1/*
2 *
3 * Copyright 2017 gRPC authors.
4 *
5 * Licensed under the Apache License, Version 2.0 (the "License");
6 * you may not use this file except in compliance with the License.
7 * You may obtain a copy of the License at
8 *
9 *     http://www.apache.org/licenses/LICENSE-2.0
10 *
11 * Unless required by applicable law or agreed to in writing, software
12 * distributed under the License is distributed on an "AS IS" BASIS,
13 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
14 * See the License for the specific language governing permissions and
15 * limitations under the License.
16 *
17 */
18
19// Package roundrobin defines a roundrobin balancer. Roundrobin balancer is
20// installed as one of the default balancers in gRPC, users don't need to
21// explicitly install this balancer.
22package roundrobin
23
24import (
25	"sync"
26
27	"google.golang.org/grpc/balancer"
28	"google.golang.org/grpc/balancer/base"
29	"google.golang.org/grpc/grpclog"
30	"google.golang.org/grpc/internal/grpcrand"
31)
32
33// Name is the name of round_robin balancer.
34const Name = "round_robin"
35
36var logger = grpclog.Component("roundrobin")
37
38// newBuilder creates a new roundrobin balancer builder.
39func newBuilder() balancer.Builder {
40	return base.NewBalancerBuilder(Name, &rrPickerBuilder{}, base.Config{HealthCheck: true})
41}
42
43func init() {
44	balancer.Register(newBuilder())
45}
46
47type rrPickerBuilder struct{}
48
49func (*rrPickerBuilder) Build(info base.PickerBuildInfo) balancer.Picker {
50	logger.Infof("roundrobinPicker: newPicker called with info: %v", info)
51	if len(info.ReadySCs) == 0 {
52		return base.NewErrPicker(balancer.ErrNoSubConnAvailable)
53	}
54	var scs []balancer.SubConn
55	for sc := range info.ReadySCs {
56		scs = append(scs, sc)
57	}
58	return &rrPicker{
59		subConns: scs,
60		// Start at a random index, as the same RR balancer rebuilds a new
61		// picker when SubConn states change, and we don't want to apply excess
62		// load to the first server in the list.
63		next: grpcrand.Intn(len(scs)),
64	}
65}
66
67type rrPicker struct {
68	// subConns is the snapshot of the roundrobin balancer when this picker was
69	// created. The slice is immutable. Each Get() will do a round robin
70	// selection from it and return the selected SubConn.
71	subConns []balancer.SubConn
72
73	mu   sync.Mutex
74	next int
75}
76
77func (p *rrPicker) Pick(balancer.PickInfo) (balancer.PickResult, error) {
78	p.mu.Lock()
79	sc := p.subConns[p.next]
80	p.next = (p.next + 1) % len(p.subConns)
81	p.mu.Unlock()
82	return balancer.PickResult{SubConn: sc}, nil
83}
84