics.md b/doc/operation-and-maintenance/MongooseIM-metrics.md
index ea17a54d558..98c8c601d10 100644
--- a/doc/operation-and-maintenance/MongooseIM-metrics.md
+++ b/doc/operation-and-maintenance/MongooseIM-metrics.md
@@ -50,16 +50,17 @@ All metrics are divided into the following groups:
 
     <h3>`histogram`</h3>
 
-    A histogram collects values and groups them in buckets.
+    A histogram collects values over a 60-second sliding window and exposes the number of observations and their sum (as counters), along with the 50th, 75th, 90th, 95th, 99th, and 99.9th percentiles.
 
     **Example:**
         ```
-        # TYPE xmpp_element_in_byte_size histogram
+        # TYPE xmpp_element_in_byte_size summary
         # HELP xmpp_element_in_byte_size Event: xmpp_element_in, Metric: byte_size
-        xmpp_element_in_byte_size_bucket{connection_type="c2s",host_type="localhost",le="1"} 0
+        xmpp_element_in_byte_size_count{connection_type="c2s",host_type="localhost"} 0
+        xmpp_element_in_byte_size_sum{connection_type="c2s",host_type="localhost"} 0
+        xmpp_element_in_byte_size{connection_type="c2s",host_type="localhost",quantile="0.5"} 0
         ...
-        xmpp_element_in_byte_size_bucket{connection_type="c2s",host_type="localhost",le="1073741824"} 0
-        xmpp_element_in_byte_size_bucket{connection_type="c2s",host_type="localhost",le="+Inf"} 0
+        xmpp_element_in_byte_size{connection_type="c2s",host_type="localhost",quantile="0.999"} 0
         ```
 
 === "Exometer"
diff --git a/src/instrument/mongoose_instrument_prometheus.erl b/src/instrument/mongoose_instrument_prometheus.erl
index df73da95700..e073867bcc6 100644
--- a/src/instrument/mongoose_instrument_prometheus.erl
+++ b/src/instrument/mongoose_instrument_prometheus.erl
@@ -57,7 +57,7 @@ declare_metric(MetricSpec, counter) ->
 declare_metric(MetricSpec, spiral) ->
     prometheus_counter:declare(MetricSpec);
 declare_metric(MetricSpec, histogram) ->
-    prometheus_quantile_summary:declare([{quantiles, [0.1, 0.3, 0.5, 0.9, 0.95, 0.99]} | MetricSpec]).
+    prometheus_quantile_summary:declare([{quantiles, [0.5, 0.75, 0.90, 0.95, 0.99, 0.999]} | MetricSpec]).
 
 -spec reset_metric(name(), [mongoose_instrument:label_value()],
                    mongoose_instrument:metric_type()) -> boolean().

From 42797053f857dd9789ff5eec634d014b4d02b349 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Micha=C5=82=20Dobranowski?= <mdbrnowski@gmail.com>
Date: Thu, 4 Dec 2025 16:21:39 +0100
Subject: [PATCH 06/24] Set `error` and `bound` in quantile_summary

---
 doc/operation-and-maintenance/MongooseIM-metrics.md | 2 +-
 src/instrument/mongoose_instrument_prometheus.erl   | 8 +++++++-
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/doc/operation-and-maintenance/MongooseIM-metrics.md b/doc/operation-and-maintenance/MongooseIM-metrics.md
index 98c8c601d10..2c07f607842 100644
--- a/doc/operation-and-maintenance/MongooseIM-metrics.md
+++ b/doc/operation-and-maintenance/MongooseIM-metrics.md
@@ -50,7 +50,7 @@ All metrics are divided into the following groups:
 
     <h3>`histogram`</h3>
 
-    A histogram collects values over a 60-second sliding window and exposes the number of observations and their sum (as counters), along with the 50th, 75th, 90th, 95th, 99th, and 99.9th percentiles.
+    A histogram collects values and exposes the number of observations and their sum (as counters), along with the 50th, 75th, 90th, 95th, 99th, and 99.9th percentiles with 1% accuracy.
 
     **Example:**
         ```
diff --git a/src/instrument/mongoose_instrument_prometheus.erl b/src/instrument/mongoose_instrument_prometheus.erl
index e073867bcc6..650b1c642cf 100644
--- a/src/instrument/mongoose_instrument_prometheus.erl
+++ b/src/instrument/mongoose_instrument_prometheus.erl
@@ -57,7 +57,13 @@ declare_metric(MetricSpec, counter) ->
 declare_metric(MetricSpec, spiral) ->
     prometheus_counter:declare(MetricSpec);
 declare_metric(MetricSpec, histogram) ->
-    prometheus_quantile_summary:declare([{quantiles, [0.5, 0.75, 0.90, 0.95, 0.99, 0.999]} | MetricSpec]).
+    prometheus_quantile_summary:declare([
+        {quantiles, [0.5, 0.75, 0.90, 0.95, 0.99, 0.999]},
+        {error, 0.01},
+        %% Measuring in µs suffices for actions lasting up to a day (with 1% accuracy).
+        %% Measuring in bytes suffices for sizes up to 81 GB (with 1% accuracy).
+        {bound, 1260}
+    | MetricSpec]).
 
 -spec reset_metric(name(), [mongoose_instrument:label_value()],
                    mongoose_instrument:metric_type()) -> boolean().

From 587a84c901e7cc940c543bd17111febc07fc0a27 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Micha=C5=82=20Dobranowski?= <mdbrnowski@gmail.com>
Date: Wed, 17 Dec 2025 10:45:15 +0100
Subject: [PATCH 07/24] Update docs

---
 doc/operation-and-maintenance/MongooseIM-metrics.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/doc/operation-and-maintenance/MongooseIM-metrics.md b/doc/operation-and-maintenance/MongooseIM-metrics.md
index 2c07f607842..4887225c599 100644
--- a/doc/operation-and-maintenance/MongooseIM-metrics.md
+++ b/doc/operation-and-maintenance/MongooseIM-metrics.md
@@ -50,7 +50,7 @@ All metrics are divided into the following groups:
 
     <h3>`histogram`</h3>
 
-    A histogram collects values and exposes the number of observations and their sum (as counters), along with the 50th, 75th, 90th, 95th, 99th, and 99.9th percentiles with 1% accuracy.
+    A histogram collects values over a 60-second sliding window and exposes the number of observations and their sum (as counters), along with the 50th, 75th, 90th, 95th, 99th, and 99.9th percentiles with 1% accuracy.
 
     **Example:**
         ```

From f851c51bc24f7781fa0a4031b5f46204b075ba70 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Micha=C5=82=20Dobranowski?= <mdbrnowski@gmail.com>
Date: Wed, 17 Dec 2025 11:18:04 +0100
Subject: [PATCH 08/24] Add support for sliding window (histogram metrics)

---
 rebar.config                                  |   1 +
 rebar.lock                                    |   2 +-
 .../mongoose_instrument_prometheus.erl        |  19 +-
 .../mongoose_prometheus_sliding_window.erl    | 243 ++++++++++++++++++
 4 files changed, 260 insertions(+), 5 deletions(-)
 create mode 100644 src/instrument/mongoose_prometheus_sliding_window.erl

diff --git a/rebar.config b/rebar.config
index 42b7861f38b..2993aa3fe21 100644
--- a/rebar.config
+++ b/rebar.config
@@ -56,6 +56,7 @@
   {exometer_report_statsd, {git, "https://github.com/esl/exometer_report_statsd.git", {branch, "master"}}},
   {syslogger, "0.3.0"},
   {flatlog, "0.1.2"},
+  {ddskerl, "0.4.2"},
   {prometheus, "6.1.1"},
   {prometheus_cowboy, "0.2.0"},
 
diff --git a/rebar.lock b/rebar.lock
index bcaed5cdb60..98dbee6ce43 100644
--- a/rebar.lock
+++ b/rebar.lock
@@ -19,7 +19,7 @@
  {<<"credentials_obfuscation">>,
   {pkg,<<"credentials_obfuscation">>,<<"3.5.0">>},
   1},
- {<<"ddskerl">>,{pkg,<<"ddskerl">>,<<"0.4.2">>},1},
+ {<<"ddskerl">>,{pkg,<<"ddskerl">>,<<"0.4.2">>},0},
  {<<"eini">>,{pkg,<<"eini">>,<<"1.2.9">>},1},
  {<<"epgsql">>,{pkg,<<"epgsql">>,<<"4.7.1">>},0},
  {<<"eredis">>,{pkg,<<"eredis">>,<<"1.7.1">>},0},
diff --git a/src/instrument/mongoose_instrument_prometheus.erl b/src/instrument/mongoose_instrument_prometheus.erl
index 650b1c642cf..a125c14fd59 100644
--- a/src/instrument/mongoose_instrument_prometheus.erl
+++ b/src/instrument/mongoose_instrument_prometheus.erl
@@ -13,6 +13,13 @@
 start(#{}) ->
     Apps = [prometheus, prometheus_httpd, prometheus_cowboy],
     {ok, _} = application:ensure_all_started(Apps, permanent),
+    %% Start sliding window manager for histogram metrics
+    case whereis(mongoose_prometheus_sliding_window) of
+        undefined ->
+            {ok, _} = mongoose_prometheus_sliding_window:start_link();
+        _ ->
+            ok
+    end,
     ok.
 
 -spec set_up(mongoose_instrument:event_name(), mongoose_instrument:labels(),
@@ -57,13 +64,15 @@ declare_metric(MetricSpec, counter) ->
 declare_metric(MetricSpec, spiral) ->
     prometheus_counter:declare(MetricSpec);
 declare_metric(MetricSpec, histogram) ->
-    prometheus_quantile_summary:declare([
-        {quantiles, [0.5, 0.75, 0.90, 0.95, 0.99, 0.999]},
-        {error, 0.01},
+    SWResult = mongoose_prometheus_sliding_window:declare(MetricSpec),
+    PromResult = prometheus_quantile_summary:declare([
+        {quantiles, mongoose_prometheus_sliding_window:default_quantiles()},
+        {error, mongoose_prometheus_sliding_window:default_error()},
         %% Measuring in µs suffices for actions lasting up to a day (with 1% accuracy).
         %% Measuring in bytes suffices for sizes up to 81 GB (with 1% accuracy).
         {bound, 1260}
-    | MetricSpec]).
+    | MetricSpec]),
+    SWResult or PromResult.
 
 -spec reset_metric(name(), [mongoose_instrument:label_value()],
                    mongoose_instrument:metric_type()) -> boolean().
@@ -74,6 +83,7 @@ reset_metric(Name, LabelValues, counter) ->
 reset_metric(Name, LabelValues, spiral) ->
     prometheus_counter:remove(Name, LabelValues);
 reset_metric(Name, LabelValues, histogram) ->
+    mongoose_prometheus_sliding_window:remove(Name, LabelValues),
     prometheus_quantile_summary:remove(Name, LabelValues).
 
 -spec initialize_metric(name(), [mongoose_instrument:label_value()],
@@ -136,4 +146,5 @@ update_metric(Name, Labels, counter, Value) when is_integer(Value) ->
 update_metric(Name, Labels, spiral, Value) when is_integer(Value), Value >= 0 ->
     ok = prometheus_counter:inc(Name, Labels, Value);
 update_metric(Name, Labels, histogram, Value) when is_integer(Value) ->
+    ok = mongoose_prometheus_sliding_window:observe(Name, Labels, Value),
     ok = prometheus_quantile_summary:observe(Name, Labels, Value).
diff --git a/src/instrument/mongoose_prometheus_sliding_window.erl b/src/instrument/mongoose_prometheus_sliding_window.erl
new file mode 100644
index 00000000000..d317baeefdb
--- /dev/null
+++ b/src/instrument/mongoose_prometheus_sliding_window.erl
@@ -0,0 +1,243 @@
+-module(mongoose_prometheus_sliding_window).
+
+%% Manages a sliding window of 20 DDSketch instances (3 seconds each = 60 seconds total)
+%% Each window uses ddskerl to store quantile summaries
+
+-export([declare/1,
+         observe/3,
+         value/2,
+         remove/2]).
+
+-export([start_link/0,
+         init/1,
+         handle_call/3,
+         handle_cast/2,
+         handle_info/2,
+         terminate/2,
+         code_change/3]).
+
+-export([default_quantiles/0, default_error/0]).
+
+-behaviour(gen_server).
+
+-define(WINDOW_COUNT, 20).
+-define(WINDOW_DURATION_MS, 3000). % 3 seconds per window
+-define(TOTAL_WINDOW_MS, (?WINDOW_COUNT * ?WINDOW_DURATION_MS)). % 60 seconds total
+
+-spec default_quantiles() -> [number()].
+default_quantiles() -> [0.5, 0.75, 0.90, 0.95, 0.99, 0.999].
+
+-spec default_error() -> number().
+default_error() -> 0.01.
+
+-record(state, {
+    timer_ref :: reference() | undefined,
+    metrics = #{} :: #{name() => #{label_values() => metric_state()}}
+}).
+
+-type name() :: string().
+-type label_values() :: [mongoose_instrument:label_value()].
+-type metric_state() :: #{windows => [{ddskerl_ets:ddsketch(), non_neg_integer()}],
+                          current_index => non_neg_integer()}.
+
+%% Public API
+
+-spec declare(proplists:proplist()) -> boolean().
+declare(MetricSpec) ->
+    Name = proplists:get_value(name, MetricSpec),
+    gen_server:call(?MODULE, {declare, Name, MetricSpec}).
+
+-spec observe(name(), label_values(), number()) -> ok.
+observe(Name, LabelValues, Value) ->
+    gen_server:cast(?MODULE, {observe, Name, LabelValues, Value}).
+
+-spec value(name(), label_values()) ->
+    undefined | {non_neg_integer(), number(), [{number(), number()}]}.
+value(Name, LabelValues) ->
+    gen_server:call(?MODULE, {value, Name, LabelValues}).
+
+-spec remove(name(), label_values()) -> boolean().
+remove(Name, LabelValues) ->
+    gen_server:call(?MODULE, {remove, Name, LabelValues}).
+
+%% gen_server callbacks
+
+start_link() ->
+    gen_server:start_link({local, ?MODULE}, ?MODULE, [], []).
+
+init([]) ->
+    TimerRef = erlang:start_timer(?WINDOW_DURATION_MS, self(), rotate),
+    {ok, #state{timer_ref = TimerRef}}.
+
+handle_call({declare, Name, _MetricSpec}, _From, State) ->
+    case maps:is_key(Name, State#state.metrics) of
+        true ->
+            {reply, false, State};
+        false ->
+            NewMetrics = maps:put(Name, #{}, State#state.metrics),
+            {reply, true, State#state{metrics = NewMetrics}}
+    end;
+
+handle_call({observe, Name, LabelValues, Value}, _From, State) ->
+    NewState = do_observe(Name, LabelValues, Value, State),
+    {reply, ok, NewState};
+
+handle_call({value, Name, LabelValues}, _From, State) ->
+    {Result, NewState} = get_value(Name, LabelValues, State),
+    {reply, Result, NewState};
+
+handle_call({remove, Name, LabelValues}, _From, State) ->
+    NewState = do_remove(Name, LabelValues, State),
+    {reply, true, NewState};
+
+handle_call(_Request, _From, State) ->
+    {reply, ok, State}.
+
+handle_cast({observe, Name, LabelValues, Value}, State) ->
+    NewState = do_observe(Name, LabelValues, Value, State),
+    {noreply, NewState};
+
+handle_cast(_Msg, State) ->
+    {noreply, State}.
+
+handle_info({timeout, _TimerRef, rotate}, State) ->
+    %% The rotation is handled in ensure_windows_rotated, so we just reschedule
+    TimerRef = erlang:start_timer(?WINDOW_DURATION_MS, self(), rotate),
+    {noreply, State#state{timer_ref = TimerRef}};
+
+handle_info(_Info, State) ->
+    {noreply, State}.
+
+terminate(_Reason, _State) ->
+    ok.
+
+code_change(_OldVsn, State, _Extra) ->
+    {ok, State}.
+
+%% Internal functions
+
+-spec do_observe(name(), label_values(), number(), #state{}) -> #state{}.
+do_observe(Name, LabelValues, Value, State) ->
+    case maps:find(Name, State#state.metrics) of
+        error ->
+            State; % metric not declared, ignore
+        {ok, MetricState} ->
+            Key = {Name, LabelValues},
+            CurrentTime = erlang:monotonic_time(millisecond),
+            {Windows, CurrentIndex} =
+                ensure_windows_rotated(Key, CurrentTime, State),
+
+            %% Get current window and add observation
+            {CurrentWindow, WindowStartTime} = lists:nth(CurrentIndex + 1, Windows),
+            UpdatedWindow = ddskerl_std:insert(CurrentWindow, Value),
+
+            %% Update windows list and metric state
+            UpdatedWindows = set_nth(CurrentIndex + 1, {UpdatedWindow, WindowStartTime}, Windows),
+            UpdatedMetricState = maps:put(Key,
+                #{windows => UpdatedWindows,
+                  current_index => CurrentIndex},
+                MetricState),
+
+            UpdatedMetrics = maps:put(Name, UpdatedMetricState, State#state.metrics),
+            State#state{metrics = UpdatedMetrics}
+    end.
+
+-spec ensure_windows_rotated({name(), label_values()}, non_neg_integer(), #state{}) ->
+    {[{ddskerl_std:ddsketch(), non_neg_integer()}], non_neg_integer()}.
+ensure_windows_rotated(Key, CurrentTime, State) ->
+    case get_metric_state(Key, State) of
+        undefined ->
+            %% Initialize new windows, all starting at current time (they'll be rotated as needed)
+            Windows = [{ddskerl_std:new(#{error => default_error()}), CurrentTime} || _ <- lists:seq(1, ?WINDOW_COUNT)],
+            {Windows, 0};
+        #{windows := Windows, current_index := CurrentIndex} ->
+            {_CurrentWindow, WindowStartTime} = lists:nth(CurrentIndex + 1, Windows),
+            Elapsed = CurrentTime - WindowStartTime,
+            if
+                Elapsed >= ?WINDOW_DURATION_MS ->
+                    NewIndex = (CurrentIndex + 1) rem ?WINDOW_COUNT,
+                    NewWindow = {ddskerl_std:new(#{error => default_error()}), CurrentTime},
+                    UpdatedWindows = set_nth(CurrentIndex + 1, NewWindow, Windows),
+                    {UpdatedWindows, NewIndex};
+                true ->
+                    {Windows, CurrentIndex}
+            end
+    end.
+
+-spec get_value(name(), label_values(), #state{}) ->
+    {undefined | {non_neg_integer(), number(), [{number(), number()}]}, #state{}}.
+get_value(Name, LabelValues, State) ->
+    Key = {Name, LabelValues},
+    case get_metric_state(Key, State) of
+        undefined ->
+            {undefined, State};
+        #{} ->
+            CurrentTime = erlang:monotonic_time(millisecond),
+            {UpdatedWindows, NewIndex} = ensure_windows_rotated(Key, CurrentTime, State),
+
+            %% Update metric state with rotated windows
+            {ok, MetricState} = maps:find(Name, State#state.metrics),
+            UpdatedMetricState = maps:put(Key,
+                #{windows => UpdatedWindows,
+                  current_index => NewIndex},
+                MetricState),
+            UpdatedMetrics = maps:put(Name, UpdatedMetricState, State#state.metrics),
+            NewState = State#state{metrics = UpdatedMetrics},
+
+            %% Filter windows to only include those within the last 60 seconds
+            CutoffTime = CurrentTime - ?TOTAL_WINDOW_MS,
+            ActiveWindows = [Window || {Window, StartTime} <- UpdatedWindows,
+                                      StartTime >= CutoffTime],
+
+            %% Merge all active windows
+            Merged = lists:foldl(fun(Window, Acc) ->
+                                    case Acc of
+                                        undefined -> Window;
+                                        _ -> ddskerl_std:merge(Acc, Window)
+                                    end
+                                end, undefined, ActiveWindows),
+
+            Result = case Merged of
+                undefined ->
+                    undefined;
+                _ ->
+                    Count = ddskerl_std:total(Merged),
+                    Sum = ddskerl_std:sum(Merged),
+                    Quantiles = [{Q, ddskerl_std:quantile(Merged, Q)} || Q <- default_quantiles()],
+                    {Count, Sum, Quantiles}
+            end,
+            {Result, NewState}
+    end.
+
+-spec do_remove(name(), label_values(), #state{}) -> #state{}.
+do_remove(Name, LabelValues, State) ->
+    Key = {Name, LabelValues},
+    case maps:find(Name, State#state.metrics) of
+        error ->
+            State;
+        {ok, MetricState} ->
+            UpdatedMetricState = maps:remove(Key, MetricState),
+            if
+                map_size(UpdatedMetricState) == 0 ->
+                    UpdatedMetrics = maps:remove(Name, State#state.metrics),
+                    State#state{metrics = UpdatedMetrics};
+                true ->
+                    UpdatedMetrics = maps:put(Name, UpdatedMetricState, State#state.metrics),
+                    State#state{metrics = UpdatedMetrics}
+            end
+    end.
+
+-spec get_metric_state({name(), label_values()}, #state{}) -> undefined | metric_state().
+get_metric_state({Name, LabelValues}, State) ->
+    case State#state.metrics of
+       #{Name := MetricState} ->
+           maps:get({Name, LabelValues}, MetricState, undefined);
+       #{} ->
+           undefined
+   end.
+
+-spec set_nth(pos_integer(), T, [T]) -> [T].
+set_nth(1, Value, [_ | Rest]) ->
+    [Value | Rest];
+set_nth(N, Value, [H | Rest]) when N > 1 ->
+    [H | set_nth(N - 1, Value, Rest)].

From 657a0fafe762f122c6537846624d1e1f8edbc906 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Micha=C5=82=20Dobranowski?= <mdbrnowski@gmail.com>
Date: Wed, 17 Dec 2025 12:17:49 +0100
Subject: [PATCH 09/24] Add ddskerl to dialyzer `plt_extra_apps`

---
 rebar.config | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/rebar.config b/rebar.config
index 2993aa3fe21..13b971ed25c 100644
--- a/rebar.config
+++ b/rebar.config
@@ -219,7 +219,7 @@
     {plt_extra_apps, [jid, cowboy, cowlib, lasse, p1_utils, ranch, gen_fsm_compat, epgsql, cqerl,
                       common_test, tools, amqp_client, jiffy, erl_csv, inets, compiler, jsx,
                       rabbit_common, mysql, eredis, erlcloud, telemetry,
-                      nksip, nklib, nkservice, nkpacket, prometheus, exometer_core,
+                      nksip, nklib, nkservice, nkpacket, ddskerl, prometheus, exometer_core,
                       mnesia, cets, cpool, tirerl, erlang_doctor]}]}.
 
 {cover_print_enabled, true}.

From 36bff41b759331684f478e1541f2da63ae4726fd Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Micha=C5=82=20Dobranowski?= <mdbrnowski@gmail.com>
Date: Thu, 18 Dec 2025 13:22:39 +0100
Subject: [PATCH 10/24] Create and register new prometheus collector

---
 .../mongoose_instrument_prometheus.erl        |  1 +
 .../mongoose_prometheus_sliding_window.erl    | 60 +++++++++++++++----
 ...se_prometheus_sliding_window_collector.erl | 57 ++++++++++++++++++
 3 files changed, 107 insertions(+), 11 deletions(-)
 create mode 100644 src/instrument/mongoose_prometheus_sliding_window_collector.erl

diff --git a/src/instrument/mongoose_instrument_prometheus.erl b/src/instrument/mongoose_instrument_prometheus.erl
index a125c14fd59..002bc62a3b7 100644
--- a/src/instrument/mongoose_instrument_prometheus.erl
+++ b/src/instrument/mongoose_instrument_prometheus.erl
@@ -20,6 +20,7 @@ start(#{}) ->
         _ ->
             ok
     end,
+    prometheus_registry:register_collector(mongoose_prometheus_sliding_window_collector),
     ok.
 
 -spec set_up(mongoose_instrument:event_name(), mongoose_instrument:labels(),
diff --git a/src/instrument/mongoose_prometheus_sliding_window.erl b/src/instrument/mongoose_prometheus_sliding_window.erl
index d317baeefdb..4205c6cb4ad 100644
--- a/src/instrument/mongoose_prometheus_sliding_window.erl
+++ b/src/instrument/mongoose_prometheus_sliding_window.erl
@@ -5,8 +5,10 @@
 
 -export([declare/1,
          observe/3,
-         value/2,
-         remove/2]).
+         values/1,
+         remove/2,
+         get_all_metric_names/0,
+         get_metric_spec/1]).
 
 -export([start_link/0,
          init/1,
@@ -32,7 +34,8 @@ default_error() -> 0.01.
 
 -record(state, {
     timer_ref :: reference() | undefined,
-    metrics = #{} :: #{name() => #{label_values() => metric_state()}}
+    metrics = #{} :: #{name() => #{label_values() => metric_state()}},
+    metric_specs = #{} :: #{name() => proplists:proplist()}
 }).
 
 -type name() :: string().
@@ -51,15 +54,22 @@ declare(MetricSpec) ->
 observe(Name, LabelValues, Value) ->
     gen_server:cast(?MODULE, {observe, Name, LabelValues, Value}).
 
--spec value(name(), label_values()) ->
-    undefined | {non_neg_integer(), number(), [{number(), number()}]}.
-value(Name, LabelValues) ->
-    gen_server:call(?MODULE, {value, Name, LabelValues}).
+-spec values(name()) -> [{label_values(), {non_neg_integer(), number(), [{number(), number()}]}}].
+values(Name) ->
+    gen_server:call(?MODULE, {values, Name}).
 
 -spec remove(name(), label_values()) -> boolean().
 remove(Name, LabelValues) ->
     gen_server:call(?MODULE, {remove, Name, LabelValues}).
 
+-spec get_all_metric_names() -> [name()].
+get_all_metric_names() ->
+    gen_server:call(?MODULE, get_all_metric_names).
+
+-spec get_metric_spec(name()) -> proplists:proplist() | undefined.
+get_metric_spec(Name) ->
+    gen_server:call(?MODULE, {get_metric_spec, Name}).
+
 %% gen_server callbacks
 
 start_link() ->
@@ -69,27 +79,36 @@ init([]) ->
     TimerRef = erlang:start_timer(?WINDOW_DURATION_MS, self(), rotate),
     {ok, #state{timer_ref = TimerRef}}.
 
-handle_call({declare, Name, _MetricSpec}, _From, State) ->
+handle_call({declare, Name, MetricSpec}, _From, State) ->
     case maps:is_key(Name, State#state.metrics) of
         true ->
             {reply, false, State};
         false ->
             NewMetrics = maps:put(Name, #{}, State#state.metrics),
-            {reply, true, State#state{metrics = NewMetrics}}
+            NewSpecs = maps:put(Name, MetricSpec, State#state.metric_specs),
+            {reply, true, State#state{metrics = NewMetrics, metric_specs = NewSpecs}}
     end;
 
 handle_call({observe, Name, LabelValues, Value}, _From, State) ->
     NewState = do_observe(Name, LabelValues, Value, State),
     {reply, ok, NewState};
 
-handle_call({value, Name, LabelValues}, _From, State) ->
-    {Result, NewState} = get_value(Name, LabelValues, State),
+handle_call({values, Name}, _From, State) ->
+    {Result, NewState} = get_values(Name, State),
     {reply, Result, NewState};
 
 handle_call({remove, Name, LabelValues}, _From, State) ->
     NewState = do_remove(Name, LabelValues, State),
     {reply, true, NewState};
 
+handle_call(get_all_metric_names, _From, State) ->
+    Names = maps:keys(State#state.metrics),
+    {reply, Names, State};
+
+handle_call({get_metric_spec, Name}, _From, State) ->
+    Spec = maps:get(Name, State#state.metric_specs, undefined),
+    {reply, Spec, State};
+
 handle_call(_Request, _From, State) ->
     {reply, ok, State}.
 
@@ -209,6 +228,25 @@ get_value(Name, LabelValues, State) ->
             {Result, NewState}
     end.
 
+-spec get_values(name(), #state{}) -> {[{label_values(), {non_neg_integer(), number(), [{number(), number()}]}}], #state{}}.
+get_values(Name, State) ->
+    case maps:find(Name, State#state.metrics) of
+        error ->
+            {[], State};
+        {ok, MetricState} ->
+            Keys = maps:keys(MetricState),
+            {ResultsRev, FinalState} = lists:foldl(
+                fun({_, LabelValues}, {AccRes, S}) ->
+                    {Res, S2} = get_value(Name, LabelValues, S),
+                    NewAcc = case Res of
+                        undefined -> AccRes;
+                        _ -> [{LabelValues, Res} | AccRes]
+                    end,
+                    {NewAcc, S2}
+                end, {[], State}, Keys),
+            {lists:reverse(ResultsRev), FinalState}
+    end.
+
 -spec do_remove(name(), label_values(), #state{}) -> #state{}.
 do_remove(Name, LabelValues, State) ->
     Key = {Name, LabelValues},
diff --git a/src/instrument/mongoose_prometheus_sliding_window_collector.erl b/src/instrument/mongoose_prometheus_sliding_window_collector.erl
new file mode 100644
index 00000000000..47d747027b5
--- /dev/null
+++ b/src/instrument/mongoose_prometheus_sliding_window_collector.erl
@@ -0,0 +1,57 @@
+-module(mongoose_prometheus_sliding_window_collector).
+
+-behaviour(prometheus_collector).
+
+-export([deregister_cleanup/1,
+         collect_mf/2]).
+
+%% Collector behavior callbacks
+
+deregister_cleanup(_) ->
+    ok.
+
+collect_mf(_Registry, Callback) ->
+    %% Get all metric names from the sliding window manager
+    MetricNames = mongoose_prometheus_sliding_window:get_all_metric_names(),
+    lists:foreach(fun(Name) ->
+                      collect_metric_family(Name, Callback)
+                  end, MetricNames),
+    ok.
+
+%% Internal functions
+
+collect_metric_family(Name, Callback) ->
+    Values = mongoose_prometheus_sliding_window:values(Name),
+    case Values of
+        [] ->
+            ok;
+        _ ->
+            SWName = sliding_window_metric_name(Name),
+            MetricSpec = mongoose_prometheus_sliding_window:get_metric_spec(Name),
+            Help = case MetricSpec of
+                       undefined ->
+                           "60-second sliding window quantile summary for " ++ Name;
+                       Spec ->
+                           BaseHelp = proplists:get_value(help, Spec, ""),
+                           BaseHelp ++ " (60s sliding window)"
+                   end,
+            LabelKeys = case MetricSpec of
+                            undefined -> [];
+                            Spec2 -> proplists:get_value(labels, Spec2, [])
+                        end,
+            %% Convert to Prometheus format
+            Metrics = [create_summary_metric(LabelKeys, LabelValues, Count, Sum, Quantiles)
+                       || {LabelValues, {Count, Sum, Quantiles}} <- Values],
+            %% Create and send the metric family
+            MF = prometheus_model_helpers:create_mf(SWName, Help, summary, Metrics),
+            Callback(MF)
+    end.
+
+sliding_window_metric_name(Name) ->
+    Name ++ "_60s".
+
+create_summary_metric(LabelKeys, LabelValues, Count, Sum, Quantiles) ->
+    %% Convert label keys and values to label pairs format
+    LabelPairs = lists:zip(LabelKeys, LabelValues),
+    QuantilePairs = [{Q, V} || {Q, V} <- Quantiles],
+    prometheus_model_helpers:summary_metric(LabelPairs, Count, Sum, QuantilePairs).

From 6f69268f751c62d6c61c29b64ffcc63fed0c0540 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Micha=C5=82=20Dobranowski?= <mdbrnowski@gmail.com>
Date: Fri, 19 Dec 2025 13:18:00 +0100
Subject: [PATCH 11/24] Make sure that sliding_window is started

---
 .../mongoose_prometheus_sliding_window.erl    | 21 +++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/src/instrument/mongoose_prometheus_sliding_window.erl b/src/instrument/mongoose_prometheus_sliding_window.erl
index 4205c6cb4ad..c4c6e4805a0 100644
--- a/src/instrument/mongoose_prometheus_sliding_window.erl
+++ b/src/instrument/mongoose_prometheus_sliding_window.erl
@@ -47,27 +47,33 @@ default_error() -> 0.01.
 
 -spec declare(proplists:proplist()) -> boolean().
 declare(MetricSpec) ->
+    ok = ensure_started(),
     Name = proplists:get_value(name, MetricSpec),
     gen_server:call(?MODULE, {declare, Name, MetricSpec}).
 
 -spec observe(name(), label_values(), number()) -> ok.
 observe(Name, LabelValues, Value) ->
+    ok = ensure_started(),
     gen_server:cast(?MODULE, {observe, Name, LabelValues, Value}).
 
 -spec values(name()) -> [{label_values(), {non_neg_integer(), number(), [{number(), number()}]}}].
 values(Name) ->
+    ok = ensure_started(),
     gen_server:call(?MODULE, {values, Name}).
 
 -spec remove(name(), label_values()) -> boolean().
 remove(Name, LabelValues) ->
+    ok = ensure_started(),
     gen_server:call(?MODULE, {remove, Name, LabelValues}).
 
 -spec get_all_metric_names() -> [name()].
 get_all_metric_names() ->
+    ok = ensure_started(),
     gen_server:call(?MODULE, get_all_metric_names).
 
 -spec get_metric_spec(name()) -> proplists:proplist() | undefined.
 get_metric_spec(Name) ->
+    ok = ensure_started(),
     gen_server:call(?MODULE, {get_metric_spec, Name}).
 
 %% gen_server callbacks
@@ -133,6 +139,21 @@ terminate(_Reason, _State) ->
 code_change(_OldVsn, State, _Extra) ->
     {ok, State}.
 
+%% Internal helpers
+
+-spec ensure_started() -> ok.
+ensure_started() ->
+    case whereis(?MODULE) of
+        undefined ->
+            case start_link() of
+                {ok, _Pid} -> ok;
+                {error, {already_started, _Pid}} -> ok;
+                {error, Reason} -> exit(Reason)
+            end;
+        _Pid ->
+            ok
+    end.
+
 %% Internal functions
 
 -spec do_observe(name(), label_values(), number(), #state{}) -> #state{}.

From bdd9b9423e6a4f30bd90e863fcbe9f598b67a303 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Micha=C5=82=20Dobranowski?= <mdbrnowski@gmail.com>
Date: Mon, 22 Dec 2025 13:57:11 +0100
Subject: [PATCH 12/24] Change `ddskerl_std` -> `ddskerl_ets`

---
 .../mongoose_instrument_prometheus.erl        |   2 +-
 .../mongoose_prometheus_sliding_window.erl    | 118 +++++++++++++++---
 2 files changed, 99 insertions(+), 21 deletions(-)

diff --git a/src/instrument/mongoose_instrument_prometheus.erl b/src/instrument/mongoose_instrument_prometheus.erl
index 002bc62a3b7..a982f32270e 100644
--- a/src/instrument/mongoose_instrument_prometheus.erl
+++ b/src/instrument/mongoose_instrument_prometheus.erl
@@ -71,7 +71,7 @@ declare_metric(MetricSpec, histogram) ->
         {error, mongoose_prometheus_sliding_window:default_error()},
         %% Measuring in µs suffices for actions lasting up to a day (with 1% accuracy).
         %% Measuring in bytes suffices for sizes up to 81 GB (with 1% accuracy).
-        {bound, 1260}
+        {bound, mongoose_prometheus_sliding_window:default_bound()}
     | MetricSpec]),
     SWResult or PromResult.
 
diff --git a/src/instrument/mongoose_prometheus_sliding_window.erl b/src/instrument/mongoose_prometheus_sliding_window.erl
index c4c6e4805a0..e9ecc84e949 100644
--- a/src/instrument/mongoose_prometheus_sliding_window.erl
+++ b/src/instrument/mongoose_prometheus_sliding_window.erl
@@ -18,7 +18,7 @@
          terminate/2,
          code_change/3]).
 
--export([default_quantiles/0, default_error/0]).
+-export([default_quantiles/0, default_error/0, default_bound/0]).
 
 -behaviour(gen_server).
 
@@ -26,21 +26,29 @@
 -define(WINDOW_DURATION_MS, 3000). % 3 seconds per window
 -define(TOTAL_WINDOW_MS, (?WINDOW_COUNT * ?WINDOW_DURATION_MS)). % 60 seconds total
 
+
 -spec default_quantiles() -> [number()].
 default_quantiles() -> [0.5, 0.75, 0.90, 0.95, 0.99, 0.999].
 
 -spec default_error() -> number().
 default_error() -> 0.01.
 
+-spec default_bound() -> non_neg_integer().
+default_bound() -> 1260.
+
 -record(state, {
     timer_ref :: reference() | undefined,
-    metrics = #{} :: #{name() => #{label_values() => metric_state()}},
+    ets_table :: ets:tab() | undefined,
+    metrics = #{} :: #{name() => #{{name(), label_values()} => metric_state()}},
     metric_specs = #{} :: #{name() => proplists:proplist()}
 }).
 
 -type name() :: string().
 -type label_values() :: [mongoose_instrument:label_value()].
--type metric_state() :: #{windows => [{ddskerl_ets:ddsketch(), non_neg_integer()}],
+-type window_data() :: #{sketch := ddskerl_ets:ddsketch(),
+                         ref := ets:tab(),
+                         name := term()}.
+-type metric_state() :: #{windows => [{window_data(), non_neg_integer()}],
                           current_index => non_neg_integer()}.
 
 %% Public API
@@ -82,8 +90,9 @@ start_link() ->
     gen_server:start_link({local, ?MODULE}, ?MODULE, [], []).
 
 init([]) ->
+    EtsTable = ets:new(?MODULE, [set, private, {read_concurrency, true}, {write_concurrency, true}]),
     TimerRef = erlang:start_timer(?WINDOW_DURATION_MS, self(), rotate),
-    {ok, #state{timer_ref = TimerRef}}.
+    {ok, #state{timer_ref = TimerRef, ets_table = EtsTable}}.
 
 handle_call({declare, Name, MetricSpec}, _From, State) ->
     case maps:is_key(Name, State#state.metrics) of
@@ -169,7 +178,9 @@ do_observe(Name, LabelValues, Value, State) ->
 
             %% Get current window and add observation
             {CurrentWindow, WindowStartTime} = lists:nth(CurrentIndex + 1, Windows),
-            UpdatedWindow = ddskerl_std:insert(CurrentWindow, Value),
+            Sketch = maps:get(sketch, CurrentWindow),
+            UpdatedSketch = ddskerl_ets:insert(Sketch, Value),
+            UpdatedWindow = CurrentWindow#{sketch := UpdatedSketch},
 
             %% Update windows list and metric state
             UpdatedWindows = set_nth(CurrentIndex + 1, {UpdatedWindow, WindowStartTime}, Windows),
@@ -183,12 +194,12 @@ do_observe(Name, LabelValues, Value, State) ->
     end.
 
 -spec ensure_windows_rotated({name(), label_values()}, non_neg_integer(), #state{}) ->
-    {[{ddskerl_std:ddsketch(), non_neg_integer()}], non_neg_integer()}.
+    {[{window_data(), non_neg_integer()}], non_neg_integer()}.
 ensure_windows_rotated(Key, CurrentTime, State) ->
     case get_metric_state(Key, State) of
         undefined ->
             %% Initialize new windows, all starting at current time (they'll be rotated as needed)
-            Windows = [{ddskerl_std:new(#{error => default_error()}), CurrentTime} || _ <- lists:seq(1, ?WINDOW_COUNT)],
+            Windows = [new_window(Key, Index, CurrentTime, State) || Index <- lists:seq(0, ?WINDOW_COUNT - 1)],
             {Windows, 0};
         #{windows := Windows, current_index := CurrentIndex} ->
             {_CurrentWindow, WindowStartTime} = lists:nth(CurrentIndex + 1, Windows),
@@ -196,8 +207,9 @@ ensure_windows_rotated(Key, CurrentTime, State) ->
             if
                 Elapsed >= ?WINDOW_DURATION_MS ->
                     NewIndex = (CurrentIndex + 1) rem ?WINDOW_COUNT,
-                    NewWindow = {ddskerl_std:new(#{error => default_error()}), CurrentTime},
-                    UpdatedWindows = set_nth(CurrentIndex + 1, NewWindow, Windows),
+                    {CurrentWindow, _} = lists:nth(CurrentIndex + 1, Windows),
+                    ResetWindow = reset_window(CurrentWindow),
+                    UpdatedWindows = set_nth(CurrentIndex + 1, {ResetWindow, CurrentTime}, Windows),
                     {UpdatedWindows, NewIndex};
                 true ->
                     {Windows, CurrentIndex}
@@ -230,21 +242,26 @@ get_value(Name, LabelValues, State) ->
                                       StartTime >= CutoffTime],
 
             %% Merge all active windows
-            Merged = lists:foldl(fun(Window, Acc) ->
-                                    case Acc of
-                                        undefined -> Window;
-                                        _ -> ddskerl_std:merge(Acc, Window)
-                                    end
-                                end, undefined, ActiveWindows),
+            WindowTuples = [window_tuple(Window) || Window <- ActiveWindows],
+            Merged = lists:foldl(fun
+                                      (undefined, Acc) -> Acc;
+                                      (Val, undefined) -> Val;
+                                      (Val, Acc) -> ddskerl_ets:merge_tuples(Acc, Val)
+                                  end, undefined, WindowTuples),
 
             Result = case Merged of
                 undefined ->
                     undefined;
                 _ ->
-                    Count = ddskerl_std:total(Merged),
-                    Sum = ddskerl_std:sum(Merged),
-                    Quantiles = [{Q, ddskerl_std:quantile(Merged, Q)} || Q <- default_quantiles()],
-                    {Count, Sum, Quantiles}
+                    Count = ddskerl_ets:total_tuple(Merged),
+                    case Count of
+                        0 ->
+                            undefined;
+                        _ ->
+                            Sum = ddskerl_ets:sum_tuple(Merged),
+                            Quantiles = [{Q, ddskerl_ets:quantile_tuple(Merged, Q)} || Q <- default_quantiles()],
+                            {Count, Sum, Quantiles}
+                    end
             end,
             {Result, NewState}
     end.
@@ -275,7 +292,7 @@ do_remove(Name, LabelValues, State) ->
         error ->
             State;
         {ok, MetricState} ->
-            UpdatedMetricState = maps:remove(Key, MetricState),
+            UpdatedMetricState = remove_metric_state(Key, MetricState),
             if
                 map_size(UpdatedMetricState) == 0 ->
                     UpdatedMetrics = maps:remove(Name, State#state.metrics),
@@ -300,3 +317,64 @@ set_nth(1, Value, [_ | Rest]) ->
     [Value | Rest];
 set_nth(N, Value, [H | Rest]) when N > 1 ->
     [H | set_nth(N - 1, Value, Rest)].
+
+%% Window helpers
+
+-spec metric_options(name(), #state{}) -> {number(), non_neg_integer()}.
+metric_options(Name, State) ->
+    MetricSpec = maps:get(Name, State#state.metric_specs, []),
+    Error = proplists:get_value(error, MetricSpec, default_error()),
+    Bound = proplists:get_value(bound, MetricSpec, default_bound()),
+    {Error, Bound}.
+
+-spec window_name({name(), label_values()}, non_neg_integer()) -> term().
+window_name({Name, LabelValues}, Index) ->
+    {Name, LabelValues, Index}.
+
+-spec new_window({name(), label_values()}, non_neg_integer(), non_neg_integer(), #state{}) ->
+    {window_data(), non_neg_integer()}.
+new_window(Key = {Name, _LabelValues}, Index, StartTime, State) ->
+    {Error, Bound} = metric_options(Name, State),
+    WindowName = window_name(Key, Index),
+    Sketch = ddskerl_ets:new(#{ets_table => State#state.ets_table,
+                               name => WindowName,
+                               error => Error,
+                               bound => Bound}),
+    WindowData = #{sketch => Sketch,
+                   ref => State#state.ets_table,
+                   name => WindowName},
+    {WindowData, StartTime}.
+
+-spec reset_window(window_data()) -> window_data().
+reset_window(WindowData) ->
+    Sketch = maps:get(sketch, WindowData),
+    ResetSketch = ddskerl_ets:reset(Sketch),
+    WindowData#{sketch := ResetSketch}.
+
+-spec window_tuple(window_data()) -> ddskerl_ets:object() | undefined.
+window_tuple(WindowData) ->
+    Ref = maps:get(ref, WindowData),
+    Name = maps:get(name, WindowData),
+    case ets:lookup(Ref, Name) of
+        [Val] -> Val;
+        [] -> undefined
+    end.
+
+-spec remove_metric_state({name(), label_values()}, #{}) -> #{}.
+remove_metric_state(Key, MetricState) ->
+    UpdatedMetricState =
+        case maps:find(Key, MetricState) of
+            error ->
+                MetricState;
+            {ok, #{windows := Windows}} ->
+                lists:foreach(fun({Window, _}) -> delete_window(Window) end, Windows),
+                maps:remove(Key, MetricState)
+        end,
+    UpdatedMetricState.
+
+-spec delete_window(window_data()) -> ok.
+delete_window(WindowData) ->
+    Ref = maps:get(ref, WindowData),
+    Name = maps:get(name, WindowData),
+    ets:delete(Ref, Name),
+    ok.

From 3400ad4d7861fb85c90e6c7d5267e0e3437e1388 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Micha=C5=82=20Dobranowski?= <mdbrnowski@gmail.com>
Date: Tue, 23 Dec 2025 13:54:21 +0100
Subject: [PATCH 13/24] Upgrade ddskerl (and prometheus)

---
 rebar.config |  4 ++--
 rebar.lock   | 12 ++++++------
 2 files changed, 8 insertions(+), 8 deletions(-)

diff --git a/rebar.config b/rebar.config
index 13b971ed25c..c597110540e 100644
--- a/rebar.config
+++ b/rebar.config
@@ -56,8 +56,8 @@
   {exometer_report_statsd, {git, "https://github.com/esl/exometer_report_statsd.git", {branch, "master"}}},
   {syslogger, "0.3.0"},
   {flatlog, "0.1.2"},
-  {ddskerl, "0.4.2"},
-  {prometheus, "6.1.1"},
+  {ddskerl, "0.4.3"},
+  {prometheus, "6.1.2"},
   {prometheus_cowboy, "0.2.0"},
 
   %%% Stateless libraries
diff --git a/rebar.lock b/rebar.lock
index 98dbee6ce43..68bac0aae92 100644
--- a/rebar.lock
+++ b/rebar.lock
@@ -19,7 +19,7 @@
  {<<"credentials_obfuscation">>,
   {pkg,<<"credentials_obfuscation">>,<<"3.5.0">>},
   1},
- {<<"ddskerl">>,{pkg,<<"ddskerl">>,<<"0.4.2">>},0},
+ {<<"ddskerl">>,{pkg,<<"ddskerl">>,<<"0.4.3">>},0},
  {<<"eini">>,{pkg,<<"eini">>,<<"1.2.9">>},1},
  {<<"epgsql">>,{pkg,<<"epgsql">>,<<"4.7.1">>},0},
  {<<"eredis">>,{pkg,<<"eredis">>,<<"1.7.1">>},0},
@@ -87,7 +87,7 @@
   0},
  {<<"parse_trans">>,{pkg,<<"parse_trans">>,<<"3.4.0">>},1},
  {<<"pooler">>,{pkg,<<"pooler">>,<<"1.5.3">>},1},
- {<<"prometheus">>,{pkg,<<"prometheus">>,<<"6.1.1">>},0},
+ {<<"prometheus">>,{pkg,<<"prometheus">>,<<"6.1.2">>},0},
  {<<"prometheus_cowboy">>,{pkg,<<"prometheus_cowboy">>,<<"0.2.0">>},0},
  {<<"prometheus_httpd">>,{pkg,<<"prometheus_httpd">>,<<"2.1.15">>},1},
  {<<"quickrand">>,{pkg,<<"quickrand">>,<<"2.0.7">>},1},
@@ -131,7 +131,7 @@
  {<<"cowlib">>, <<"623791C56C1CC9DF54A71A9C55147A401549917F00A2E48A6AE12B812C586CED">>},
  {<<"cpool">>, <<"76222AA1DAC0F8089707167BD69D221EDA63DB65B8BD67DBF6E133075392EEDC">>},
  {<<"credentials_obfuscation">>, <<"61E282ADFB4439486B3994FAAEC69543C7EE6CC7E70C6340E8853FD9DEAF8219">>},
- {<<"ddskerl">>, <<"A51A90BE9AC9B36A94017670BED479C623B10CA9D4BDA1EDF3A0E48CAEEADA2A">>},
+ {<<"ddskerl">>, <<"BB97B90EEF1B5906520CEBDD38DA29B18A770B5567F6297927F53566EAE9BEBB">>},
  {<<"eini">>, <<"FCC3CBD49BBDD9A1D9735C7365DAFFCD84481CCE81E6CB80537883AA44AC4895">>},
  {<<"epgsql">>, <<"D4E47CAE46C18C8AFA88E34D59A9B4BAE16368D7CE1EB3DA24FA755EB28393EB">>},
  {<<"eredis">>, <<"39E31AA02ADCD651C657F39AAFD4D31A9B2F63C6C700DC9CECE98D4BC3C897AB">>},
@@ -163,7 +163,7 @@
  {<<"p1_utils">>, <<"67B0C4AC9FA3BA3EF563B31AA111B0A004439A37FAC85E027F1C3617E1C7EC6C">>},
  {<<"parse_trans">>, <<"BB87AC362A03CA674EBB7D9D498F45C03256ADED7214C9101F7035EF44B798C7">>},
  {<<"pooler">>, <<"898CD1FA301FC42D4A8ED598CE139B71CA85B54C16AB161152B5CC5FBDCFA1A8">>},
- {<<"prometheus">>, <<"3C9D8E2D4FCF948450550693F6A82FAE013D3665BA10AA55955B64C2875FADB3">>},
+ {<<"prometheus">>, <<"E5C3C567CD8B0994425920763405635211183C15BDC47D0CD524807DDE20CA1D">>},
  {<<"prometheus_cowboy">>, <<"526F75D9850A9125496F78BCEECCA0F237BC7B403C976D44508543AE5967DAD9">>},
  {<<"prometheus_httpd">>, <<"8F767D819A5D36275EAB9264AFF40D87279151646776069BF69FBDBBD562BD75">>},
  {<<"quickrand">>, <<"D2BD76676A446E6A058D678444B7FDA1387B813710D1AF6D6E29BB92186C8820">>},
@@ -198,7 +198,7 @@
  {<<"cowlib">>, <<"0AF652D1550C8411C3B58EED7A035A7FB088C0B86AFF6BC504B0BC3B7F791AA2">>},
  {<<"cpool">>, <<"430E18DF4A9D584EB1ED0D196A87CC02E878AF5B4888BFDC9B65F86A96480E30">>},
  {<<"credentials_obfuscation">>, <<"843ADBE3246861CE0F1A0FA3222F384834EB31DEFD8D6B9CBA7AFD2977C957BC">>},
- {<<"ddskerl">>, <<"63F907373D7E548151D584D4DA8A38928FD26EC9477B94C0FFAAD87D7CB69FE7">>},
+ {<<"ddskerl">>, <<"4E0F6047C6A002CE38F6EA155276DD918CD635FD0A5EDB9E0B46AEB6F7AFF2C2">>},
  {<<"eini">>, <<"DA64AE8DB7C2F502E6F20CDF44CD3D9BE364412B87FF49FEBF282540F673DFCB">>},
  {<<"epgsql">>, <<"B6D86B7DC42C8555B1D4E20880E5099D6D6D053148000E188E548F98E4E01836">>},
  {<<"eredis">>, <<"7C2B54C566FED55FEEF3341CA79B0100A6348FD3F162184B7ED5118D258C3CC1">>},
@@ -230,7 +230,7 @@
  {<<"p1_utils">>, <<"D0379E8C1156B98BD64F8129C1DE022FCCA4F2FDB7486CE73BF0ED2C3376B04C">>},
  {<<"parse_trans">>, <<"F99E368830BEA44552224E37E04943A54874F08B8590485DE8D13832B63A2DC3">>},
  {<<"pooler">>, <<"058D85C5081289B90E97E4DDDBC3BB5A3B4A19A728AB3BC88C689EFCC36A07C7">>},
- {<<"prometheus">>, <<"C3ECC7A35676948089E28FF4383CCB85DCEF447F0982C01E5630A084008C456A">>},
+ {<<"prometheus">>, <<"109662001328112B860DE112D49B575310BE8FF33FE04BF0482EEC4B1E5A1278">>},
  {<<"prometheus_cowboy">>, <<"2C7EB12F4B970D91E3B47BAAD0F138F6ADC34E53EEB0AE18068FF0AFAB441B24">>},
  {<<"prometheus_httpd">>, <<"67736D000745184D5013C58A63E947821AB90CB9320BC2E6AE5D3061C6FFE039">>},
  {<<"quickrand">>, <<"B8ACBF89A224BC217C3070CA8BEBC6EB236DBE7F9767993B274084EA044D35F0">>},

From 92b5da291a7911746682e758feda90b65eef3c13 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Micha=C5=82=20Dobranowski?= <mdbrnowski@gmail.com>
Date: Fri, 16 Jan 2026 10:34:08 +0100
Subject: [PATCH 14/24] Fix format and remove unused function

---
 src/instrument/mongoose_prometheus_sliding_window.erl | 11 -----------
 1 file changed, 11 deletions(-)

diff --git a/src/instrument/mongoose_prometheus_sliding_window.erl b/src/instrument/mongoose_prometheus_sliding_window.erl
index e9ecc84e949..1b5ae252e80 100644
--- a/src/instrument/mongoose_prometheus_sliding_window.erl
+++ b/src/instrument/mongoose_prometheus_sliding_window.erl
@@ -103,34 +103,24 @@ handle_call({declare, Name, MetricSpec}, _From, State) ->
             NewSpecs = maps:put(Name, MetricSpec, State#state.metric_specs),
             {reply, true, State#state{metrics = NewMetrics, metric_specs = NewSpecs}}
     end;
-
-handle_call({observe, Name, LabelValues, Value}, _From, State) ->
-    NewState = do_observe(Name, LabelValues, Value, State),
-    {reply, ok, NewState};
-
 handle_call({values, Name}, _From, State) ->
     {Result, NewState} = get_values(Name, State),
     {reply, Result, NewState};
-
 handle_call({remove, Name, LabelValues}, _From, State) ->
     NewState = do_remove(Name, LabelValues, State),
     {reply, true, NewState};
-
 handle_call(get_all_metric_names, _From, State) ->
     Names = maps:keys(State#state.metrics),
     {reply, Names, State};
-
 handle_call({get_metric_spec, Name}, _From, State) ->
     Spec = maps:get(Name, State#state.metric_specs, undefined),
     {reply, Spec, State};
-
 handle_call(_Request, _From, State) ->
     {reply, ok, State}.
 
 handle_cast({observe, Name, LabelValues, Value}, State) ->
     NewState = do_observe(Name, LabelValues, Value, State),
     {noreply, NewState};
-
 handle_cast(_Msg, State) ->
     {noreply, State}.
 
@@ -138,7 +128,6 @@ handle_info({timeout, _TimerRef, rotate}, State) ->
     %% The rotation is handled in ensure_windows_rotated, so we just reschedule
     TimerRef = erlang:start_timer(?WINDOW_DURATION_MS, self(), rotate),
     {noreply, State#state{timer_ref = TimerRef}};
-
 handle_info(_Info, State) ->
     {noreply, State}.
 

From 8c7223bcfc44a623900a37b350ad89c5b21198e1 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Micha=C5=82=20Dobranowski?= <mdbrnowski@gmail.com>
Date: Wed, 21 Jan 2026 11:31:40 +0100
Subject: [PATCH 15/24] Add sliding window to the supervision tree

---
 src/ejabberd_sup.erl                          |  2 ++
 .../mongoose_instrument_prometheus.erl        |  7 ----
 .../mongoose_prometheus_sliding_window.erl    | 33 ++++++++-----------
 test/mongoose_instrument_metrics_SUITE.erl    |  7 +++-
 test/prometheus_endpoint_SUITE.erl            |  3 +-
 5 files changed, 23 insertions(+), 29 deletions(-)

diff --git a/src/ejabberd_sup.erl b/src/ejabberd_sup.erl
index 7750ed1103f..87662fedb51 100644
--- a/src/ejabberd_sup.erl
+++ b/src/ejabberd_sup.erl
@@ -45,6 +45,7 @@ start_link() ->
 init(noargs) ->
     Hooks = worker_spec(gen_hook),
     Instrument = worker_spec(mongoose_instrument),
+    PrometheusSlidingWindow = mongoose_prometheus_sliding_window:child_spec(),
     Cleaner = worker_spec(mongoose_cleaner),
     Router = worker_spec(ejabberd_router),
     S2S = worker_spec(ejabberd_s2s),
@@ -70,6 +71,7 @@ init(noargs) ->
            PG,
            Hooks,
            Instrument,
+           PrometheusSlidingWindow,
            Cleaner,
            SMBackendSupervisor,
            OutgoingPoolsSupervisor
diff --git a/src/instrument/mongoose_instrument_prometheus.erl b/src/instrument/mongoose_instrument_prometheus.erl
index a982f32270e..7d29073903c 100644
--- a/src/instrument/mongoose_instrument_prometheus.erl
+++ b/src/instrument/mongoose_instrument_prometheus.erl
@@ -13,13 +13,6 @@
 start(#{}) ->
     Apps = [prometheus, prometheus_httpd, prometheus_cowboy],
     {ok, _} = application:ensure_all_started(Apps, permanent),
-    %% Start sliding window manager for histogram metrics
-    case whereis(mongoose_prometheus_sliding_window) of
-        undefined ->
-            {ok, _} = mongoose_prometheus_sliding_window:start_link();
-        _ ->
-            ok
-    end,
     prometheus_registry:register_collector(mongoose_prometheus_sliding_window_collector),
     ok.
 
diff --git a/src/instrument/mongoose_prometheus_sliding_window.erl b/src/instrument/mongoose_prometheus_sliding_window.erl
index 1b5ae252e80..fdb68b66c55 100644
--- a/src/instrument/mongoose_prometheus_sliding_window.erl
+++ b/src/instrument/mongoose_prometheus_sliding_window.erl
@@ -3,7 +3,8 @@
 %% Manages a sliding window of 20 DDSketch instances (3 seconds each = 60 seconds total)
 %% Each window uses ddskerl to store quantile summaries
 
--export([declare/1,
+-export([child_spec/0,
+         declare/1,
          observe/3,
          values/1,
          remove/2,
@@ -20,6 +21,8 @@
 
 -export([default_quantiles/0, default_error/0, default_bound/0]).
 
+-ignore_xref([start_link/0]).
+
 -behaviour(gen_server).
 
 -define(WINDOW_COUNT, 20).
@@ -55,33 +58,27 @@ default_bound() -> 1260.
 
 -spec declare(proplists:proplist()) -> boolean().
 declare(MetricSpec) ->
-    ok = ensure_started(),
     Name = proplists:get_value(name, MetricSpec),
     gen_server:call(?MODULE, {declare, Name, MetricSpec}).
 
 -spec observe(name(), label_values(), number()) -> ok.
 observe(Name, LabelValues, Value) ->
-    ok = ensure_started(),
     gen_server:cast(?MODULE, {observe, Name, LabelValues, Value}).
 
 -spec values(name()) -> [{label_values(), {non_neg_integer(), number(), [{number(), number()}]}}].
 values(Name) ->
-    ok = ensure_started(),
     gen_server:call(?MODULE, {values, Name}).
 
 -spec remove(name(), label_values()) -> boolean().
 remove(Name, LabelValues) ->
-    ok = ensure_started(),
     gen_server:call(?MODULE, {remove, Name, LabelValues}).
 
 -spec get_all_metric_names() -> [name()].
 get_all_metric_names() ->
-    ok = ensure_started(),
     gen_server:call(?MODULE, get_all_metric_names).
 
 -spec get_metric_spec(name()) -> proplists:proplist() | undefined.
 get_metric_spec(Name) ->
-    ok = ensure_started(),
     gen_server:call(?MODULE, {get_metric_spec, Name}).
 
 %% gen_server callbacks
@@ -137,20 +134,16 @@ terminate(_Reason, _State) ->
 code_change(_OldVsn, State, _Extra) ->
     {ok, State}.
 
-%% Internal helpers
+%% Child spec for supervision tree
 
--spec ensure_started() -> ok.
-ensure_started() ->
-    case whereis(?MODULE) of
-        undefined ->
-            case start_link() of
-                {ok, _Pid} -> ok;
-                {error, {already_started, _Pid}} -> ok;
-                {error, Reason} -> exit(Reason)
-            end;
-        _Pid ->
-            ok
-    end.
+-spec child_spec() -> supervisor:child_spec().
+child_spec() ->
+    #{id => ?MODULE,
+      start => {?MODULE, start_link, []},
+      restart => permanent,
+      shutdown => timer:seconds(5),
+      type => worker,
+      modules => [?MODULE]}.
 
 %% Internal functions
 
diff --git a/test/mongoose_instrument_metrics_SUITE.erl b/test/mongoose_instrument_metrics_SUITE.erl
index eaa28c3a707..bd8931972c1 100644
--- a/test/mongoose_instrument_metrics_SUITE.erl
+++ b/test/mongoose_instrument_metrics_SUITE.erl
@@ -80,10 +80,15 @@ init_per_group(Group, Config) ->
     mongoose_config:set_opts(#{hosts => [?HOST_TYPE],
                                host_types => [?HOST_TYPE2],
                                instrumentation => opts(Group)}),
-    Config1 = async_helper:start(Config, mongoose_instrument, start_link, []),
+    Config1 = async_helper:start(Config, extra_processes(Group) ++
+                                 [{mongoose_instrument, start_link, []}]),
     mongoose_instrument:persist(),
     Config1 ++ extra_config(Group).
 
+extra_processes(prometheus) -> [{mongoose_prometheus_sliding_window, start_link, []}];
+extra_processes(prometheus_and_exometer) -> [{mongoose_prometheus_sliding_window, start_link, []}];
+extra_processes(_) -> [].
+
 end_per_group(Group, Config) ->
     async_helper:stop_all(Config),
     mongoose_config:erase_opts(),
diff --git a/test/prometheus_endpoint_SUITE.erl b/test/prometheus_endpoint_SUITE.erl
index b063481e1e7..1e589a8ed19 100644
--- a/test/prometheus_endpoint_SUITE.erl
+++ b/test/prometheus_endpoint_SUITE.erl
@@ -23,7 +23,8 @@ init_per_suite(Config) ->
     mongoose_config:set_opts(opts()),
     Config1 = async_helper:start(Config, [{mongoose_instrument, start_link, []},
                                           {mim_ct_sup, start_link, [ejabberd_sup]},
-                                          {mongoose_listener_sup, start_link, []}]),
+                                          {mongoose_listener_sup, start_link, []},
+                                          {mongoose_prometheus_sliding_window, start_link, []}]),
     mongoose_listener:start(),
     Config1.
 

From 185e948674510c9f64687130ba9b7be93c8cb147 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Micha=C5=82=20Dobranowski?= <mdbrnowski@gmail.com>
Date: Wed, 21 Jan 2026 15:25:05 +0100
Subject: [PATCH 16/24] Make window parameters configurable

---
 .../mongoose_prometheus_sliding_window.erl    | 26 +++++++++++--------
 ...se_prometheus_sliding_window_collector.erl |  9 ++++---
 2 files changed, 21 insertions(+), 14 deletions(-)

diff --git a/src/instrument/mongoose_prometheus_sliding_window.erl b/src/instrument/mongoose_prometheus_sliding_window.erl
index fdb68b66c55..59c04ea3604 100644
--- a/src/instrument/mongoose_prometheus_sliding_window.erl
+++ b/src/instrument/mongoose_prometheus_sliding_window.erl
@@ -1,7 +1,7 @@
 -module(mongoose_prometheus_sliding_window).
 
-%% Manages a sliding window of 20 DDSketch instances (3 seconds each = 60 seconds total)
-%% Each window uses ddskerl to store quantile summaries
+%% Manages a sliding window using DDSketch instances for quantile summaries.
+%% Window parameters are configurable via WINDOW_SIZE_MS and WINDOW_STEP_MS constants.
 
 -export([child_spec/0,
          declare/1,
@@ -19,15 +19,16 @@
          terminate/2,
          code_change/3]).
 
--export([default_quantiles/0, default_error/0, default_bound/0]).
+-export([default_quantiles/0, default_error/0, default_bound/0, window_size_s/0]).
 
 -ignore_xref([start_link/0]).
 
 -behaviour(gen_server).
 
--define(WINDOW_COUNT, 20).
--define(WINDOW_DURATION_MS, 3000). % 3 seconds per window
--define(TOTAL_WINDOW_MS, (?WINDOW_COUNT * ?WINDOW_DURATION_MS)). % 60 seconds total
+%% Configurable window parameters
+-define(WINDOW_SIZE_MS, 60000). % Total window size: 60 seconds
+-define(WINDOW_STEP_MS, 3000).  % Step size: 3 seconds per sub-window
+-define(WINDOW_COUNT, (?WINDOW_SIZE_MS div ?WINDOW_STEP_MS)). % Number of sub-windows
 
 
 -spec default_quantiles() -> [number()].
@@ -39,6 +40,9 @@ default_error() -> 0.01.
 -spec default_bound() -> non_neg_integer().
 default_bound() -> 1260.
 
+-spec window_size_s() -> pos_integer().
+window_size_s() -> ?WINDOW_SIZE_MS div 1000.
+
 -record(state, {
     timer_ref :: reference() | undefined,
     ets_table :: ets:tab() | undefined,
@@ -88,7 +92,7 @@ start_link() ->
 
 init([]) ->
     EtsTable = ets:new(?MODULE, [set, private, {read_concurrency, true}, {write_concurrency, true}]),
-    TimerRef = erlang:start_timer(?WINDOW_DURATION_MS, self(), rotate),
+    TimerRef = erlang:start_timer(?WINDOW_STEP_MS, self(), rotate),
     {ok, #state{timer_ref = TimerRef, ets_table = EtsTable}}.
 
 handle_call({declare, Name, MetricSpec}, _From, State) ->
@@ -123,7 +127,7 @@ handle_cast(_Msg, State) ->
 
 handle_info({timeout, _TimerRef, rotate}, State) ->
     %% The rotation is handled in ensure_windows_rotated, so we just reschedule
-    TimerRef = erlang:start_timer(?WINDOW_DURATION_MS, self(), rotate),
+    TimerRef = erlang:start_timer(?WINDOW_STEP_MS, self(), rotate),
     {noreply, State#state{timer_ref = TimerRef}};
 handle_info(_Info, State) ->
     {noreply, State}.
@@ -187,7 +191,7 @@ ensure_windows_rotated(Key, CurrentTime, State) ->
             {_CurrentWindow, WindowStartTime} = lists:nth(CurrentIndex + 1, Windows),
             Elapsed = CurrentTime - WindowStartTime,
             if
-                Elapsed >= ?WINDOW_DURATION_MS ->
+                Elapsed >= ?WINDOW_STEP_MS ->
                     NewIndex = (CurrentIndex + 1) rem ?WINDOW_COUNT,
                     {CurrentWindow, _} = lists:nth(CurrentIndex + 1, Windows),
                     ResetWindow = reset_window(CurrentWindow),
@@ -218,8 +222,8 @@ get_value(Name, LabelValues, State) ->
             UpdatedMetrics = maps:put(Name, UpdatedMetricState, State#state.metrics),
             NewState = State#state{metrics = UpdatedMetrics},
 
-            %% Filter windows to only include those within the last 60 seconds
-            CutoffTime = CurrentTime - ?TOTAL_WINDOW_MS,
+            %% Filter windows to only include those within the last window size
+            CutoffTime = CurrentTime - ?WINDOW_SIZE_MS,
             ActiveWindows = [Window || {Window, StartTime} <- UpdatedWindows,
                                       StartTime >= CutoffTime],
 
diff --git a/src/instrument/mongoose_prometheus_sliding_window_collector.erl b/src/instrument/mongoose_prometheus_sliding_window_collector.erl
index 47d747027b5..fc759c2eafb 100644
--- a/src/instrument/mongoose_prometheus_sliding_window_collector.erl
+++ b/src/instrument/mongoose_prometheus_sliding_window_collector.erl
@@ -28,12 +28,14 @@ collect_metric_family(Name, Callback) ->
         _ ->
             SWName = sliding_window_metric_name(Name),
             MetricSpec = mongoose_prometheus_sliding_window:get_metric_spec(Name),
+            WindowSizeS = mongoose_prometheus_sliding_window:window_size_s(),
+            WindowSizeSStr = integer_to_list(WindowSizeS),
             Help = case MetricSpec of
                        undefined ->
-                           "60-second sliding window quantile summary for " ++ Name;
+                           WindowSizeSStr ++ "-second sliding window quantile summary for " ++ Name;
                        Spec ->
                            BaseHelp = proplists:get_value(help, Spec, ""),
-                           BaseHelp ++ " (60s sliding window)"
+                           BaseHelp ++ " (" ++ WindowSizeSStr ++ "s sliding window)"
                    end,
             LabelKeys = case MetricSpec of
                             undefined -> [];
@@ -48,7 +50,8 @@ collect_metric_family(Name, Callback) ->
     end.
 
 sliding_window_metric_name(Name) ->
-    Name ++ "_60s".
+    WindowSizeS = mongoose_prometheus_sliding_window:window_size_s(),
+    Name ++ "_" ++ integer_to_list(WindowSizeS) ++ "s".
 
 create_summary_metric(LabelKeys, LabelValues, Count, Sum, Quantiles) ->
     %% Convert label keys and values to label pairs format

From 1141781c56fe930c34500449a4636f0ba6d07c3e Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Micha=C5=82=20Dobranowski?= <mdbrnowski@gmail.com>
Date: Thu, 22 Jan 2026 14:15:28 +0100
Subject: [PATCH 17/24] Remove old quantile_summary metrics

use only sliding_window; fix `do_remove` function
---
 .../mongoose_instrument_prometheus.erl        | 16 ++-----
 .../mongoose_prometheus_sliding_window.erl    | 43 ++++++++++++-------
 ...se_prometheus_sliding_window_collector.erl | 13 ++----
 test/mongoose_instrument_metrics_SUITE.erl    | 30 ++++++-------
 4 files changed, 49 insertions(+), 53 deletions(-)

diff --git a/src/instrument/mongoose_instrument_prometheus.erl b/src/instrument/mongoose_instrument_prometheus.erl
index 7d29073903c..6e74991cebf 100644
--- a/src/instrument/mongoose_instrument_prometheus.erl
+++ b/src/instrument/mongoose_instrument_prometheus.erl
@@ -58,15 +58,7 @@ declare_metric(MetricSpec, counter) ->
 declare_metric(MetricSpec, spiral) ->
     prometheus_counter:declare(MetricSpec);
 declare_metric(MetricSpec, histogram) ->
-    SWResult = mongoose_prometheus_sliding_window:declare(MetricSpec),
-    PromResult = prometheus_quantile_summary:declare([
-        {quantiles, mongoose_prometheus_sliding_window:default_quantiles()},
-        {error, mongoose_prometheus_sliding_window:default_error()},
-        %% Measuring in µs suffices for actions lasting up to a day (with 1% accuracy).
-        %% Measuring in bytes suffices for sizes up to 81 GB (with 1% accuracy).
-        {bound, mongoose_prometheus_sliding_window:default_bound()}
-    | MetricSpec]),
-    SWResult or PromResult.
+    mongoose_prometheus_sliding_window:declare(MetricSpec).
 
 -spec reset_metric(name(), [mongoose_instrument:label_value()],
                    mongoose_instrument:metric_type()) -> boolean().
@@ -77,8 +69,7 @@ reset_metric(Name, LabelValues, counter) ->
 reset_metric(Name, LabelValues, spiral) ->
     prometheus_counter:remove(Name, LabelValues);
 reset_metric(Name, LabelValues, histogram) ->
-    mongoose_prometheus_sliding_window:remove(Name, LabelValues),
-    prometheus_quantile_summary:remove(Name, LabelValues).
+    mongoose_prometheus_sliding_window:remove(Name, LabelValues).
 
 -spec initialize_metric(name(), [mongoose_instrument:label_value()],
                         mongoose_instrument:metric_type()) -> ok.
@@ -140,5 +131,4 @@ update_metric(Name, Labels, counter, Value) when is_integer(Value) ->
 update_metric(Name, Labels, spiral, Value) when is_integer(Value), Value >= 0 ->
     ok = prometheus_counter:inc(Name, Labels, Value);
 update_metric(Name, Labels, histogram, Value) when is_integer(Value) ->
-    ok = mongoose_prometheus_sliding_window:observe(Name, Labels, Value),
-    ok = prometheus_quantile_summary:observe(Name, Labels, Value).
+    ok = mongoose_prometheus_sliding_window:observe(Name, Labels, Value).
diff --git a/src/instrument/mongoose_prometheus_sliding_window.erl b/src/instrument/mongoose_prometheus_sliding_window.erl
index 59c04ea3604..943bf84c822 100644
--- a/src/instrument/mongoose_prometheus_sliding_window.erl
+++ b/src/instrument/mongoose_prometheus_sliding_window.erl
@@ -6,6 +6,7 @@
 -export([child_spec/0,
          declare/1,
          observe/3,
+         value/2,
          values/1,
          remove/2,
          get_all_metric_names/0,
@@ -19,9 +20,7 @@
          terminate/2,
          code_change/3]).
 
--export([default_quantiles/0, default_error/0, default_bound/0, window_size_s/0]).
-
--ignore_xref([start_link/0]).
+-ignore_xref([start_link/0, value/2]).
 
 -behaviour(gen_server).
 
@@ -40,9 +39,6 @@ default_error() -> 0.01.
 -spec default_bound() -> non_neg_integer().
 default_bound() -> 1260.
 
--spec window_size_s() -> pos_integer().
-window_size_s() -> ?WINDOW_SIZE_MS div 1000.
-
 -record(state, {
     timer_ref :: reference() | undefined,
     ets_table :: ets:tab() | undefined,
@@ -69,9 +65,17 @@ declare(MetricSpec) ->
 observe(Name, LabelValues, Value) ->
     gen_server:cast(?MODULE, {observe, Name, LabelValues, Value}).
 
+-spec value(name(), label_values()) -> undefined | {non_neg_integer(), number(), [{number(), number()}]}.
+value(Name, LabelValues) ->
+    gen_server:call(?MODULE, {value, Name, LabelValues}).
+
 -spec values(name()) -> [{label_values(), {non_neg_integer(), number(), [{number(), number()}]}}].
 values(Name) ->
-    gen_server:call(?MODULE, {values, Name}).
+    [{LV, V} || LV <- get_label_values(Name), V <- [value(Name, LV)], V =/= undefined].
+
+-spec get_label_values(name()) -> [label_values()].
+get_label_values(Name) ->
+    gen_server:call(?MODULE, {get_label_values, Name}).
 
 -spec remove(name(), label_values()) -> boolean().
 remove(Name, LabelValues) ->
@@ -104,6 +108,12 @@ handle_call({declare, Name, MetricSpec}, _From, State) ->
             NewSpecs = maps:put(Name, MetricSpec, State#state.metric_specs),
             {reply, true, State#state{metrics = NewMetrics, metric_specs = NewSpecs}}
     end;
+handle_call({value, Name, LabelValues}, _From, State) ->
+    {Result, NewState} = get_value(Name, LabelValues, State),
+    {reply, Result, NewState};
+handle_call({get_label_values, Name}, _From, State) ->
+    Result = do_get_label_values(Name, State),
+    {reply, Result, State};
 handle_call({values, Name}, _From, State) ->
     {Result, NewState} = get_values(Name, State),
     {reply, Result, NewState};
@@ -271,6 +281,15 @@ get_values(Name, State) ->
             {lists:reverse(ResultsRev), FinalState}
     end.
 
+-spec do_get_label_values(name(), #state{}) -> [label_values()].
+do_get_label_values(Name, State) ->
+    case State#state.metrics of
+       #{Name := MetricState} ->
+           [LabelValues || {_, LabelValues} <- maps:keys(MetricState)];
+       #{} ->
+           []
+   end.
+
 -spec do_remove(name(), label_values(), #state{}) -> #state{}.
 do_remove(Name, LabelValues, State) ->
     Key = {Name, LabelValues},
@@ -279,14 +298,8 @@ do_remove(Name, LabelValues, State) ->
             State;
         {ok, MetricState} ->
             UpdatedMetricState = remove_metric_state(Key, MetricState),
-            if
-                map_size(UpdatedMetricState) == 0 ->
-                    UpdatedMetrics = maps:remove(Name, State#state.metrics),
-                    State#state{metrics = UpdatedMetrics};
-                true ->
-                    UpdatedMetrics = maps:put(Name, UpdatedMetricState, State#state.metrics),
-                    State#state{metrics = UpdatedMetrics}
-            end
+            UpdatedMetrics = maps:put(Name, UpdatedMetricState, State#state.metrics),
+            State#state{metrics = UpdatedMetrics}
     end.
 
 -spec get_metric_state({name(), label_values()}, #state{}) -> undefined | metric_state().
diff --git a/src/instrument/mongoose_prometheus_sliding_window_collector.erl b/src/instrument/mongoose_prometheus_sliding_window_collector.erl
index fc759c2eafb..7dd46613338 100644
--- a/src/instrument/mongoose_prometheus_sliding_window_collector.erl
+++ b/src/instrument/mongoose_prometheus_sliding_window_collector.erl
@@ -28,14 +28,9 @@ collect_metric_family(Name, Callback) ->
         _ ->
             SWName = sliding_window_metric_name(Name),
             MetricSpec = mongoose_prometheus_sliding_window:get_metric_spec(Name),
-            WindowSizeS = mongoose_prometheus_sliding_window:window_size_s(),
-            WindowSizeSStr = integer_to_list(WindowSizeS),
             Help = case MetricSpec of
-                       undefined ->
-                           WindowSizeSStr ++ "-second sliding window quantile summary for " ++ Name;
-                       Spec ->
-                           BaseHelp = proplists:get_value(help, Spec, ""),
-                           BaseHelp ++ " (" ++ WindowSizeSStr ++ "s sliding window)"
+                       undefined -> "";
+                       Spec -> proplists:get_value(help, Spec, "")
                    end,
             LabelKeys = case MetricSpec of
                             undefined -> [];
@@ -49,9 +44,7 @@ collect_metric_family(Name, Callback) ->
             Callback(MF)
     end.
 
-sliding_window_metric_name(Name) ->
-    WindowSizeS = mongoose_prometheus_sliding_window:window_size_s(),
-    Name ++ "_" ++ integer_to_list(WindowSizeS) ++ "s".
+sliding_window_metric_name(Name) -> Name.
 
 create_summary_metric(LabelKeys, LabelValues, Count, Sum, Quantiles) ->
     %% Convert label keys and values to label pairs format
diff --git a/test/mongoose_instrument_metrics_SUITE.erl b/test/mongoose_instrument_metrics_SUITE.erl
index bd8931972c1..2f5b1182457 100644
--- a/test/mongoose_instrument_metrics_SUITE.erl
+++ b/test/mongoose_instrument_metrics_SUITE.erl
@@ -11,7 +11,7 @@
 -define(HOST_TYPE2, <<"test type">>).
 
 -define(assertRelEqual(Expect, Actual, Tolerance),
-    (fun() -> 
+    (fun() ->
         __Exp = (Expect),
         __Act = (Actual),
         __Tol = (Tolerance),
@@ -21,7 +21,7 @@
                 __RelErr = abs(__Exp - __Act) / __Exp,
                 __Check = case __RelErr < __Tol of
                     true -> ok;
-                    false -> {fail, [{expected, __Exp}, {actual, __Act}, 
+                    false -> {fail, [{expected, __Exp}, {actual, __Act},
                                         {rel_error, __RelErr}, {tolerance, __Tol}]}
                 end,
                 ?assertEqual(ok, __Check)
@@ -208,13 +208,13 @@ prometheus_histogram_is_created_and_updated(Config) ->
     ok = mongoose_instrument:set_up(Event, ?LABELS, #{metrics => #{time => histogram}}),
 
     %% Prometheus histogram shows no value if there is no data
-    ?assertEqual(undefined, prometheus_quantile_summary:value(Metric, [?HOST_TYPE])),
+    ?assertEqual(undefined, mongoose_prometheus_sliding_window:value(Metric, [?HOST_TYPE])),
     ok = mongoose_instrument:execute(Event, ?LABELS, #{time => 1}),
-    ?assertMatch({1, 1, _}, prometheus_quantile_summary:value(Metric, [?HOST_TYPE])),
+    ?assertMatch({1, 1, _}, mongoose_prometheus_sliding_window:value(Metric, [?HOST_TYPE])),
     ok = mongoose_instrument:execute(Event, ?LABELS, #{time => 1}),
-    ?assertMatch({2, 2, _}, prometheus_quantile_summary:value(Metric, [?HOST_TYPE])),
+    ?assertMatch({2, 2, _}, mongoose_prometheus_sliding_window:value(Metric, [?HOST_TYPE])),
     ok = mongoose_instrument:execute(Event, ?LABELS, #{time => 2}),
-    ?assertMatch({3, 4, _}, prometheus_quantile_summary:value(Metric, [?HOST_TYPE])).
+    ?assertMatch({3, 4, _}, mongoose_prometheus_sliding_window:value(Metric, [?HOST_TYPE])).
 
 prometheus_histogram_is_calculated_correctly(Config) ->
     Event = ?config(event, Config),
@@ -222,14 +222,14 @@ prometheus_histogram_is_calculated_correctly(Config) ->
     ok = mongoose_instrument:set_up(Event, ?LABELS, #{metrics => #{time => histogram}}),
 
     %% Prometheus histogram shows no value if there is no data
-    ?assertEqual(undefined, prometheus_quantile_summary:value(Metric, [?HOST_TYPE])),
+    ?assertEqual(undefined, mongoose_prometheus_sliding_window:value(Metric, [?HOST_TYPE])),
     Values = [1, 2, 2, 2, 2, 2, 2, 9, 9, 10],
     lists:foreach(fun(V) -> ok = mongoose_instrument:execute(Event, ?LABELS, #{time => V}) end, Values),
     Sum = lists:sum(Values),
     Length = length(Values),
-    ?assertMatch({Length, Sum, _}, prometheus_quantile_summary:value(Metric, [?HOST_TYPE])),
+    ?assertMatch({Length, Sum, _}, mongoose_prometheus_sliding_window:value(Metric, [?HOST_TYPE])),
     %% Check some quantiles
-    {_, _, Quantiles} = prometheus_quantile_summary:value(Metric, [?HOST_TYPE]),
+    {_, _, Quantiles} = mongoose_prometheus_sliding_window:value(Metric, [?HOST_TYPE]),
     Tolerance = 0.01,
     ?assertRelEqual(2, proplists:get_value(0.5, Quantiles), Tolerance),
     ?assertRelEqual(9, proplists:get_value(0.9, Quantiles), Tolerance),
@@ -243,8 +243,8 @@ prometheus_histogram_is_updated_separately_for_different_labels(Config) ->
     ok = mongoose_instrument:set_up(Event, ?LABELS2, #{metrics => #{time => histogram}}),
     ok = mongoose_instrument:execute(Event, ?LABELS, #{time => 1}),
     ok = mongoose_instrument:execute(Event, ?LABELS2, #{time => 2}),
-    ?assertMatch({1, 1, _}, prometheus_quantile_summary:value(Metric, [?HOST_TYPE])),
-    ?assertMatch({1, 2, _}, prometheus_quantile_summary:value(Metric, [?HOST_TYPE2])).
+    ?assertMatch({1, 1, _}, mongoose_prometheus_sliding_window:value(Metric, [?HOST_TYPE])),
+    ?assertMatch({1, 2, _}, mongoose_prometheus_sliding_window:value(Metric, [?HOST_TYPE2])).
 
 multiple_prometheus_metrics_are_updated(Config) ->
     Event = ?config(event, Config),
@@ -255,18 +255,18 @@ multiple_prometheus_metrics_are_updated(Config) ->
     %% Update both metrics
     ok = mongoose_instrument:execute(Event, ?LABELS, #{count => 1, time => 2}),
     ?assertEqual(1, prometheus_counter:value(Counter, [?HOST_TYPE])),
-    HistogramValue = prometheus_quantile_summary:value(Histogram, [?HOST_TYPE]),
+    HistogramValue = mongoose_prometheus_sliding_window:value(Histogram, [?HOST_TYPE]),
     ?assertMatch({1, 2, _}, HistogramValue),
 
     %% Update only one metric
     ok = mongoose_instrument:execute(Event, ?LABELS, #{count => 2}),
     ?assertEqual(3, prometheus_counter:value(Counter, [?HOST_TYPE])),
-    ?assertEqual(HistogramValue, prometheus_quantile_summary:value(Histogram, [?HOST_TYPE])),
+    ?assertEqual(HistogramValue, mongoose_prometheus_sliding_window:value(Histogram, [?HOST_TYPE])),
 
     %% No update
     ok = mongoose_instrument:execute(Event, ?LABELS, #{something => irrelevant}),
     ?assertEqual(3, prometheus_counter:value(Counter, [?HOST_TYPE])),
-    ?assertEqual(HistogramValue, prometheus_quantile_summary:value(Histogram, [?HOST_TYPE])).
+    ?assertEqual(HistogramValue, mongoose_prometheus_sliding_window:value(Histogram, [?HOST_TYPE])).
 
 exometer_skips_non_metric_event(Config) ->
     Event = ?config(event, Config),
@@ -430,7 +430,7 @@ prometheus_and_exometer_metrics_are_updated(Config) ->
     ?assertEqual({ok, [{count, 1}]}, exometer:get_value([?HOST_TYPE, Event, count], count)),
     ?assertEqual({ok, [{mean, 2}]}, exometer:get_value([?HOST_TYPE, Event, time], mean)),
     ?assertEqual(1, prometheus_counter:value(prom_name(Event, count), [?HOST_TYPE])),
-    ?assertMatch({1, 2, _}, prometheus_quantile_summary:value(prom_name(Event, time), [?HOST_TYPE])).
+    ?assertMatch({1, 2, _}, mongoose_prometheus_sliding_window:value(prom_name(Event, time), [?HOST_TYPE])).
 
 %% Helpers
 

From 4e4d34c1d8dfb0d06c2e51489b30f3a3a13b00ba Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Micha=C5=82=20Dobranowski?= <mdbrnowski@gmail.com>
Date: Thu, 22 Jan 2026 15:48:13 +0100
Subject: [PATCH 18/24] Raise and error when metric is unknown & clean up

---
 .../mongoose_prometheus_sliding_window.erl    | 24 +------------------
 1 file changed, 1 insertion(+), 23 deletions(-)

diff --git a/src/instrument/mongoose_prometheus_sliding_window.erl b/src/instrument/mongoose_prometheus_sliding_window.erl
index 943bf84c822..91160636eb3 100644
--- a/src/instrument/mongoose_prometheus_sliding_window.erl
+++ b/src/instrument/mongoose_prometheus_sliding_window.erl
@@ -114,9 +114,6 @@ handle_call({value, Name, LabelValues}, _From, State) ->
 handle_call({get_label_values, Name}, _From, State) ->
     Result = do_get_label_values(Name, State),
     {reply, Result, State};
-handle_call({values, Name}, _From, State) ->
-    {Result, NewState} = get_values(Name, State),
-    {reply, Result, NewState};
 handle_call({remove, Name, LabelValues}, _From, State) ->
     NewState = do_remove(Name, LabelValues, State),
     {reply, true, NewState};
@@ -165,7 +162,7 @@ child_spec() ->
 do_observe(Name, LabelValues, Value, State) ->
     case maps:find(Name, State#state.metrics) of
         error ->
-            State; % metric not declared, ignore
+            erlang:error({unknown_metric, Name});
         {ok, MetricState} ->
             Key = {Name, LabelValues},
             CurrentTime = erlang:monotonic_time(millisecond),
@@ -262,25 +259,6 @@ get_value(Name, LabelValues, State) ->
             {Result, NewState}
     end.
 
--spec get_values(name(), #state{}) -> {[{label_values(), {non_neg_integer(), number(), [{number(), number()}]}}], #state{}}.
-get_values(Name, State) ->
-    case maps:find(Name, State#state.metrics) of
-        error ->
-            {[], State};
-        {ok, MetricState} ->
-            Keys = maps:keys(MetricState),
-            {ResultsRev, FinalState} = lists:foldl(
-                fun({_, LabelValues}, {AccRes, S}) ->
-                    {Res, S2} = get_value(Name, LabelValues, S),
-                    NewAcc = case Res of
-                        undefined -> AccRes;
-                        _ -> [{LabelValues, Res} | AccRes]
-                    end,
-                    {NewAcc, S2}
-                end, {[], State}, Keys),
-            {lists:reverse(ResultsRev), FinalState}
-    end.
-
 -spec do_get_label_values(name(), #state{}) -> [label_values()].
 do_get_label_values(Name, State) ->
     case State#state.metrics of

From 7d52e9b2a9d832fdc9b31e764a10a3faccc5c524 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Micha=C5=82=20Dobranowski?= <mdbrnowski@gmail.com>
Date: Fri, 30 Jan 2026 15:25:57 +0100
Subject: [PATCH 19/24] Init sliding_window only if prometheus is enabled

---
 src/ejabberd_sup.erl                                  | 11 ++++++++---
 src/instrument/mongoose_prometheus_sliding_window.erl |  4 ++++
 2 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/src/ejabberd_sup.erl b/src/ejabberd_sup.erl
index 87662fedb51..a3d00db9924 100644
--- a/src/ejabberd_sup.erl
+++ b/src/ejabberd_sup.erl
@@ -45,7 +45,6 @@ start_link() ->
 init(noargs) ->
     Hooks = worker_spec(gen_hook),
     Instrument = worker_spec(mongoose_instrument),
-    PrometheusSlidingWindow = mongoose_prometheus_sliding_window:child_spec(),
     Cleaner = worker_spec(mongoose_cleaner),
     Router = worker_spec(ejabberd_router),
     S2S = worker_spec(ejabberd_s2s),
@@ -70,8 +69,8 @@ init(noargs) ->
           [StartIdServer,
            PG,
            Hooks,
-           Instrument,
-           PrometheusSlidingWindow,
+           Instrument
+           ] ++ prometheus_sliding_window_spec() ++ [
            Cleaner,
            SMBackendSupervisor,
            OutgoingPoolsSupervisor
@@ -124,6 +123,12 @@ template_supervisor_spec(Name, Module) ->
 supervisor_spec(Mod) ->
     {Mod, {Mod, start_link, []}, permanent, infinity, supervisor, [Mod]}.
 
+prometheus_sliding_window_spec() ->
+    case mongoose_config:get_opt([instrumentation, prometheus], undefined) of
+        undefined -> [];
+        #{} -> [mongoose_prometheus_sliding_window:child_spec()]
+    end.
+
 worker_spec(Mod) ->
     worker_spec(Mod, []).
 
diff --git a/src/instrument/mongoose_prometheus_sliding_window.erl b/src/instrument/mongoose_prometheus_sliding_window.erl
index 91160636eb3..5c7ca9912c0 100644
--- a/src/instrument/mongoose_prometheus_sliding_window.erl
+++ b/src/instrument/mongoose_prometheus_sliding_window.erl
@@ -36,6 +36,10 @@ default_quantiles() -> [0.5, 0.75, 0.90, 0.95, 0.99, 0.999].
 -spec default_error() -> number().
 default_error() -> 0.01.
 
+%% Setting the default bound to 1260 means that:
+%% * measuring in µs suffices for actions lasting up to a day (with 1% accuracy),
+%% * measuring in bytes suffices for sizes up to 81 GB (with 1% accuracy).
+%% See: https://hexdocs.pm/ddskerl/ddskerl_ets.html
 -spec default_bound() -> non_neg_integer().
 default_bound() -> 1260.
 

From 10d9817ca2788830ee134803eb34717b3af0b948 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Micha=C5=82=20Dobranowski?= <mdbrnowski@gmail.com>
Date: Thu, 5 Feb 2026 11:42:43 +0100
Subject: [PATCH 20/24] Fix rotating windows, add tests

---
 .../mongoose_prometheus_sliding_window.erl    | 89 ++++++++++---------
 test/mongoose_instrument_metrics_SUITE.erl    | 30 +++++++
 2 files changed, 78 insertions(+), 41 deletions(-)

diff --git a/src/instrument/mongoose_prometheus_sliding_window.erl b/src/instrument/mongoose_prometheus_sliding_window.erl
index 5c7ca9912c0..b75b96159c5 100644
--- a/src/instrument/mongoose_prometheus_sliding_window.erl
+++ b/src/instrument/mongoose_prometheus_sliding_window.erl
@@ -1,7 +1,7 @@
 -module(mongoose_prometheus_sliding_window).
 
 %% Manages a sliding window using DDSketch instances for quantile summaries.
-%% Window parameters are configurable via WINDOW_SIZE_MS and WINDOW_STEP_MS constants.
+%% Window parameters are configurable via window_size_ms and window_step_ms constants.
 
 -export([child_spec/0,
          declare/1,
@@ -20,15 +20,24 @@
          terminate/2,
          code_change/3]).
 
--ignore_xref([start_link/0, value/2]).
+-export([window_size_ms/0, window_step_ms/0, window_count/0]). % Exported for mocking in tests
+
+-ignore_xref([start_link/0, value/2, window_size_ms/0, window_step_ms/0, window_count/0]).
 
 -behaviour(gen_server).
 
-%% Configurable window parameters
--define(WINDOW_SIZE_MS, 60000). % Total window size: 60 seconds
--define(WINDOW_STEP_MS, 3000).  % Step size: 3 seconds per sub-window
--define(WINDOW_COUNT, (?WINDOW_SIZE_MS div ?WINDOW_STEP_MS)). % Number of sub-windows
+%% Window parameters
+
+-spec window_size_ms() -> pos_integer().
+window_size_ms() -> 60000. % Total window size: 60 seconds
+
+-spec window_step_ms() -> pos_integer().
+window_step_ms() -> 3000.  % Step size: 3 seconds per sub-window
 
+-spec window_count() -> pos_integer().
+window_count() -> ?MODULE:window_size_ms() div ?MODULE:window_step_ms().
+
+%% DDSketch parameters
 
 -spec default_quantiles() -> [number()].
 default_quantiles() -> [0.5, 0.75, 0.90, 0.95, 0.99, 0.999].
@@ -100,7 +109,7 @@ start_link() ->
 
 init([]) ->
     EtsTable = ets:new(?MODULE, [set, private, {read_concurrency, true}, {write_concurrency, true}]),
-    TimerRef = erlang:start_timer(?WINDOW_STEP_MS, self(), rotate),
+    TimerRef = erlang:start_timer(?MODULE:window_step_ms(), self(), rotate),
     {ok, #state{timer_ref = TimerRef, ets_table = EtsTable}}.
 
 handle_call({declare, Name, MetricSpec}, _From, State) ->
@@ -137,9 +146,10 @@ handle_cast(_Msg, State) ->
     {noreply, State}.
 
 handle_info({timeout, _TimerRef, rotate}, State) ->
-    %% The rotation is handled in ensure_windows_rotated, so we just reschedule
-    TimerRef = erlang:start_timer(?WINDOW_STEP_MS, self(), rotate),
-    {noreply, State#state{timer_ref = TimerRef}};
+    CurrentTime = erlang:monotonic_time(millisecond),
+    NewState = rotate_all_windows(CurrentTime, State),
+    TimerRef = erlang:start_timer(?MODULE:window_step_ms(), self(), rotate),
+    {noreply, NewState#state{timer_ref = TimerRef}};
 handle_info(_Info, State) ->
     {noreply, State}.
 
@@ -171,7 +181,7 @@ do_observe(Name, LabelValues, Value, State) ->
             Key = {Name, LabelValues},
             CurrentTime = erlang:monotonic_time(millisecond),
             {Windows, CurrentIndex} =
-                ensure_windows_rotated(Key, CurrentTime, State),
+                ensure_windows_initialized(Key, CurrentTime, State),
 
             %% Get current window and add observation
             {CurrentWindow, WindowStartTime} = lists:nth(CurrentIndex + 1, Windows),
@@ -190,27 +200,17 @@ do_observe(Name, LabelValues, Value, State) ->
             State#state{metrics = UpdatedMetrics}
     end.
 
--spec ensure_windows_rotated({name(), label_values()}, non_neg_integer(), #state{}) ->
+-spec ensure_windows_initialized({name(), label_values()}, non_neg_integer(), #state{}) ->
     {[{window_data(), non_neg_integer()}], non_neg_integer()}.
-ensure_windows_rotated(Key, CurrentTime, State) ->
+ensure_windows_initialized(Key, CurrentTime, State) ->
+    WindowCount = ?MODULE:window_count(),
     case get_metric_state(Key, State) of
         undefined ->
-            %% Initialize new windows, all starting at current time (they'll be rotated as needed)
-            Windows = [new_window(Key, Index, CurrentTime, State) || Index <- lists:seq(0, ?WINDOW_COUNT - 1)],
+            %% Initialize new windows, all starting at current time
+            Windows = [new_window(Key, Index, CurrentTime, State) || Index <- lists:seq(0, WindowCount - 1)],
             {Windows, 0};
         #{windows := Windows, current_index := CurrentIndex} ->
-            {_CurrentWindow, WindowStartTime} = lists:nth(CurrentIndex + 1, Windows),
-            Elapsed = CurrentTime - WindowStartTime,
-            if
-                Elapsed >= ?WINDOW_STEP_MS ->
-                    NewIndex = (CurrentIndex + 1) rem ?WINDOW_COUNT,
-                    {CurrentWindow, _} = lists:nth(CurrentIndex + 1, Windows),
-                    ResetWindow = reset_window(CurrentWindow),
-                    UpdatedWindows = set_nth(CurrentIndex + 1, {ResetWindow, CurrentTime}, Windows),
-                    {UpdatedWindows, NewIndex};
-                true ->
-                    {Windows, CurrentIndex}
-            end
+            {Windows, CurrentIndex}
     end.
 
 -spec get_value(name(), label_values(), #state{}) ->
@@ -220,22 +220,12 @@ get_value(Name, LabelValues, State) ->
     case get_metric_state(Key, State) of
         undefined ->
             {undefined, State};
-        #{} ->
+        #{windows := Windows} ->
             CurrentTime = erlang:monotonic_time(millisecond),
-            {UpdatedWindows, NewIndex} = ensure_windows_rotated(Key, CurrentTime, State),
-
-            %% Update metric state with rotated windows
-            {ok, MetricState} = maps:find(Name, State#state.metrics),
-            UpdatedMetricState = maps:put(Key,
-                #{windows => UpdatedWindows,
-                  current_index => NewIndex},
-                MetricState),
-            UpdatedMetrics = maps:put(Name, UpdatedMetricState, State#state.metrics),
-            NewState = State#state{metrics = UpdatedMetrics},
 
             %% Filter windows to only include those within the last window size
-            CutoffTime = CurrentTime - ?WINDOW_SIZE_MS,
-            ActiveWindows = [Window || {Window, StartTime} <- UpdatedWindows,
+            CutoffTime = CurrentTime - ?MODULE:window_size_ms(),
+            ActiveWindows = [Window || {Window, StartTime} <- Windows,
                                       StartTime >= CutoffTime],
 
             %% Merge all active windows
@@ -260,7 +250,7 @@ get_value(Name, LabelValues, State) ->
                             {Count, Sum, Quantiles}
                     end
             end,
-            {Result, NewState}
+            {Result, State}
     end.
 
 -spec do_get_label_values(name(), #state{}) -> [label_values()].
@@ -326,6 +316,23 @@ new_window(Key = {Name, _LabelValues}, Index, StartTime, State) ->
                    name => WindowName},
     {WindowData, StartTime}.
 
+-spec rotate_all_windows(non_neg_integer(), #state{}) -> #state{}.
+rotate_all_windows(CurrentTime, State) ->
+    WindowCount = ?MODULE:window_count(),
+    UpdatedMetrics = maps:map(
+        fun(_Name, MetricState) ->
+            maps:map(
+                fun(_Key, #{windows := Windows, current_index := CurrentIndex}) ->
+                    %% Move to next window and reset it
+                    NewIndex = (CurrentIndex + 1) rem WindowCount,
+                    {WindowToReset, _OldTime} = lists:nth(NewIndex + 1, Windows),
+                    ResetWindow = reset_window(WindowToReset),
+                    UpdatedWindows = set_nth(NewIndex + 1, {ResetWindow, CurrentTime}, Windows),
+                    #{windows => UpdatedWindows, current_index => NewIndex}
+                end, MetricState)
+        end, State#state.metrics),
+    State#state{metrics = UpdatedMetrics}.
+
 -spec reset_window(window_data()) -> window_data().
 reset_window(WindowData) ->
     Sketch = maps:get(sketch, WindowData),
diff --git a/test/mongoose_instrument_metrics_SUITE.erl b/test/mongoose_instrument_metrics_SUITE.erl
index 2f5b1182457..99d8ab98232 100644
--- a/test/mongoose_instrument_metrics_SUITE.erl
+++ b/test/mongoose_instrument_metrics_SUITE.erl
@@ -50,6 +50,7 @@ groups() ->
                                prometheus_histogram_is_created_and_updated,
                                prometheus_histogram_is_calculated_correctly,
                                prometheus_histogram_is_updated_separately_for_different_labels,
+                               prometheus_histogram_sliding_window_expires_data,
                                multiple_prometheus_metrics_are_updated]},
      {exometer, [parallel], [exometer_skips_non_metric_event,
                              exometer_gauge_is_created_and_updated,
@@ -246,6 +247,35 @@ prometheus_histogram_is_updated_separately_for_different_labels(Config) ->
     ?assertMatch({1, 1, _}, mongoose_prometheus_sliding_window:value(Metric, [?HOST_TYPE])),
     ?assertMatch({1, 2, _}, mongoose_prometheus_sliding_window:value(Metric, [?HOST_TYPE2])).
 
+prometheus_histogram_sliding_window_expires_data(Config) ->
+    Event = ?config(event, Config),
+    Metric = prom_name(Event, time),
+
+    OriginalWindowStep = mongoose_prometheus_sliding_window:window_step_ms(),
+
+    %% We mock the window parameters to use small values and wait a short time.
+    meck:new(mongoose_prometheus_sliding_window, [passthrough]),
+    meck:expect(mongoose_prometheus_sliding_window, window_size_ms, fun() -> 1000 end),
+    meck:expect(mongoose_prometheus_sliding_window, window_step_ms, fun() -> 100 end),
+
+    %% Wait a bit for the mocked module to be used
+    timer:sleep(OriginalWindowStep),
+
+    try
+        ok = mongoose_instrument:set_up(Event, ?LABELS, #{metrics => #{time => histogram}}),
+        ok = mongoose_instrument:execute(Event, ?LABELS, #{time => 10}),
+        ?assertMatch({1, 10, _}, mongoose_prometheus_sliding_window:value(Metric, [?HOST_TYPE])),
+        timer:sleep(500),
+        ok = mongoose_instrument:execute(Event, ?LABELS, #{time => 20}),
+        ?assertMatch({2, 30, _}, mongoose_prometheus_sliding_window:value(Metric, [?HOST_TYPE])),
+        timer:sleep(700), % Wait enough for the first value to expire
+        ?assertMatch({1, 20, _}, mongoose_prometheus_sliding_window:value(Metric, [?HOST_TYPE])),
+        timer:sleep(500), % Wait enough for the second value to expire
+        ?assertEqual(undefined, mongoose_prometheus_sliding_window:value(Metric, [?HOST_TYPE]))
+    after
+        meck:unload(mongoose_prometheus_sliding_window)
+    end.
+
 multiple_prometheus_metrics_are_updated(Config) ->
     Event = ?config(event, Config),
     Counter = prom_name(Event, count),

From 604909c0ee179400b42a9e38f1c037d03579c679 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Micha=C5=82=20Dobranowski?= <mdbrnowski@gmail.com>
Date: Wed, 11 Feb 2026 15:17:55 +0100
Subject: [PATCH 21/24] Check quantile endpoint too

---
 test/prometheus_endpoint_SUITE.erl | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/test/prometheus_endpoint_SUITE.erl b/test/prometheus_endpoint_SUITE.erl
index 1e589a8ed19..ac601d78f07 100644
--- a/test/prometheus_endpoint_SUITE.erl
+++ b/test/prometheus_endpoint_SUITE.erl
@@ -55,7 +55,7 @@ test_metrics(Event, Labels) ->
     check_counter(Event, requests, Labels, 1, Scraped), % 'spiral' is a Prometheus counter
     check_gauge(Event, sessions, Labels, 5, Scraped), % 'counter' is a Prometheus gauge
     check_gauge(Event, seconds, Labels, 10, Scraped),
-    check_histogram(Event, time, Labels, #{count => 1, sum => 2}, Scraped).
+    check_histogram(Event, time, Labels, #{count => 1, sum => 2, median => 2}, Scraped).
 
 %% Checks for the parsed metrics
 
@@ -72,14 +72,17 @@ check_gauge(Event, Metric, Labels, ExpValue, Scraped) ->
     ?assertEqual({Labels, ExpValue}, Value).
 
 check_histogram(Event, Metric, Labels, ExpValues, Scraped) ->
-    #{count := ExpCount, sum := ExpSum} = ExpValues,
+    #{count := ExpCount, sum := ExpSum, median := ExpMedian} = ExpValues,
     [Type, Help | _] = get_metric([Event, Metric], Scraped),
     ?assertEqual({<<"TYPE">>, <<"summary">>}, Type),
     ?assertEqual({<<"HELP">>, help(Event, Metric)}, Help),
     [Count] = get_metric([Event, Metric, count], Scraped),
     ?assertEqual({Labels, ExpCount}, Count),
     [Sum] = get_metric([Event, Metric, sum], Scraped),
-    ?assertEqual({Labels, ExpSum}, Sum).
+    ?assertEqual({Labels, ExpSum}, Sum),
+    MedianLabels = Labels#{quantile => <<"0.5">>},
+    [{MedianLabels, Median}] = get_metric([Event, Metric], MedianLabels, Scraped),
+    ?assert(abs(Median - ExpMedian) < 0.01).
 
 help(Event, Metric) ->
     <<"Event: ", (atom_to_binary(Event))/binary, ", Metric: ", (atom_to_binary(Metric))/binary>>.
@@ -89,6 +92,11 @@ get_metric(Parts, Scraped) when is_list(Parts) ->
 get_metric(Name, Scraped) when is_binary(Name) ->
     [{LabelsOrKey, Value} || {N, LabelsOrKey, Value} <- Scraped, N =:= Name].
 
+get_metric(Parts, Labels, Scraped) when is_list(Parts) ->
+    get_metric(binary_name(Parts), Labels, Scraped);
+get_metric(Name, Labels, Scraped) when is_binary(Name) ->
+    [{L, Value} || {N, L, Value} <- Scraped, N =:= Name, is_map(L), maps:merge(L, Labels) =:= L].
+
 binary_name(Atoms) ->
     list_to_binary(string:join([atom_to_list(Atom) || Atom <- Atoms], "_")).
 

From e018a13e2fccb39309cfd61c6b3e93af05ace087 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Micha=C5=82=20Dobranowski?= <mdbrnowski@gmail.com>
Date: Wed, 11 Feb 2026 16:52:39 +0100
Subject: [PATCH 22/24] Use `timer:send_interval` instead of `start_timer`

and restart gen_server instead of waiting during tests
---
 .../mongoose_prometheus_sliding_window.erl           | 12 +++++++-----
 test/mongoose_instrument_metrics_SUITE.erl           |  7 +++----
 2 files changed, 10 insertions(+), 9 deletions(-)

diff --git a/src/instrument/mongoose_prometheus_sliding_window.erl b/src/instrument/mongoose_prometheus_sliding_window.erl
index b75b96159c5..f44628d6807 100644
--- a/src/instrument/mongoose_prometheus_sliding_window.erl
+++ b/src/instrument/mongoose_prometheus_sliding_window.erl
@@ -53,7 +53,7 @@ default_error() -> 0.01.
 default_bound() -> 1260.
 
 -record(state, {
-    timer_ref :: reference() | undefined,
+    timer_ref :: {ok, timer:tref()} | {error, term()} | undefined,
     ets_table :: ets:tab() | undefined,
     metrics = #{} :: #{name() => #{{name(), label_values()} => metric_state()}},
     metric_specs = #{} :: #{name() => proplists:proplist()}
@@ -109,7 +109,7 @@ start_link() ->
 
 init([]) ->
     EtsTable = ets:new(?MODULE, [set, private, {read_concurrency, true}, {write_concurrency, true}]),
-    TimerRef = erlang:start_timer(?MODULE:window_step_ms(), self(), rotate),
+    TimerRef = timer:send_interval(?MODULE:window_step_ms(), rotate),
     {ok, #state{timer_ref = TimerRef, ets_table = EtsTable}}.
 
 handle_call({declare, Name, MetricSpec}, _From, State) ->
@@ -145,14 +145,16 @@ handle_cast({observe, Name, LabelValues, Value}, State) ->
 handle_cast(_Msg, State) ->
     {noreply, State}.
 
-handle_info({timeout, _TimerRef, rotate}, State) ->
+handle_info(rotate, State) ->
     CurrentTime = erlang:monotonic_time(millisecond),
     NewState = rotate_all_windows(CurrentTime, State),
-    TimerRef = erlang:start_timer(?MODULE:window_step_ms(), self(), rotate),
-    {noreply, NewState#state{timer_ref = TimerRef}};
+    {noreply, NewState};
 handle_info(_Info, State) ->
     {noreply, State}.
 
+terminate(_Reason, #state{timer_ref = {ok, TRef}}) ->
+    timer:cancel(TRef),
+    ok;
 terminate(_Reason, _State) ->
     ok.
 
diff --git a/test/mongoose_instrument_metrics_SUITE.erl b/test/mongoose_instrument_metrics_SUITE.erl
index 99d8ab98232..47ab245ba32 100644
--- a/test/mongoose_instrument_metrics_SUITE.erl
+++ b/test/mongoose_instrument_metrics_SUITE.erl
@@ -251,15 +251,14 @@ prometheus_histogram_sliding_window_expires_data(Config) ->
     Event = ?config(event, Config),
     Metric = prom_name(Event, time),
 
-    OriginalWindowStep = mongoose_prometheus_sliding_window:window_step_ms(),
-
     %% We mock the window parameters to use small values and wait a short time.
     meck:new(mongoose_prometheus_sliding_window, [passthrough]),
     meck:expect(mongoose_prometheus_sliding_window, window_size_ms, fun() -> 1000 end),
     meck:expect(mongoose_prometheus_sliding_window, window_step_ms, fun() -> 100 end),
 
-    %% Wait a bit for the mocked module to be used
-    timer:sleep(OriginalWindowStep),
+    %% Restart the gen_server to make sure that the new window parameters are applied
+    ok = gen_server:stop(mongoose_prometheus_sliding_window),
+    {ok, _} = mongoose_prometheus_sliding_window:start_link(),
 
     try
         ok = mongoose_instrument:set_up(Event, ?LABELS, #{metrics => #{time => histogram}}),

From 1b64dc88ef0fa5667fbd48dc0b857ef887bc688b Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Micha=C5=82=20Dobranowski?= <mdbrnowski@gmail.com>
Date: Wed, 11 Feb 2026 17:17:32 +0100
Subject: [PATCH 23/24] Remove some redundant code

---
 .../mongoose_prometheus_sliding_window.erl    | 81 +++++++------------
 1 file changed, 31 insertions(+), 50 deletions(-)

diff --git a/src/instrument/mongoose_prometheus_sliding_window.erl b/src/instrument/mongoose_prometheus_sliding_window.erl
index f44628d6807..16505d90759 100644
--- a/src/instrument/mongoose_prometheus_sliding_window.erl
+++ b/src/instrument/mongoose_prometheus_sliding_window.erl
@@ -64,7 +64,7 @@ default_bound() -> 1260.
 -type window_data() :: #{sketch := ddskerl_ets:ddsketch(),
                          ref := ets:tab(),
                          name := term()}.
--type metric_state() :: #{windows => [{window_data(), non_neg_integer()}],
+-type metric_state() :: #{windows => [window_data()],
                           current_index => non_neg_integer()}.
 
 %% Public API
@@ -122,8 +122,8 @@ handle_call({declare, Name, MetricSpec}, _From, State) ->
             {reply, true, State#state{metrics = NewMetrics, metric_specs = NewSpecs}}
     end;
 handle_call({value, Name, LabelValues}, _From, State) ->
-    {Result, NewState} = get_value(Name, LabelValues, State),
-    {reply, Result, NewState};
+    Result = get_value(Name, LabelValues, State),
+    {reply, Result, State};
 handle_call({get_label_values, Name}, _From, State) ->
     Result = do_get_label_values(Name, State),
     {reply, Result, State};
@@ -146,8 +146,7 @@ handle_cast(_Msg, State) ->
     {noreply, State}.
 
 handle_info(rotate, State) ->
-    CurrentTime = erlang:monotonic_time(millisecond),
-    NewState = rotate_all_windows(CurrentTime, State),
+    NewState = rotate_all_windows(State),
     {noreply, NewState};
 handle_info(_Info, State) ->
     {noreply, State}.
@@ -181,18 +180,17 @@ do_observe(Name, LabelValues, Value, State) ->
             erlang:error({unknown_metric, Name});
         {ok, MetricState} ->
             Key = {Name, LabelValues},
-            CurrentTime = erlang:monotonic_time(millisecond),
             {Windows, CurrentIndex} =
-                ensure_windows_initialized(Key, CurrentTime, State),
+                ensure_windows_initialized(Key, State),
 
             %% Get current window and add observation
-            {CurrentWindow, WindowStartTime} = lists:nth(CurrentIndex + 1, Windows),
+            CurrentWindow = lists:nth(CurrentIndex + 1, Windows),
             Sketch = maps:get(sketch, CurrentWindow),
             UpdatedSketch = ddskerl_ets:insert(Sketch, Value),
             UpdatedWindow = CurrentWindow#{sketch := UpdatedSketch},
 
             %% Update windows list and metric state
-            UpdatedWindows = set_nth(CurrentIndex + 1, {UpdatedWindow, WindowStartTime}, Windows),
+            UpdatedWindows = set_nth(CurrentIndex + 1, UpdatedWindow, Windows),
             UpdatedMetricState = maps:put(Key,
                 #{windows => UpdatedWindows,
                   current_index => CurrentIndex},
@@ -202,43 +200,31 @@ do_observe(Name, LabelValues, Value, State) ->
             State#state{metrics = UpdatedMetrics}
     end.
 
--spec ensure_windows_initialized({name(), label_values()}, non_neg_integer(), #state{}) ->
-    {[{window_data(), non_neg_integer()}], non_neg_integer()}.
-ensure_windows_initialized(Key, CurrentTime, State) ->
+-spec ensure_windows_initialized({name(), label_values()}, #state{}) -> {[window_data()], non_neg_integer()}.
+ensure_windows_initialized(Key, State) ->
     WindowCount = ?MODULE:window_count(),
     case get_metric_state(Key, State) of
         undefined ->
-            %% Initialize new windows, all starting at current time
-            Windows = [new_window(Key, Index, CurrentTime, State) || Index <- lists:seq(0, WindowCount - 1)],
+            Windows = [new_window(Key, Index, State) || Index <- lists:seq(0, WindowCount - 1)],
             {Windows, 0};
         #{windows := Windows, current_index := CurrentIndex} ->
             {Windows, CurrentIndex}
     end.
 
--spec get_value(name(), label_values(), #state{}) ->
-    {undefined | {non_neg_integer(), number(), [{number(), number()}]}, #state{}}.
+-spec get_value(name(), label_values(), #state{}) -> undefined | {non_neg_integer(), number(), [{number(), number()}]}.
 get_value(Name, LabelValues, State) ->
     Key = {Name, LabelValues},
     case get_metric_state(Key, State) of
         undefined ->
-            {undefined, State};
+            undefined;
         #{windows := Windows} ->
-            CurrentTime = erlang:monotonic_time(millisecond),
-
-            %% Filter windows to only include those within the last window size
-            CutoffTime = CurrentTime - ?MODULE:window_size_ms(),
-            ActiveWindows = [Window || {Window, StartTime} <- Windows,
-                                      StartTime >= CutoffTime],
-
-            %% Merge all active windows
-            WindowTuples = [window_tuple(Window) || Window <- ActiveWindows],
+            WindowTuples = [window_tuple(Window) || Window <- Windows],
             Merged = lists:foldl(fun
                                       (undefined, Acc) -> Acc;
                                       (Val, undefined) -> Val;
                                       (Val, Acc) -> ddskerl_ets:merge_tuples(Acc, Val)
                                   end, undefined, WindowTuples),
-
-            Result = case Merged of
+            case Merged of
                 undefined ->
                     undefined;
                 _ ->
@@ -251,8 +237,7 @@ get_value(Name, LabelValues, State) ->
                             Quantiles = [{Q, ddskerl_ets:quantile_tuple(Merged, Q)} || Q <- default_quantiles()],
                             {Count, Sum, Quantiles}
                     end
-            end,
-            {Result, State}
+            end
     end.
 
 -spec do_get_label_values(name(), #state{}) -> [label_values()].
@@ -304,22 +289,20 @@ metric_options(Name, State) ->
 window_name({Name, LabelValues}, Index) ->
     {Name, LabelValues, Index}.
 
--spec new_window({name(), label_values()}, non_neg_integer(), non_neg_integer(), #state{}) ->
-    {window_data(), non_neg_integer()}.
-new_window(Key = {Name, _LabelValues}, Index, StartTime, State) ->
+-spec new_window({name(), label_values()}, non_neg_integer(), #state{}) -> window_data().
+new_window(Key = {Name, _LabelValues}, Index, State) ->
     {Error, Bound} = metric_options(Name, State),
     WindowName = window_name(Key, Index),
     Sketch = ddskerl_ets:new(#{ets_table => State#state.ets_table,
                                name => WindowName,
                                error => Error,
                                bound => Bound}),
-    WindowData = #{sketch => Sketch,
-                   ref => State#state.ets_table,
-                   name => WindowName},
-    {WindowData, StartTime}.
+    #{sketch => Sketch,
+      ref => State#state.ets_table,
+      name => WindowName}.
 
--spec rotate_all_windows(non_neg_integer(), #state{}) -> #state{}.
-rotate_all_windows(CurrentTime, State) ->
+-spec rotate_all_windows(#state{}) -> #state{}.
+rotate_all_windows(State) ->
     WindowCount = ?MODULE:window_count(),
     UpdatedMetrics = maps:map(
         fun(_Name, MetricState) ->
@@ -327,9 +310,9 @@ rotate_all_windows(CurrentTime, State) ->
                 fun(_Key, #{windows := Windows, current_index := CurrentIndex}) ->
                     %% Move to next window and reset it
                     NewIndex = (CurrentIndex + 1) rem WindowCount,
-                    {WindowToReset, _OldTime} = lists:nth(NewIndex + 1, Windows),
+                    WindowToReset = lists:nth(NewIndex + 1, Windows),
                     ResetWindow = reset_window(WindowToReset),
-                    UpdatedWindows = set_nth(NewIndex + 1, {ResetWindow, CurrentTime}, Windows),
+                    UpdatedWindows = set_nth(NewIndex + 1, ResetWindow, Windows),
                     #{windows => UpdatedWindows, current_index => NewIndex}
                 end, MetricState)
         end, State#state.metrics),
@@ -352,15 +335,13 @@ window_tuple(WindowData) ->
 
 -spec remove_metric_state({name(), label_values()}, #{}) -> #{}.
 remove_metric_state(Key, MetricState) ->
-    UpdatedMetricState =
-        case maps:find(Key, MetricState) of
-            error ->
-                MetricState;
-            {ok, #{windows := Windows}} ->
-                lists:foreach(fun({Window, _}) -> delete_window(Window) end, Windows),
-                maps:remove(Key, MetricState)
-        end,
-    UpdatedMetricState.
+    case maps:find(Key, MetricState) of
+        error ->
+            MetricState;
+        {ok, #{windows := Windows}} ->
+            lists:foreach(fun(Window) -> delete_window(Window) end, Windows),
+            maps:remove(Key, MetricState)
+    end.
 
 -spec delete_window(window_data()) -> ok.
 delete_window(WindowData) ->

From 1b68f82c72aeee91fd01a29cffc990f56bebc594 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Micha=C5=82=20Dobranowski?= <mdbrnowski@gmail.com>
Date: Wed, 11 Feb 2026 17:51:01 +0100
Subject: [PATCH 24/24] Create a new sequential test group

---
 test/mongoose_instrument_metrics_SUITE.erl | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/test/mongoose_instrument_metrics_SUITE.erl b/test/mongoose_instrument_metrics_SUITE.erl
index 47ab245ba32..b3293226cd8 100644
--- a/test/mongoose_instrument_metrics_SUITE.erl
+++ b/test/mongoose_instrument_metrics_SUITE.erl
@@ -33,6 +33,7 @@
 
 all() ->
     [{group, prometheus},
+     {group, prometheus_sliding_window}, % separate, because it can't be run in parallel
      {group, exometer},
      {group, exometer_global},
      {group, prometheus_and_exometer}
@@ -50,8 +51,8 @@ groups() ->
                                prometheus_histogram_is_created_and_updated,
                                prometheus_histogram_is_calculated_correctly,
                                prometheus_histogram_is_updated_separately_for_different_labels,
-                               prometheus_histogram_sliding_window_expires_data,
                                multiple_prometheus_metrics_are_updated]},
+     {prometheus_sliding_window, [], [prometheus_histogram_sliding_window_expires_data]},
      {exometer, [parallel], [exometer_skips_non_metric_event,
                              exometer_gauge_is_created_and_updated,
                              exometer_gauge_is_updated_separately_for_different_labels,
@@ -87,6 +88,7 @@ init_per_group(Group, Config) ->
     Config1 ++ extra_config(Group).
 
 extra_processes(prometheus) -> [{mongoose_prometheus_sliding_window, start_link, []}];
+extra_processes(prometheus_sliding_window) -> [{mongoose_prometheus_sliding_window, start_link, []}];
 extra_processes(prometheus_and_exometer) -> [{mongoose_prometheus_sliding_window, start_link, []}];
 extra_processes(_) -> [].
 
@@ -103,12 +105,15 @@ end_per_testcase(_Case, _Config) ->
     log_helper:unsubscribe().
 
 apps(prometheus) -> [prometheus, prometheus_httpd, prometheus_cowboy];
+apps(prometheus_sliding_window) -> apps(prometheus);
 apps(exometer) -> [exometer_core];
 apps(exometer_global) -> [exometer_core];
 apps(prometheus_and_exometer) -> apps(prometheus) ++ apps(exometer).
 
 opts(prometheus) ->
     #{prometheus => #{}};
+opts(prometheus_sliding_window) ->
+    opts(prometheus);
 opts(exometer) ->
     #{exometer => #{all_metrics_are_global => false, report => #{}}};
 opts(exometer_global) ->