conn, err := grpc.Dial("example:8009", grpc.WithInsecure())
if err != nil {
  panic(err)
}

scheme:[//[user[:password]@]host[:port]][/path][?query][#fragment]

type Builder interface {
 // Build creates a new resolver for the given target.
 //
 // gRPC dial calls Build synchronously, and fails if the returned error is
 // not nil.
 Build(target Target, cc ClientConn, opts BuildOptions) (Resolver, error)
 // Scheme returns the scheme supported by this resolver.
 // Scheme is defined at https://github.com/grpc/grpc/blob/master/doc/naming.md.
 Scheme() string
}

type Resolver interface {
 // ResolveNow will be called by gRPC to try to resolve the target name
 // again. It's just a hint, resolver can ignore this if it's not necessary.
 //
 // It could be called multiple times concurrently.
  // ResolveNow 嘗試再一次對域名進行解析，在個人實踐中，服務端進程掛掉會觸發該調用
 ResolveNow(ResolveNowOptions)
  // Close closes the resolver.
  // 資源釋放。
 Close()
}

# builder.go
package resolver

import "google.golang.org/grpc/resolver"

var _ resolver.Builder = Builder{}

type Builder struct {
 addrsStore map[string][]string
}

func NewResolverBuilder(addrsStore map[string][]string) *Builder {
 return &Builder{addrsStore: addrsStore}
}

func (b Builder) Build(target resolver.Target, cc resolver.ClientConn, opts resolver.BuildOptions) (resolver.Resolver, error) {
 r := &Resolver{
  target:     target,
  cc:         cc,
  addrsStore: b.addrsStore,
 }
 r.Start()
 return r, nil
}

func (b Builder) Scheme() string {
 return "example"
}

# resolver.go
package resolver

import (
 "google.golang.org/grpc/resolver"
)

var _ resolver.Resolver = &Resolver{}

// impl google.golang.org/grpc/resolver.Resolver
type Resolver struct {
 target resolver.Target
 cc     resolver.ClientConn

 addrsStore map[string][]string
}

func (r *Resolver) Start() {
 // 在靜態路由表中查詢此 Endpoint 對應 addrs
 var addrs []resolver.Address
 for _, addr := range r.addrsStore[r.target.URL.Opaque] {
  addrs = append(addrs, resolver.Address{Addr: addr})
 }

 r.cc.UpdateState(resolver.State{
  Addresses: addrs,
    // 設置負載均衡策略為round_robin
  ServiceConfig: r.cc.ParseServiceConfig(
    `{"loadBalancingPolicy":"round_robin"}`),
 })
}

func (r *Resolver) ResolveNow(resolver.ResolveNowOptions) {

}

func (r *Resolver) Close() {

}

# main.go
package main

import (
 "context"
 "log"

 rs "github.com/liangwt/note/grpc/name_resolver_lb_example/client/resolver"
 pb "github.com/liangwt/note/grpc/name_resolver_lb_example/ecommerce"
 "google.golang.org/grpc"
 "google.golang.org/grpc/resolver"
)

func main() {
 resolver.Register(rs.NewResolverBuilder(map[string][]string{
  "cluster@callee": {
   "127.0.0.1:8009",
  },
 }))

 conn, err := grpc.Dial("example:cluster@callee", grpc.WithInsecure())
 if err != nil {
  panic(err)
 }
  
 // ...
}

conn, err := grpc.Dial("example:cluster@callee",
  grpc.WithInsecure(),
  grpc.WithDefaultServiceConfig(
   `{"loadBalancingPolicy":"round_robin"}`,
  ),
 )

_ "google.golang.org/grpc/xds" // To install the xds resolvers and balancers.

conn, err := grpc.Dial("xds:///localhost:50051", grpc.WithTransportCredentials(creds))
if err != nil {
  panic(err)
}

ctx, cancelFn := context.WithCancel(context.Background())
defer cancelFn()

c := pb.NewOrderManagementClient(conn)
res, err := c.AddOrder(ctx, &order)

// Builder creates a balancer.
type Builder interface {
 // Build creates a new balancer with the ClientConn.
 Build(cc ClientConn, opts BuildOptions) Balancer
 // Name returns the name of balancers built by this builder.
 // It will be used to pick balancers (for example in service config).
 Name() string
}

type Balancer interface {
 // UpdateClientConnState is called by gRPC when the state of the ClientConn
 // changes.  If the error returned is ErrBadResolverState, the ClientConn
 // will begin calling ResolveNow on the active name resolver with
 // exponential backoff until a subsequent call to UpdateClientConnState
 // returns a nil error.  Any other errors are currently ignored.
 UpdateClientConnState(ClientConnState) error
 // ResolverError is called by gRPC when the name resolver reports an error.
 ResolverError(error)
 // UpdateSubConnState is called by gRPC when the state of a SubConn
 // changes.
 UpdateSubConnState(SubConn, SubConnState)
 // Close closes the balancer. The balancer is not required to call
 // ClientConn.RemoveSubConn for its existing SubConns.
 Close()
}

func NewBalancerBuilder(name string, pb PickerBuilder, config Config) balancer.Builder

// PickerBuilder creates balancer.Picker.
type PickerBuilder interface {
 // Build returns a picker that will be used by gRPC to pick a SubConn.
 Build(info PickerBuildInfo) balancer.Picker
}

type Picker interface {
  // 子連接選擇
 Pick(info PickInfo) (PickResult, error)
}

# builder.go
package balancer

import (
 "google.golang.org/grpc/balancer"
 "google.golang.org/grpc/balancer/base"
)

var _ base.PickerBuilder = &Builder{}

type Builder struct {
}

func NewBalancerBuilder() balancer.Builder {
 return base.NewBalancerBuilder("random_picker", &Builder{}, base.Config{HealthCheck: true})
}

func (b *Builder) Build(info base.PickerBuildInfo) balancer.Picker {
 if len(info.ReadySCs) == 0 {
  return base.NewErrPicker(balancer.ErrNoSubConnAvailable)
 }

 var scs []balancer.SubConn
 for subConn := range info.ReadySCs {
  scs = append(scs, subConn)
 }

 return &Picker{
  subConns: scs,
 }
}

// picker.go
package balancer

import (
 "math/rand"

 "google.golang.org/grpc/balancer"
)

var _ balancer.Picker = &Picker{}

type Picker struct {
 subConns []balancer.SubConn
}

func (p *Picker) Pick(info balancer.PickInfo) (balancer.PickResult, error) {
 index := rand.Intn(len(p.subConns))
 sc := p.subConns[index]
 return balancer.PickResult{SubConn: sc}, nil
}

#client.go
package main

import (
 "context"
 "log"

 bl "github.com/liangwt/note/grpc/name_resolver_lb_example/client/balancer"
 rs "github.com/liangwt/note/grpc/name_resolver_lb_example/client/resolver"
 pb "github.com/liangwt/note/grpc/name_resolver_lb_example/ecommerce"
 "google.golang.org/grpc"
 "google.golang.org/grpc/balancer"
 "google.golang.org/grpc/resolver"
)

func main() {
 resolver.Register(rs.NewResolverBuilder(map[string][]string{
  "cluster@callee": {
   "127.0.0.1:8009",
   "127.0.0.1:8010",
  },
 }))

 balancer.Register(bl.NewBalancerBuilder())

 conn, err := grpc.Dial("example:cluster@callee",
  grpc.WithInsecure(),
  grpc.WithDefaultServiceConfig(
   `{"loadBalancingPolicy":"random_picker"}`,
  ),
 )
 if err != nil {
  panic(err)
 }
 
  ....
}