前言

grpc默認(rèn)支持兩種負(fù)載均衡算法pick_first 和 round_robin

輪詢法round_robin不能滿足因服務(wù)器配置不同而承擔(dān)不同負(fù)載量，這篇文章將介紹如何實(shí)現(xiàn)自定義負(fù)載均衡策略--加權(quán)隨機(jī)。

加權(quán)隨機(jī)法可以根據(jù)服務(wù)器的處理能力而分配不同的權(quán)重，從而實(shí)現(xiàn)處理能力高的服務(wù)器可承擔(dān)更多的請(qǐng)求，處理能力低的服務(wù)器少承擔(dān)請(qǐng)求。

上一篇我們實(shí)現(xiàn)resolverBuilder接口，用來解析服務(wù)器地址，而負(fù)載均衡算法就是在這個(gè)接口基礎(chǔ)上篩選出一個(gè)地址，這個(gè)過程是在客戶端發(fā)送請(qǐng)求的時(shí)候進(jìn)行。

以下示例基于grpc v1.26.0版本，更高的版本不兼容etcd，不方便測(cè)試，而接口名或者參數(shù)也會(huì)不同，不過原理是相似的

自定義負(fù)載均衡策略

使用自定義的負(fù)載均衡策略主要實(shí)現(xiàn)V2PickerBuilder和V2Picker這兩個(gè)接口

type V2PickerBuilder interface {
    Build(info PickerBuildInfo) balancer.V2Picker
}

Build方法：返回一個(gè)V2選擇器，將用于gRPC選擇子連接。

type V2Picker interface {
    Pick(info PickInfo) (PickResult, error)
}

Pick方法：子連接選擇，具體的算法在這個(gè)方法內(nèi)實(shí)現(xiàn)

完整代碼：

package rpc

import (
    "google.golang.org/grpc/balancer"
    "google.golang.org/grpc/balancer/base"
    "google.golang.org/grpc/grpclog"
    "math/rand"
    "sync"
)

const WEIGHT_LB_NAME =  "weight"

func init() {
    balancer.Register(
        base.NewBalancerBuilderV2(WEIGHT_LB_NAME,&LocalBuilder{},base.Config{HealthCheck: false}))
}

type LocalBuilder struct {

}

func (*LocalBuilder) Build(info base.PickerBuildInfo) balancer.V2Picker {
    if len(info.ReadySCs) == 0 {
        return base.NewErrPickerV2(balancer.ErrNoSubConnAvailable)
    }
    var scs []balancer.SubConn
    for subConn,addr := range info.ReadySCs {
        weight := addr.Address.Attributes.Value("weight").(int)
        if weight <= 0 {
            weight = 1
        }
        for i := 0; i < weight; i++ {
            scs = append(scs, subConn)
        }
    }
    return &localPick{
        subConn: scs,
    }
}

type localPick struct {
    subConn []balancer.SubConn
    mu sync.Mutex
}

func  (l *localPick)Pick(info balancer.PickInfo) (r balancer.PickResult, err error){
    l.mu.Lock()
    index := rand.Intn(len(l.subConn))
    r.SubConn = l.subConn[index]
    l.mu.Lock()
    return r,nil
}

結(jié)合示例再看一下Build(info PickerBuildInfo) balancer.V2Picker這個(gè)方法的作用：

這個(gè)函數(shù)的返回值就是我們還要實(shí)現(xiàn)的第二個(gè)接口，所以這里主要看一下參數(shù)

type PickerBuildInfo struct {
    // ReadySCs is a map from all ready SubConns to the Addresses used to
    // create them.
    ReadySCs map[balancer.SubConn]
}

type SubConnInfo struct {
    Address resolver.Address // the address used to create this SubConn
}

ReadySCs:是所有可用的子連接
balancer.SubConn:是一個(gè)子連接的結(jié)構(gòu)，這里我們不用關(guān)心這個(gè)值，在pick里面直接返回就可以了。
SubConnInfo:里面是一個(gè)Address的結(jié)構(gòu)

type Address struct {
    Addr string
    ServerName string

    // Attributes contains arbitrary data about this address intended for
    // consumption by the load balancing policy.
    Attributes *attributes.Attributes  
    Type AddressType
    Metadata interface{}
}
...
...
type Attributes struct {
    m map[interface{}]interface{}
}

這個(gè)Address就是上一篇中resolverBuilder中設(shè)置的值，所以這兩個(gè)功能是有聯(lián)系的，Attributes的m是一個(gè)map，剛剛好保存我們需要的權(quán)重weight

結(jié)下來就是Pick方法
Pick(info balancer.PickInfo) (r balancer.PickResult, err error)

type PickInfo struct {
    // FullMethodName is the method name that NewClientStream() is called
    // with. The canonical format is /service/Method.
    FullMethodName string    //eg : /User/GetInfo
    // Ctx is the RPC's context, and may contain relevant RPC-level information
    // like the outgoing header metadata.
    Ctx context.Context
}

type PickResult struct {
    SubConn SubConn
    Done func(DoneInfo)
}

FullMethodName:對(duì)應(yīng)XXXpb.go中生成的FullMethod: "/Memo/GetMemo",
Ctx: 如注釋所說，包含metadata
PickResult: 返回的子連接
Done:rpc完成之后調(diào)用Done

使用新建的策略

func main()  {
    grpc.UseCompressor(gzip.Name)
    conn, err := grpc.Dial(
        fmt.Sprintf("%s:///%s", "game", baseService),
        //grpc.WithDefaultServiceConfig(fmt.Sprintf(`{"LoadBalancingPolicy": "%s"}`, roundrobin.Name)),
        grpc.WithDefaultServiceConfig(fmt.Sprintf(`{"LoadBalancingPolicy": "%s"}`, rpc.WEIGHT_LB_NAME)),
        grpc.WithInsecure(),
    )  
      ...
}