Implementation of A68 random_subsetting LB policy.

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 defines a random subsetting balancer.
+//
+// To install random subsetting balancer, import this package as:
+//
+//	import _ "google.golang.org/grpc/balancer/randomsubsetting"
+package randomsubsetting
+
+import (
+	"encoding/json"
+	"errors"
+	"fmt"
+	"sort"
+	"time"
+
+	"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"
+)
+
+const (
+	// Name is the name of the random subsetting load balancer.
+	Name = "random_subsetting"
+)
+
+var (
+	logger = grpclog.Component(Name)
+)
+
+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{
+		cc:    cc,
+		hashf: xxhash.NewWithSeed(uint64(time.Now().UnixNano())),
+	}
+	// Create a logger with a prefix specific to this balancer instance.
+	b.logger = prefixLogger(b)
+
+	b.logger.Infof("Created")
+	b.child = gracefulswitch.NewBalancer(cc, bOpts)
+	return b
+}
+
+// LBConfig is the config for the outlier detection balancer.
+type LBConfig struct {
+	serviceconfig.LoadBalancingConfig `json:"-"`
+
+	SubsetSize uint64 `json:"subset_size,omitempty"`
+
+	ChildPolicy *iserviceconfig.BalancerConfig `json:"child_policy,omitempty"`
+}
+
+func (bb) ParseConfig(s json.RawMessage) (serviceconfig.LoadBalancingConfig, error) {
+	lbCfg := &LBConfig{
+		// Default top layer values.
+		SubsetSize: 10,
+	}
+
+	if err := json.Unmarshal(s, lbCfg); err != nil { // Validates child config if present as well.
+		return nil, fmt.Errorf("subsetting: unable to unmarshal LBconfig: %s, error: %v", string(s), err)
+	}
+
+	// if someonw needs subsetSize == 1, he should use pick_first instead
+	if lbCfg.SubsetSize < 2 {
+		return nil, errors.New("subsetting: subsetSize must be >= 2")
+	}
+
+	return lbCfg, nil
+}
+
+func (bb) Name() string {
+	return Name
+}
+
+type subsettingBalancer struct {
+	cc     balancer.ClientConn
+	logger *internalgrpclog.PrefixLogger
+	cfg    *LBConfig
+	hashf  *xxhash.Digest
+	child  *gracefulswitch.Balancer
+}
+
+func (b *subsettingBalancer) UpdateClientConnState(s balancer.ClientConnState) error {
+	lbCfg, ok := s.BalancerConfig.(*LBConfig)
+	if !ok {
+		b.logger.Errorf("received config with unexpected type %T: %v", s.BalancerConfig, s.BalancerConfig)
+		return balancer.ErrBadResolverState
+	}
+
+	// Reject whole config if child policy doesn't exist, don't persist it for
+	// later.
+	bb := balancer.Get(lbCfg.ChildPolicy.Name)
+	if bb == nil {
+		return fmt.Errorf("subsetting: child balancer %q not registered", lbCfg.ChildPolicy.Name)
+	}
+
+	if b.cfg == nil || b.cfg.ChildPolicy.Name != lbCfg.ChildPolicy.Name {
+		err := b.child.SwitchTo(bb)
+		if err != nil {
+			return fmt.Errorf("subsetting: error switching to child of type %q: %v", lbCfg.ChildPolicy.Name, err)
+		}
+	}
+	b.cfg = lbCfg
+
+	err := b.child.UpdateClientConnState(balancer.ClientConnState{
+		ResolverState:  b.prepareChildResolverState(s.ResolverState),
+		BalancerConfig: b.cfg.ChildPolicy.Config,
+	})
+
+	return err
+}
+
+type AddressWithHash struct {
+	hash uint64
+	addr resolver.Address
+}
+
+// implements the subsetting algorithm, as described in A68: https://github.com/grpc/proposal/pull/423
+func (b *subsettingBalancer) prepareChildResolverState(s resolver.State) resolver.State {
+	addresses := s.Addresses
+	backendCount := len(addresses)
+	if backendCount <= int(b.cfg.SubsetSize) {
+		return s
+	}
+
+	addressesSet := make([]AddressWithHash, backendCount)
+	// calculate hash for each endpoint
+	for i, endpoint := range addresses {
+
+		b.hashf.Write([]byte(s.Addresses[0].String()))
+		addressesSet[i] = AddressWithHash{
+			hash: b.hashf.Sum64(),
+			addr: endpoint,
+		}
+	}
+	// sort addresses by hash
+	sort.Slice(addressesSet, func(i, j int) bool {
+		return addressesSet[i].hash < addressesSet[j].hash
+	})
+
+	b.logger.Infof("resulting subset: %v", addressesSet[:b.cfg.SubsetSize])
+
+	// Convert back to resolver.addresses
+	addressesSubset := make([]resolver.Address, b.cfg.SubsetSize)
+	for _, eh := range addressesSet[:b.cfg.SubsetSize] {
+		addressesSubset = append(addressesSubset, eh.addr)
+	}
+
+	return resolver.State{
+		Addresses:     addressesSubset,
+		ServiceConfig: s.ServiceConfig,
+		Attributes:    s.Attributes,
+	}
+}
+
+func (b *subsettingBalancer) ResolverError(err error) {
+	b.child.ResolverError(err)
+}
+
+func (b *subsettingBalancer) UpdateSubConnState(sc balancer.SubConn, state balancer.SubConnState) {
+	b.child.UpdateSubConnState(sc, state)
+}
+
+func (b *subsettingBalancer) Close() {
+	b.child.Close()
+}
+
+func (b *subsettingBalancer) ExitIdle() {
+	b.child.ExitIdle()
+}