balancer/randomsubsetting: Extend the unit tests in the randomsubsetting package.

balancer/randomsubsetting/randomsubsetting_ext_test.go

@@ -0,0 +1,112 @@
+/*
+ *
+ * Copyright 2025 gRPC authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ */
+
+package randomsubsetting
+
+import (
+	"fmt"
+	"testing"
+
+	xxhash "github.com/cespare/xxhash/v2"
+	"google.golang.org/grpc/resolver"
+
+	_ "google.golang.org/grpc/balancer/roundrobin" // For round_robin LB policy in tests
+)
+
+func (s) TestSubsettingEndpointsDomain(t *testing.T) {
+
+	testCases := []struct {
+		endpoints  []resolver.Endpoint
+		subsetSize uint32
+		want       uint32
+	}{
+		{
+			endpoints:  makeEndpoints(4),
+			subsetSize: 0,
+			want:       0,
+		},
+		{
+			endpoints:  makeEndpoints(3),
+			subsetSize: 4,
+			want:       3,
+		},
+		{
+			endpoints:  makeEndpoints(5),
+			subsetSize: 3,
+			want:       3,
+		},
+		{
+			endpoints:  []resolver.Endpoint{},
+			subsetSize: 1,
+			want:       0,
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(fmt.Sprint(tc.subsetSize), func(t *testing.T) {
+			b := &subsettingBalancer{
+				cfg:        &lbConfig{SubsetSize: tc.subsetSize},
+				hashSeed:   0,
+				hashDigest: xxhash.New(),
+			}
+			subsetOfEndpoints := b.calculateSubset(tc.endpoints)
+			got := len(subsetOfEndpoints)
+			if fmt.Sprint(got) != fmt.Sprint(tc.want) {
+				t.Fatalf("subset size=%v; endpoints(%v) = %v; want %v", tc.want, tc.endpoints, got, tc.want)
+			}
+		})
+	}
+}
+
+func (s) TestUniformDistributionOfEndpoints(t *testing.T) {
+
+	var (
+		subsetSize = 4
+		iteration  = 1600
+		diff       int
+	)
+
+	endpoints := makeEndpoints(16)
+	expected := iteration / len(endpoints) * subsetSize
+	diff = expected / 7 // allow ~14% difference
+	EndpointCount := make(map[string]int, len(endpoints))
+
+	for i := 0; i < iteration; i++ {
+		t.Run(fmt.Sprint(subsetSize), func(t *testing.T) {
+			t.Helper()
+			lb := &subsettingBalancer{
+				cfg:        &lbConfig{SubsetSize: uint32(subsetSize)},
+				hashSeed:   uint64(i ^ 3 + iteration*i + subsetSize),
+				hashDigest: xxhash.New(),
+			}
+			subsetOfEndpoints := lb.calculateSubset(endpoints)
+
+			for _, ep := range subsetOfEndpoints {
+				EndpointCount[ep.Addresses[0].Addr]++
+			}
+		})
+	}
+	// Verify the distribution is uniform within a small diff range.
+	// The expected count for each endpoint is: iteration / total_endpoints * subset_size
+	// e.g. 1600 / 16 * 4 = 400 +/- diff
+	for epAddr, count := range EndpointCount {
+		if count < expected-diff || count > expected+diff {
+			t.Fatalf("endpoint %v selected %v times; expected <=> %v", epAddr, count, expected)
+		}
+	}
+}