snapshot of https://github.com/grpc/grpc-go.git (master, 2026-09-15; blobs > 131072B omitted)

balancer/randomsubsetting/randomsubsetting.go

@@ -0,0 +1,201 @@
+/*
+ *
+ * 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 implements the random_subsetting LB policy specified
+// here: https://github.com/grpc/proposal/blob/master/A68-random-subsetting.md
+//
+// To install the LB policy, import this package as:
+//
+//	import _ "google.golang.org/grpc/balancer/randomsubsetting"
+//
+// # Experimental
+//
+// Notice: This package is EXPERIMENTAL and may be changed or removed in a
+// later release.
+package randomsubsetting
+
+import (
+	"cmp"
+	"encoding/json"
+	"fmt"
+	"math/rand/v2"
+	"slices"
+
+	xxhash "github.com/cespare/xxhash/v2"
+	"google.golang.org/grpc/balancer"
+	"google.golang.org/grpc/grpclog"
+	"google.golang.org/grpc/internal/balancer/gracefulswitch"
+	internalgrpclog "google.golang.org/grpc/internal/grpclog"
+	iserviceconfig "google.golang.org/grpc/internal/serviceconfig"
+	"google.golang.org/grpc/resolver"
+	"google.golang.org/grpc/serviceconfig"
+)
+
+// Name is the name of the random subsetting load balancer.
+const Name = "random_subsetting_experimental"
+
+var (
+	logger     = grpclog.Component(Name)
+	randUint64 = rand.Uint64
+)
+
+func prefixLogger(p *subsettingBalancer) *internalgrpclog.PrefixLogger {
+	return internalgrpclog.NewPrefixLogger(logger, fmt.Sprintf("[random-subsetting-lb %p] ", p))
+}
+
+func init() {
+	balancer.Register(bb{})
+}
+
+type bb struct{}
+
+func (bb) Build(cc balancer.ClientConn, bOpts balancer.BuildOptions) balancer.Balancer {
+	b := &subsettingBalancer{
+		Balancer:   gracefulswitch.NewBalancer(cc, bOpts),
+		hashSeed:   randUint64(),
+		hashDigest: xxhash.New(),
+	}
+	b.logger = prefixLogger(b)
+	b.logger.Infof("Created")
+	return b
+}
+
+type lbConfig struct {
+	serviceconfig.LoadBalancingConfig `json:"-"`
+
+	SubsetSize  uint32                         `json:"subsetSize,omitempty"`
+	ChildPolicy *iserviceconfig.BalancerConfig `json:"childPolicy,omitempty"`
+}
+
+func (bb) ParseConfig(s json.RawMessage) (serviceconfig.LoadBalancingConfig, error) {
+	lbCfg := &lbConfig{}
+
+	// Ensure that the specified child policy is registered and validates its
+	// config, if present.
+	if err := json.Unmarshal(s, lbCfg); err != nil {
+		return nil, fmt.Errorf("randomsubsetting: json.Unmarshal failed for configuration: %s with error: %v", string(s), err)
+	}
+	if lbCfg.SubsetSize == 0 {
+		return nil, fmt.Errorf("randomsubsetting: SubsetSize must be greater than 0")
+	}
+	if lbCfg.ChildPolicy == nil {
+		return nil, fmt.Errorf("randomsubsetting: ChildPolicy must be specified")
+	}
+
+	return lbCfg, nil
+}
+
+func (bb) Name() string {
+	return Name
+}
+
+type subsettingBalancer struct {
+	*gracefulswitch.Balancer
+
+	logger     *internalgrpclog.PrefixLogger
+	cfg        *lbConfig
+	hashSeed   uint64
+	hashDigest *xxhash.Digest
+}
+
+func (b *subsettingBalancer) UpdateClientConnState(s balancer.ClientConnState) error {
+	lbCfg, ok := s.BalancerConfig.(*lbConfig)
+	if !ok {
+		b.logger.Warningf("Received config with unexpected type %T: %v", s.BalancerConfig, s.BalancerConfig)
+		return balancer.ErrBadResolverState
+	}
+
+	// Build config for the gracefulswitch balancer. It is safe to ignore
+	// JSON marshaling errors here, since the config was already validated
+	// as part of ParseConfig().
+	cfg := []map[string]any{{lbCfg.ChildPolicy.Name: lbCfg.ChildPolicy.Config}}
+	cfgJSON, _ := json.Marshal(cfg)
+	parsedCfg, err := gracefulswitch.ParseConfig(cfgJSON)
+	if err != nil {
+		return fmt.Errorf("randomsubsetting: error switching to child of type %q: %v", lbCfg.ChildPolicy.Name, err)
+	}
+	b.cfg = lbCfg
+	endpoints := resolver.State{
+		Endpoints:     b.calculateSubset(s.ResolverState.Endpoints),
+		ServiceConfig: s.ResolverState.ServiceConfig,
+		Attributes:    s.ResolverState.Attributes,
+	}
+
+	return b.Balancer.UpdateClientConnState(balancer.ClientConnState{
+		ResolverState:  endpoints,
+		BalancerConfig: parsedCfg,
+	})
+}
+
+// calculateSubset implements the subsetting algorithm, as described in A68:
+// https://github.com/grpc/proposal/blob/master/A68-random-subsetting.md#subsetting-algorithm
+func (b *subsettingBalancer) calculateSubset(endpoints []resolver.Endpoint) []resolver.Endpoint {
+	// A helper struct to hold an endpoint and its hash.
+	type endpointWithHash struct {
+		hash uint64
+		ep   resolver.Endpoint
+	}
+
+	// Empty endpoints cannot be hashed and are ignored by child policies.
+	// Remove them here so they do not count toward the requested subset size.
+	isEmpty := func(endpoint resolver.Endpoint) bool {
+		return len(endpoint.Addresses) == 0
+	}
+	if slices.ContainsFunc(endpoints, isEmpty) {
+		endpoints = slices.DeleteFunc(slices.Clone(endpoints), isEmpty)
+	}
+
+	subsetSize := b.cfg.SubsetSize
+	if len(endpoints) <= int(subsetSize) {
+		return endpoints
+	}
+
+	hashedEndpoints := make([]endpointWithHash, len(endpoints))
+	for i, endpoint := range endpoints {
+		// For every endpoint in the list, compute a hash with previously
+		// generated seed - A68.
+		//
+		// The xxhash package's Sum64() function does not allow setting a seed.
+		// This means that we need to reset the digest with the seed for every
+		// endpoint. Without this, an endpoint will not retain the same hash
+		// across resolver updates.
+		//
+		// Note that we only hash the first address of the endpoint, as per A68.
+		b.hashDigest.ResetWithSeed(b.hashSeed)
+		b.hashDigest.WriteString(endpoint.Addresses[0].String())
+		hashedEndpoints[i] = endpointWithHash{
+			hash: b.hashDigest.Sum64(),
+			ep:   endpoint,
+		}
+	}
+
+	slices.SortFunc(hashedEndpoints, func(a, b endpointWithHash) int {
+		// Note: This uses the standard library cmp package, not the
+		// github.com/google/go-cmp/cmp package. The latter is intended for
+		// testing purposes only.
+		return cmp.Compare(a.hash, b.hash)
+	})
+
+	// Convert back to resolver.Endpoints
+	endpointSubset := make([]resolver.Endpoint, subsetSize)
+	for i, endpoint := range hashedEndpoints[:subsetSize] {
+		endpointSubset[i] = endpoint.ep
+	}
+
+	return endpointSubset
+}