Source: roundrobin.go in package google.golang.org/grpc/balancer/roundrobin

* * Copyright 2017 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 roundrobin defines a roundrobin balancer. Roundrobin balancer is installed as one of the default balancers in gRPC, users don't need to explicitly install this balancer.

package roundrobin

import (
	"sync"

	"google.golang.org/grpc/balancer"
	"google.golang.org/grpc/balancer/base"
	"google.golang.org/grpc/grpclog"
	"google.golang.org/grpc/internal/grpcrand"
)

Name is the name of round_robin balancer.

const Name = "round_robin"

var logger = grpclog.Component("roundrobin")

newBuilder creates a new roundrobin balancer builder.

func newBuilder() balancer.Builder {
	return base.NewBalancerBuilder(Name, &rrPickerBuilder{}, base.Config{HealthCheck: true})
}

func init() {
	balancer.Register(newBuilder())
}

type rrPickerBuilder struct{}

func (*rrPickerBuilder) Build(info base.PickerBuildInfo) balancer.Picker {
	logger.Infof("roundrobinPicker: newPicker called with info: %v", info)
	if len(info.ReadySCs) == 0 {
		return base.NewErrPicker(balancer.ErrNoSubConnAvailable)
	}
	var scs []balancer.SubConn
	for sc := range info.ReadySCs {
		scs = append(scs, sc)
	}
	return &rrPicker{

Start at a random index, as the same RR balancer rebuilds a new picker when SubConn states change, and we don't want to apply excess load to the first server in the list.

		next: grpcrand.Intn(len(scs)),
	}
}

subConns is the snapshot of the roundrobin balancer when this picker was created. The slice is immutable. Each Get() will do a round robin selection from it and return the selected SubConn.

	subConns []balancer.SubConn

	mu   sync.Mutex
	next int
}

func (p *rrPicker) Pick(balancer.PickInfo) (balancer.PickResult, error) {
	p.mu.Lock()
	sc := p.subConns[p.next]
	p.next = (p.next + 1) % len(p.subConns)
	p.mu.Unlock()
	return balancer.PickResult{SubConn: sc}, nil