diff --git a/Gemfile b/Gemfile
index b4eede485d3..fd91efb1071 100644
--- a/Gemfile
+++ b/Gemfile
@@ -6,7 +6,6 @@ gem 'clockwork', require: false
 gem 'cloudfront-signer'
 gem 'concurrent-ruby'
 gem 'digest-xxhash'
-gem 'eventmachine', '~> 1.2.7'
 gem 'fluent-logger'
 gem 'googleapis-common-protos', '>= 1.8.0'
 gem 'hashdiff'
@@ -38,7 +37,6 @@ gem 'sinatra-contrib'
 gem 'statsd-ruby', '~> 1.5.0'
 gem 'steno'
 gem 'talentbox-delayed_job_sequel', '~> 4.3.0'
-gem 'thin'
 gem 'uri', '~> 1.1'
 gem 'vmstat', '~> 2.3'
 
diff --git a/Gemfile.lock b/Gemfile.lock
index 89796d205ba..e31ae39ffe6 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -129,7 +129,6 @@ GEM
       bigdecimal
       rexml
     crass (1.0.6)
-    daemons (1.4.1)
     date (3.5.0)
     debug (1.11.0)
       irb (~> 1.10)
@@ -144,7 +143,6 @@ GEM
     drb (2.2.3)
     erb (5.1.3)
     erubi (1.13.1)
-    eventmachine (1.2.7)
     excon (1.3.0)
       logger
     faraday (0.17.6)
@@ -568,11 +566,6 @@ GEM
       delayed_job (~> 4.1.0)
       sequel (>= 3.38, < 6.0)
       tzinfo
-    thin (2.0.1)
-      daemons (~> 1.0, >= 1.0.9)
-      eventmachine (~> 1.0, >= 1.0.4)
-      logger
-      rack (>= 1, < 4)
     thor (1.4.0)
     tilt (2.6.1)
     timecop (0.9.10)
@@ -625,7 +618,6 @@ DEPENDENCIES
   concurrent-ruby
   debug (~> 1.11)
   digest-xxhash
-  eventmachine (~> 1.2.7)
   fluent-logger
   fog-aliyun
   fog-aws
@@ -692,7 +684,6 @@ DEPENDENCIES
   statsd-ruby (~> 1.5.0)
   steno
   talentbox-delayed_job_sequel (~> 4.3.0)
-  thin
   timecop
   uri (~> 1.1)
   vmstat (~> 2.3)
diff --git a/app/controllers/internal/metrics_controller.rb b/app/controllers/internal/metrics_controller.rb
deleted file mode 100644
index 3d4aa75845e..00000000000
--- a/app/controllers/internal/metrics_controller.rb
+++ /dev/null
@@ -1,18 +0,0 @@
-require 'prometheus/client'
-require 'prometheus/client/formats/text'
-
-module VCAP::CloudController
-  module Internal
-    # This controller is only used when the webserver is not Puma
-    # When using Puma, the metrics are served by a separate webserver in the main process
-    class MetricsController < RestController::BaseController
-      allow_unauthenticated_access
-      get '/internal/v4/metrics', :index
-
-      def index
-        CloudController::DependencyLocator.instance.periodic_updater.update! unless VCAP::CloudController::Config.config.get(:webserver) == 'puma'
-        [200, Prometheus::Client::Formats::Text.marshal(Prometheus::Client.registry)]
-      end
-    end
-  end
-end
diff --git a/config/cloud_controller.yml b/config/cloud_controller.yml
index 3fea034bc6a..a0549a3534d 100644
--- a/config/cloud_controller.yml
+++ b/config/cloud_controller.yml
@@ -395,7 +395,7 @@ locket:
   key_file: 'spec/fixtures/certs/bbs_client.key'
 
 threadpool_size: 20
-webserver: thin
+webserver: puma
 default_app_lifecycle: buildpack
 custom_metric_tag_prefix_list: ["metric.tag.cloudfoundry.org"]
 
diff --git a/lib/cloud_controller.rb b/lib/cloud_controller.rb
index 4a3faffffd7..5875c33462a 100644
--- a/lib/cloud_controller.rb
+++ b/lib/cloud_controller.rb
@@ -1,6 +1,5 @@
 require 'sinatra'
 require 'sequel'
-require 'thin'
 require 'oj'
 require 'delayed_job'
 
diff --git a/lib/cloud_controller/config_schemas/api_schema.rb b/lib/cloud_controller/config_schemas/api_schema.rb
index ad399a1f19d..0cb25f7f5d3 100644
--- a/lib/cloud_controller/config_schemas/api_schema.rb
+++ b/lib/cloud_controller/config_schemas/api_schema.rb
@@ -318,7 +318,7 @@ class ApiSchema < VCAP::Config
           threadpool_size: Integer,
           skip_cert_verify: bool,
 
-          webserver: String, # thin or puma
+          webserver: String, # currently only puma
           optional(:puma) => {
             automatic_worker_count: bool,
             workers: Integer,
diff --git a/lib/cloud_controller/metrics_webserver.rb b/lib/cloud_controller/metrics_webserver.rb
index b2c4f4672b7..447f5d569ab 100644
--- a/lib/cloud_controller/metrics_webserver.rb
+++ b/lib/cloud_controller/metrics_webserver.rb
@@ -11,15 +11,19 @@ def initialize
       end
 
       def start(config)
-        server = Puma::Server.new(@app)
+        @server = Puma::Server.new(@app)
 
         if config.get(:nginx, :metrics_socket).nil? || config.get(:nginx, :metrics_socket).empty?
-          server.add_tcp_listener('127.0.0.1', 9395)
+          @server.add_tcp_listener('127.0.0.1', 9395)
         else
-          server.add_unix_listener(config.get(:nginx, :metrics_socket))
+          @server.add_unix_listener(config.get(:nginx, :metrics_socket))
         end
 
-        server.run
+        @server.run
+      end
+
+      def stop
+        @server.stop(true)
       end
 
       private
diff --git a/lib/cloud_controller/runner.rb b/lib/cloud_controller/runner.rb
index 0adf9741ed1..001528fb466 100644
--- a/lib/cloud_controller/runner.rb
+++ b/lib/cloud_controller/runner.rb
@@ -11,7 +11,6 @@
 require 'cloud_controller/logs/request_logs'
 require 'cloud_controller/telemetry_logger'
 require 'cloud_controller/secrets_fetcher'
-require 'cloud_controller/runners/thin_runner'
 require 'cloud_controller/runners/puma_runner'
 require 'cloud_controller/metrics_webserver'
 require 'prometheus/client/data_stores/direct_file_store'
@@ -33,7 +32,7 @@ def initialize(argv)
 
       # DB connection metrics have a label to determine whether the process accessing the connection is the
       # main or a worker process. We need to set this env variable before `setup_db` otherwise the main process
-      # will show up twice in the metrics as main and worker. Thin metrics will be labeled with main as well.
+      # will show up twice in the metrics as main and worker.
       ENV['PROCESS_TYPE'] = 'main'
 
       setup_cloud_controller
@@ -45,11 +44,7 @@ def initialize(argv)
       builder = RackAppBuilder.new
       app     = builder.build(@config, request_metrics, request_logs)
 
-      @server = if @config.get(:webserver) == 'puma'
-                  VCAP::CloudController::PumaRunner.new(@config, app, logger, periodic_updater, request_logs)
-                else
-                  VCAP::CloudController::ThinRunner.new(@config, app, logger, periodic_updater, request_logs)
-                end
+      @server = VCAP::CloudController::PumaRunner.new(@config, app, logger, periodic_updater, request_logs)
     end
 
     def logger
diff --git a/lib/cloud_controller/runners/thin_runner.rb b/lib/cloud_controller/runners/thin_runner.rb
deleted file mode 100644
index 1d00f620b64..00000000000
--- a/lib/cloud_controller/runners/thin_runner.rb
+++ /dev/null
@@ -1,73 +0,0 @@
-module VCAP::CloudController
-  class ThinRunner
-    attr_reader :logger
-
-    def initialize(config, app, logger, periodic_updater, request_logs)
-      @config = config
-      @logger = logger
-      @thin_server = if @config.get(:nginx, :use_nginx)
-                       Thin::Server.new(@config.get(:nginx, :instance_socket), signals: false)
-                     else
-                       Thin::Server.new(@config.get(:external_host), @config.get(:external_port), signals: false)
-                     end
-
-      @thin_server.app = app
-
-      # The routers proxying to us handle killing inactive connections.
-      # Set an upper limit just to be safe.
-      @thin_server.timeout = @config.get(:request_timeout_in_seconds)
-      @thin_server.threaded = true
-      @thin_server.threadpool_size = @config.get(:threadpool_size)
-      @periodic_updater = periodic_updater
-      @request_logs = request_logs
-    end
-
-    def start!
-      EM.run do
-        logger.info('starting periodic metrics updater')
-        @periodic_updater.setup_updates
-        trap_signals
-        logger.info("Starting thin server with #{EventMachine.threadpool_size} threads")
-        @thin_server.start!
-      rescue StandardError => e
-        logger.error "Encountered error: #{e}\n#{e.backtrace.join("\n")}"
-        raise e
-      end
-    end
-
-    def stop!
-      logger.info('Stopping Thin Server.')
-      @thin_server.stop if @thin_server
-      logger.info('Stopping EventMachine')
-      EM.stop
-      @request_logs.log_incomplete_requests if @request_logs
-    end
-
-    def trap_signals
-      %w[TERM INT QUIT].each do |signal|
-        trap(signal) do
-          EM.add_timer(0) do
-            logger.warn("Caught signal #{signal}")
-            stop!
-          end
-        end
-      end
-
-      trap('USR1') do
-        EM.add_timer(0) do
-          logger.warn('Collecting diagnostics')
-          collect_diagnostics
-        end
-      end
-    end
-
-    def collect_diagnostics
-      @diagnostics_dir ||= @config.get(:directories, :diagnostics)
-
-      file = VCAP::CloudController::Diagnostics.new.collect(@diagnostics_dir)
-      logger.warn("Diagnostics written to #{file}")
-    rescue StandardError => e
-      logger.warn("Failed to capture diagnostics: #{e}")
-    end
-  end
-end
diff --git a/middleware/cors.rb b/middleware/cors.rb
index f37bbed7348..ee3125f292d 100644
--- a/middleware/cors.rb
+++ b/middleware/cors.rb
@@ -33,7 +33,8 @@ def call(env)
                                    })
         end
 
-        return [200, preflight_headers, ''] if env['REQUEST_METHOD'] == 'OPTIONS'
+        # NOTE: The response body must be an enumerable for Puma. Returning an empty string does not work.
+        return [200, preflight_headers, ['']] if env['REQUEST_METHOD'] == 'OPTIONS'
 
         status, headers, body = call_app(env)
 
diff --git a/spec/request/internal/metrics_spec.rb b/spec/request/internal/metrics_spec.rb
index fdc553094a8..62284d6b43d 100644
--- a/spec/request/internal/metrics_spec.rb
+++ b/spec/request/internal/metrics_spec.rb
@@ -3,22 +3,17 @@
 RSpec.describe 'Metrics' do
   let(:user) { VCAP::CloudController::User.make }
   let(:user_header) { headers_for(user) }
-  let(:threadqueue) { double(EventMachine::Queue, size: 20, num_waiting: 0) }
-  let(:resultqueue) { double(EventMachine::Queue, size: 0, num_waiting: 1) }
+  let(:metrics_webserver) { VCAP::CloudController::MetricsWebserver.new }
+
+  delegate :app, to: :metrics_webserver
 
   before do
-    allow(EventMachine).to receive(:connection_count).and_return(123)
-
-    allow(EventMachine).to receive(:instance_variable_get) do |instance_var|
-      case instance_var
-      when :@threadqueue
-        threadqueue
-      when :@resultqueue
-        resultqueue
-      else
-        raise "Unexpected call: #{instance_var}"
-      end
-    end
+    allow_any_instance_of(VCAP::CloudController::Metrics::PeriodicUpdater).to receive(:update_webserver_stats)
+    metrics_webserver.start(TestConfig.config_instance)
+  end
+
+  after do
+    metrics_webserver.stop
   end
 
   it 'can be called several times' do
@@ -41,6 +36,7 @@
       10.times do
         VCAP::CloudController::User.make
       end
+      CloudController::DependencyLocator.instance.periodic_updater.update!
     end
 
     it 'reports the total number of users' do
@@ -54,6 +50,7 @@
 
   context 'cc_vitals' do
     it 'reports vitals' do
+      CloudController::DependencyLocator.instance.periodic_updater.update!
       get '/internal/v4/metrics', nil
 
       expect(last_response.body).to match(/cc_vitals_num_cores [1-9][0-9]*\.\d+/)
@@ -69,6 +66,8 @@
     before do
       Delayed::Job.enqueue(VCAP::CloudController::Jobs::Runtime::EventsCleanup.new(1), { queue: 'cc_api_0', run_at: Time.now + 1.day })
       Delayed::Job.enqueue(VCAP::CloudController::Jobs::Runtime::EventsCleanup.new(1), { queue: 'cc_generic', run_at: Time.now + 1.day })
+
+      CloudController::DependencyLocator.instance.periodic_updater.update!
     end
 
     after do
@@ -87,6 +86,8 @@
     before do
       Delayed::Job.enqueue(VCAP::CloudController::Jobs::Runtime::EventsCleanup.new(1), { queue: 'cc_api_0', run_at: Time.now })
       Delayed::Job.enqueue(VCAP::CloudController::Jobs::Runtime::EventsCleanup.new(1), { queue: 'cc_generic', run_at: Time.now })
+
+      CloudController::DependencyLocator.instance.periodic_updater.update!
     end
 
     after do
@@ -105,6 +106,8 @@
     before do
       Delayed::Job.enqueue(VCAP::CloudController::Jobs::Runtime::EventsCleanup.new(1), { queue: 'cc_api_0', run_at: Time.now + 1.minute })
       Delayed::Job.enqueue(VCAP::CloudController::Jobs::Runtime::EventsCleanup.new(1), { queue: 'cc_generic', run_at: Time.now + 1.minute })
+
+      CloudController::DependencyLocator.instance.periodic_updater.update!
     end
 
     after do
@@ -123,8 +126,9 @@
     before do
       Delayed::Job.enqueue(VCAP::CloudController::Jobs::Runtime::EventsCleanup.new(1), { queue: 'cc_api_0', run_at: Time.now + 1.day })
       Delayed::Job.enqueue(VCAP::CloudController::Jobs::Runtime::EventsCleanup.new(1), { queue: 'cc_generic', run_at: Time.now + 1.day })
-
       Delayed::Job.dataset.update(failed_at: Time.now.utc)
+
+      CloudController::DependencyLocator.instance.periodic_updater.update!
     end
 
     after do
diff --git a/spec/unit/controllers/internal/metric_controller_spec.rb b/spec/unit/controllers/internal/metric_controller_spec.rb
deleted file mode 100644
index b31e4db1ddc..00000000000
--- a/spec/unit/controllers/internal/metric_controller_spec.rb
+++ /dev/null
@@ -1,34 +0,0 @@
-require 'spec_helper'
-
-module VCAP::CloudController
-  module Internal
-    RSpec.describe MetricsController do
-      let(:threadqueue) { double(EventMachine::Queue, size: 20, num_waiting: 0) }
-      let(:resultqueue) { double(EventMachine::Queue, size: 0, num_waiting: 1) }
-
-      before do
-        allow(EventMachine).to receive(:connection_count).and_return(123)
-
-        allow(EventMachine).to receive(:instance_variable_get) do |instance_var|
-          case instance_var
-          when :@threadqueue
-            threadqueue
-          when :@resultqueue
-            resultqueue
-          else
-            raise "Unexpected call: #{instance_var}"
-          end
-        end
-      end
-
-      describe '#index' do
-        it 'returns a 200' do
-          get '/internal/v4/metrics'
-
-          expect(last_response.status).to eq 200
-          expect(last_response.body).to match(/cc_vitals_num_cores [1-9][0-9]*.\d+/)
-        end
-      end
-    end
-  end
-end
diff --git a/spec/unit/lib/background_job_environment_spec.rb b/spec/unit/lib/background_job_environment_spec.rb
index 1f3950100de..4127de8560c 100644
--- a/spec/unit/lib/background_job_environment_spec.rb
+++ b/spec/unit/lib/background_job_environment_spec.rb
@@ -16,7 +16,6 @@
   describe '#setup_environment' do
     before do
       allow(VCAP::CloudController::DB).to receive(:load_models)
-      allow(EM).to receive(:run).and_yield
       allow(VCAP::CloudController::ResourcePool).to receive(:new)
     end
 
diff --git a/spec/unit/lib/cloud_controller/diagnostics_spec.rb b/spec/unit/lib/cloud_controller/diagnostics_spec.rb
index 6aad33669fd..fdafad027d7 100644
--- a/spec/unit/lib/cloud_controller/diagnostics_spec.rb
+++ b/spec/unit/lib/cloud_controller/diagnostics_spec.rb
@@ -17,10 +17,6 @@ module VCAP::CloudController
 
     subject(:diagnostics) { Diagnostics.new }
 
-    before do
-      allow(EventMachine).to receive(:connection_count).and_return(17)
-    end
-
     after do
       VCAP::Request.current_id = nil
     end
diff --git a/spec/unit/lib/cloud_controller/diego/buildpack_entry_generator_spec.rb b/spec/unit/lib/cloud_controller/diego/buildpack_entry_generator_spec.rb
index fa869e91e55..27861fe2645 100644
--- a/spec/unit/lib/cloud_controller/diego/buildpack_entry_generator_spec.rb
+++ b/spec/unit/lib/cloud_controller/diego/buildpack_entry_generator_spec.rb
@@ -27,9 +27,6 @@ module Diego
       before do
         allow(blobstore_url_generator).to receive_messages(app_package_download_url: app_package_download_url, admin_buildpack_download_url: admin_buildpack_download_url,
                                                            buildpack_cache_download_url: build_artifacts_cache_download_uri)
-
-        allow(EM).to receive(:add_timer)
-        allow(EM).to receive(:defer).and_yield
       end
 
       describe '#buildpack_entries' do
diff --git a/spec/unit/lib/cloud_controller/metrics/periodic_updater_spec.rb b/spec/unit/lib/cloud_controller/metrics/periodic_updater_spec.rb
index 53aaf4dd651..c90893c845d 100644
--- a/spec/unit/lib/cloud_controller/metrics/periodic_updater_spec.rb
+++ b/spec/unit/lib/cloud_controller/metrics/periodic_updater_spec.rb
@@ -70,6 +70,16 @@ module VCAP::CloudController::Metrics
         allow(prometheus_updater).to receive(:update_log_counts)
         allow(prometheus_updater).to receive(:update_task_stats)
         allow(prometheus_updater).to receive(:update_deploying_count)
+
+        stats_hash = {
+          booted_workers: 2,
+          worker_status: [
+            { started_at: '2023-11-29T13:15:05Z', index: 0, pid: 123, last_status: { running: 1, backlog: 0, busy_threads: 0, pool_capacity: 1, requests_count: 9 } },
+            { started_at: '2023-11-29T13:15:10Z', index: 1, pid: 234, last_status: { running: 2, backlog: 1, busy_threads: 1, pool_capacity: 2, requests_count: 10 } }
+          ]
+        }
+        allow(Puma).to receive(:stats_hash).and_return(stats_hash)
+        allow(prometheus_updater).to receive(:update_webserver_stats_puma)
       end
 
       it 'bumps the number of users and sets periodic timer' do
@@ -594,38 +604,24 @@ module VCAP::CloudController::Metrics
         allow(prometheus_updater).to receive(:update_webserver_stats_puma)
       end
 
-      context 'when Puma is configured as webserver' do
-        before do
-          TestConfig.override(webserver: 'puma')
-        end
-
-        it 'sends stats to the prometheus updater' do
-          stats_hash = {
-            booted_workers: 2,
-            worker_status: [
-              { started_at: '2023-11-29T13:15:05Z', index: 0, pid: 123, last_status: { running: 1, backlog: 0, busy_threads: 0, pool_capacity: 1, requests_count: 9 } },
-              { started_at: '2023-11-29T13:15:10Z', index: 1, pid: 234, last_status: { running: 2, backlog: 1, busy_threads: 1, pool_capacity: 2, requests_count: 10 } }
-            ]
-          }
-          allow(Puma).to receive(:stats_hash).and_return(stats_hash)
-
-          periodic_updater.update_webserver_stats
-
-          expected_worker_count = 2
-          expected_worker_stats = [
-            { started_at: 1_701_263_705, index: 0, pid: 123, thread_count: 1, backlog: 0, busy_threads: 0, pool_capacity: 1, requests_count: 9 },
-            { started_at: 1_701_263_710, index: 1, pid: 234, thread_count: 2, backlog: 1, busy_threads: 1, pool_capacity: 2, requests_count: 10 }
+      it 'sends stats to the prometheus updater' do
+        stats_hash = {
+          booted_workers: 2,
+          worker_status: [
+            { started_at: '2023-11-29T13:15:05Z', index: 0, pid: 123, last_status: { running: 1, backlog: 0, busy_threads: 0, pool_capacity: 1, requests_count: 9 } },
+            { started_at: '2023-11-29T13:15:10Z', index: 1, pid: 234, last_status: { running: 2, backlog: 1, busy_threads: 1, pool_capacity: 2, requests_count: 10 } }
           ]
-          expect(prometheus_updater).to have_received(:update_webserver_stats_puma).with(expected_worker_count, expected_worker_stats)
-        end
-      end
+        }
+        allow(Puma).to receive(:stats_hash).and_return(stats_hash)
 
-      context 'when Thin is configured as webserver' do
-        it 'does not send stats to the prometheus updater' do
-          periodic_updater.update_webserver_stats
+        periodic_updater.update_webserver_stats
 
-          expect(prometheus_updater).not_to have_received(:update_webserver_stats_puma)
-        end
+        expected_worker_count = 2
+        expected_worker_stats = [
+          { started_at: 1_701_263_705, index: 0, pid: 123, thread_count: 1, backlog: 0, busy_threads: 0, pool_capacity: 1, requests_count: 9 },
+          { started_at: 1_701_263_710, index: 1, pid: 234, thread_count: 2, backlog: 1, busy_threads: 1, pool_capacity: 2, requests_count: 10 }
+        ]
+        expect(prometheus_updater).to have_received(:update_webserver_stats_puma).with(expected_worker_count, expected_worker_stats)
       end
     end
 
diff --git a/spec/unit/lib/cloud_controller/metrics/prometheus_updater_spec.rb b/spec/unit/lib/cloud_controller/metrics/prometheus_updater_spec.rb
index 274fbbbcae1..eb6b2603af3 100644
--- a/spec/unit/lib/cloud_controller/metrics/prometheus_updater_spec.rb
+++ b/spec/unit/lib/cloud_controller/metrics/prometheus_updater_spec.rb
@@ -160,10 +160,6 @@ module VCAP::CloudController::Metrics
     end
 
     describe '#update_webserver_stats_puma' do
-      before do
-        TestConfig.override(webserver: 'puma')
-      end
-
       it 'contains Puma stats' do
         worker_count = 2
         worker_stats = [
diff --git a/spec/unit/lib/cloud_controller/metrics_webserver_spec.rb b/spec/unit/lib/cloud_controller/metrics_webserver_spec.rb
index e924f54b361..47b6c6e7e48 100644
--- a/spec/unit/lib/cloud_controller/metrics_webserver_spec.rb
+++ b/spec/unit/lib/cloud_controller/metrics_webserver_spec.rb
@@ -5,6 +5,10 @@ module VCAP::CloudController
     let(:metrics_webserver) { described_class.new }
     let(:config) { double('config', get: nil) }
 
+    after do
+      metrics_webserver.stop
+    end
+
     describe '#start' do
       it 'configures and starts a Puma server' do
         allow(Puma::Server).to receive(:new).and_call_original
diff --git a/spec/unit/lib/cloud_controller/runner_spec.rb b/spec/unit/lib/cloud_controller/runner_spec.rb
index 4d8b051ba57..30edc77cc4d 100644
--- a/spec/unit/lib/cloud_controller/runner_spec.rb
+++ b/spec/unit/lib/cloud_controller/runner_spec.rb
@@ -14,12 +14,10 @@ module VCAP::CloudController
     before do
       allow(Steno).to receive(:init)
       allow(CloudController::DependencyLocator.instance).to receive(:routing_api_client).and_return(routing_api_client)
-      allow(EM).to receive(:run).and_yield
       allow(VCAP::CloudController::Metrics::PeriodicUpdater).to receive(:new).and_return(periodic_updater)
       allow(VCAP::CloudController::Metrics::RequestMetrics).to receive(:new).and_return(request_metrics)
       allow(periodic_updater).to receive(:setup_updates)
       allow(VCAP::PidFile).to receive(:new) { double(:pidfile, unlink_at_exit: nil) }
-      allow_any_instance_of(VCAP::CloudController::ThinRunner).to receive(:start!)
       allow(Puma::Server).to receive(:new).and_return(puma_server_double)
     end
 
@@ -31,13 +29,17 @@ module VCAP::CloudController
       it 'creates a pidfile' do
         expect(VCAP::PidFile).to receive(:new).with('/tmp/cloud_controller.pid')
 
+        server = double(:server)
+        allow(server).to receive(:start!)
+        expect(VCAP::CloudController::PumaRunner).to receive(:new).and_return(server)
+
         subject.run!
       end
 
       it 'starts the web server' do
         server = double(:server)
         allow(server).to receive(:start!)
-        expect(VCAP::CloudController::ThinRunner).to receive(:new).and_return(server)
+        expect(VCAP::CloudController::PumaRunner).to receive(:new).and_return(server)
 
         subject.run!
         expect(server).to have_received(:start!).once
@@ -45,25 +47,6 @@ module VCAP::CloudController
     end
 
     describe '#initialize' do
-      describe 'web server selection' do
-        context 'when thin is specifed' do
-          it 'chooses ThinRunner as the web server' do
-            expect(subject.instance_variable_get(:@server)).to be_an_instance_of(ThinRunner)
-          end
-        end
-
-        context 'when puma is specified' do
-          before do
-            TestConfig.override(webserver: 'puma')
-            allow(Config).to receive(:load_from_file).and_return(TestConfig.config_instance)
-          end
-
-          it 'chooses puma as the web server' do
-            expect(subject.instance_variable_get(:@server)).to be_an_instance_of(PumaRunner)
-          end
-        end
-      end
-
       it 'sets environment variable `PROCESS_TYPE` to `main`' do
         subject
 
@@ -149,6 +132,7 @@ module VCAP::CloudController
 
       it 'builds a rack app with request metrics and request logs handlers' do
         builder = instance_double(RackAppBuilder)
+        allow(builder).to receive(:build).and_return(double(:app))
         allow(RackAppBuilder).to receive(:new).and_return(builder)
         request_logs = double(:request_logs)
         allow(VCAP::CloudController::Logs::RequestLogs).to receive(:new).and_return(request_logs)
@@ -297,16 +281,6 @@ module VCAP::CloudController
             subject
           end
         end
-
-        context 'when the webserver is not puma' do
-          let(:test_config_overrides) { super().merge(webserver: 'thin') }
-
-          it 'does not start the MetricsWebserver' do
-            expect(MetricsWebserver).not_to receive(:new)
-
-            subject
-          end
-        end
       end
     end
   end
diff --git a/spec/unit/lib/cloud_controller/runners/thin_runner_spec.rb b/spec/unit/lib/cloud_controller/runners/thin_runner_spec.rb
deleted file mode 100644
index 70e5fa9ee9e..00000000000
--- a/spec/unit/lib/cloud_controller/runners/thin_runner_spec.rb
+++ /dev/null
@@ -1,159 +0,0 @@
-require 'spec_helper'
-
-module VCAP::CloudController
-  RSpec.describe ThinRunner do
-    let(:valid_config_file_path) { File.join(Paths::CONFIG, 'cloud_controller.yml') }
-    let(:config_file) { File.new(valid_config_file_path) }
-    let(:app) { double(:app) }
-    let(:logger) { double(:logger) }
-    let(:argv) { [] }
-    let(:periodic_updater) { double(:periodic_updater) }
-    let(:diagnostics) { instance_double(VCAP::CloudController::Diagnostics) }
-    let(:request_logs) { double(:request_logs) }
-
-    before do
-      allow(logger).to receive :info
-      allow(logger).to receive :warn
-      allow(periodic_updater).to receive :setup_updates
-      allow(EM).to receive(:run).and_yield
-      allow(EM).to receive(:add_timer).and_yield
-      allow(VCAP::CloudController::Diagnostics).to receive(:new).and_return(diagnostics)
-      allow(diagnostics).to receive(:collect)
-    end
-
-    subject do
-      config = Config.load_from_file(config_file.path, context: :api, secrets_hash: {})
-      config.set(:external_host, 'some_local_ip')
-      ThinRunner.new(config, app, logger, periodic_updater, request_logs)
-    end
-
-    it 'starts thin server on set up bind address' do
-      thin_server = double(:thin_server).as_null_object
-      expect(Thin::Server).to receive(:new).with('some_local_ip', 8181, { signals: false }).and_return(thin_server)
-      subject.start!
-      expect(subject.instance_variable_get(:@thin_server)).to eq(thin_server)
-    end
-
-    describe 'start!' do
-      let(:app) { double(:app) }
-      let(:thin_server) { OpenStruct.new(start!: nil) }
-
-      before do
-        allow(Thin::Server).to receive(:new).and_return(thin_server)
-        allow(thin_server).to receive(:start!)
-        subject.start!
-      end
-
-      it 'gets the timeout from the config' do
-        expect(thin_server.timeout).to eq(600)
-      end
-
-      it "uses thin's experimental threaded mode intentionally" do
-        expect(thin_server.threaded).to be(true)
-      end
-
-      it 'starts the thin server' do
-        expect(thin_server).to have_received(:start!)
-      end
-
-      it 'starts EventMachine' do
-        expect(EM).to have_received(:run)
-      end
-
-      it 'sets up periodic updater updates' do
-        expect(periodic_updater).to receive(:setup_updates)
-        subject.start!
-      end
-
-      it 'logs an error if an exception is raised' do
-        allow(thin_server).to receive(:start!).and_raise('we have a problem')
-        expect(subject.logger).to receive(:error)
-        expect { subject.start! }.to raise_exception('we have a problem')
-      end
-    end
-
-    describe '#stop!' do
-      let(:thin_server) { double(:thin_server) }
-
-      before do
-        subject.instance_variable_set(:@thin_server, thin_server)
-      end
-
-      it 'stops thin and EM, logs incomplete requests' do
-        expect(thin_server).to receive(:stop)
-        expect(request_logs).to receive(:log_incomplete_requests)
-        expect(EM).to receive(:stop)
-        subject.stop!
-      end
-    end
-
-    describe '#trap_signals' do
-      it 'registers TERM, INT, QUIT and USR1 handlers' do
-        expect(subject).to receive(:trap).with('TERM')
-        expect(subject).to receive(:trap).with('INT')
-        expect(subject).to receive(:trap).with('QUIT')
-        expect(subject).to receive(:trap).with('USR1')
-        subject.trap_signals
-      end
-
-      it 'calls #stop! when the handlers are triggered' do
-        callbacks = []
-
-        expect(subject).to receive(:trap).with('TERM') do |_, &blk|
-          callbacks << blk
-        end
-
-        expect(subject).to receive(:trap).with('INT') do |_, &blk|
-          callbacks << blk
-        end
-
-        expect(subject).to receive(:trap).with('QUIT') do |_, &blk|
-          callbacks << blk
-        end
-
-        expect(subject).to receive(:trap).with('USR1') do |_, &blk|
-          callbacks << blk
-        end
-
-        subject.trap_signals
-
-        expect(subject).to receive(:stop!).exactly(3).times
-
-        callbacks.each(&:call)
-      end
-    end
-
-    describe '#collect_diagnostics' do
-      callback = nil
-
-      before do
-        callback = nil
-        expect(subject).to receive(:trap).with('TERM')
-        expect(subject).to receive(:trap).with('INT')
-        expect(subject).to receive(:trap).with('QUIT')
-        expect(subject).to receive(:trap).with('USR1') do |_, &blk|
-          callback = blk
-        end
-        subject.trap_signals
-      end
-
-      let(:config_file) do
-        config = YAMLConfig.safe_load_file(valid_config_file_path)
-        config['directories'] ||= { 'tmpdir' => 'tmpdir' }
-        config['directories']['diagnostics'] = 'diagnostics/dir'
-        file = Tempfile.new('config')
-        file.write(YAML.dump(config))
-        file.rewind
-        file
-      end
-
-      it 'uses the configured directory' do
-        expect(Dir).not_to receive(:mktmpdir)
-        expect(subject).to receive(:collect_diagnostics).and_call_original
-        expect(diagnostics).to receive(:collect).with('diagnostics/dir')
-
-        callback.call
-      end
-    end
-  end
-end
diff --git a/spec/unit/lib/vcap/stats_spec.rb b/spec/unit/lib/vcap/stats_spec.rb
index bf6b78de5e7..c397bf14c3d 100644
--- a/spec/unit/lib/vcap/stats_spec.rb
+++ b/spec/unit/lib/vcap/stats_spec.rb
@@ -6,24 +6,11 @@
       allow(VCAP::Stats).to receive_messages(ps_pid: "123456 7.8\n", ps_ppid: "121212 3.4\n343434 5.6\n")
     end
 
-    it 'returns the memory bytes and cpu for the process' do
+    it 'returns the summed up memory bytes and cpu for the process and its subprocesses' do
       rss_bytes, pcpu = VCAP::Stats.process_memory_bytes_and_cpu
 
-      expect(rss_bytes).to eq(126_418_944)
-      expect(pcpu).to eq(8)
-    end
-
-    context 'when Puma is configured as webserver' do
-      before do
-        TestConfig.override(webserver: 'puma')
-      end
-
-      it 'returns the summed up memory bytes and cpu for the process and its subprocesses' do
-        rss_bytes, pcpu = VCAP::Stats.process_memory_bytes_and_cpu
-
-        expect(rss_bytes).to eq(602_216_448)
-        expect(pcpu).to eq(17)
-      end
+      expect(rss_bytes).to eq(602_216_448)
+      expect(pcpu).to eq(17)
     end
   end
 end
diff --git a/spec/unit/middleware/cors_spec.rb b/spec/unit/middleware/cors_spec.rb
index a75bb67e403..e9c9271c410 100644
--- a/spec/unit/middleware/cors_spec.rb
+++ b/spec/unit/middleware/cors_spec.rb
@@ -116,7 +116,7 @@ module Middleware
                 status, _, body = middleware.call(request_headers)
 
                 expect(status).to eq(200)
-                expect(body).to eq('')
+                expect(body).to eq([''])
               end
 
               it 'sets the Content-Type: text/plain header' do