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roundrobin.go (2520B)

      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.
     22 package roundrobin
     23 
     24 import (
     25 	"sync/atomic"
     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.
     34 const Name = "round_robin"
     35 
     36 var logger = grpclog.Component("roundrobin")
     37 
     38 // newBuilder creates a new roundrobin balancer builder.
     39 func newBuilder() balancer.Builder {
     40 	return base.NewBalancerBuilder(Name, &rrPickerBuilder{}, base.Config{HealthCheck: true})
     41 }
     42 
     43 func init() {
     44 	balancer.Register(newBuilder())
     45 }
     46 
     47 type rrPickerBuilder struct{}
     48 
     49 func (*rrPickerBuilder) Build(info base.PickerBuildInfo) balancer.Picker {
     50 	logger.Infof("roundrobinPicker: Build called with info: %v", info)
     51 	if len(info.ReadySCs) == 0 {
     52 		return base.NewErrPicker(balancer.ErrNoSubConnAvailable)
     53 	}
     54 	scs := make([]balancer.SubConn, 0, len(info.ReadySCs))
     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: uint32(grpcrand.Intn(len(scs))),
     64 	}
     65 }
     66 
     67 type 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 	next     uint32
     73 }
     74 
     75 func (p *rrPicker) Pick(balancer.PickInfo) (balancer.PickResult, error) {
     76 	subConnsLen := uint32(len(p.subConns))
     77 	nextIndex := atomic.AddUint32(&p.next, 1)
     78 
     79 	sc := p.subConns[nextIndex%subConnsLen]
     80 	return balancer.PickResult{SubConn: sc}, nil
     81 }