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example: load_balancing (#2504)
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Diff for: ‎examples/features/load_balancing/README.md

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# Load balancing
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This examples shows how `ClientConn` can pick different load balancing policies.
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Note: to show the effect of load balancers, an example resolver is installed in
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this example to get the backend addresses. It's suggested to read the name
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resolver example before this example.
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## Try it
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```
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go run server/main.go
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```
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```
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go run client/main.go
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```
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## Explanation
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Two echo servers are serving on ":50051" and ":50052". They will include their
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serving address in the response. So the server on ":50051" will reply to the RPC
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with `this is examples/load_balancing (from :50051)`.
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Two clients are created, to connect to both of these servers (they get both
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server addresses from the name resolver).
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Each client picks a different load balancer (using `grpc.WithBalancerName`):
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`pick_first` or `round_robin`. (These two policies are supported in gRPC by
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default. To add a custom balancing policy, implement the interfaces defined in
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https://godoc.org/google.golang.org/grpc/balancer).
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Note that balancers can also be switched using service config, which allows
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service owners (instead of client owners) to pick the balancer to use. Service
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config doc is available at
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https://github.com/grpc/grpc/blob/master/doc/service_config.md.
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### pick_first
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The first client is configured to use `pick_first`. `pick_first` tries to
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connect to the first address, uses it for all RPCs if it connects, or try the
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next address if it fails (and keep doing that until one connection is
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successful). Because of this, all the RPCs will be sent to the same backend. The
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responses received all show the same backend address.
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```
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this is examples/load_balancing (from :50051)
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this is examples/load_balancing (from :50051)
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this is examples/load_balancing (from :50051)
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this is examples/load_balancing (from :50051)
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this is examples/load_balancing (from :50051)
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this is examples/load_balancing (from :50051)
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this is examples/load_balancing (from :50051)
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this is examples/load_balancing (from :50051)
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this is examples/load_balancing (from :50051)
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this is examples/load_balancing (from :50051)
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```
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### round_robin
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The second client is configured to use `round_robin`. `round_robin` connects to
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all the addresses it sees, and sends an RPC to each backend one at a time in
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order. E.g. the first RPC will be sent to backend-1, the second RPC will be be
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sent to backend-2, and the third RPC will be be sent to backend-1 again.
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```
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this is examples/load_balancing (from :50051)
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this is examples/load_balancing (from :50051)
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this is examples/load_balancing (from :50052)
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this is examples/load_balancing (from :50051)
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this is examples/load_balancing (from :50052)
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this is examples/load_balancing (from :50051)
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this is examples/load_balancing (from :50052)
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this is examples/load_balancing (from :50051)
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this is examples/load_balancing (from :50052)
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this is examples/load_balancing (from :50051)
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```
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Note that it's possible to see two continues RPC sent to the same backend.
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That's because `round_robin` only picks the connections ready for RPCs. So if
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one of the two connections is not ready for some reason, all RPCs will be sent
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to the ready connection.

Diff for: ‎examples/features/load_balancing/client/main.go

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/*
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*
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* Copyright 2018 gRPC authors.
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*
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*/
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// Binary client is an example client.
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package main
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import (
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"context"
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"fmt"
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"log"
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"time"
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"google.golang.org/grpc"
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ecpb "google.golang.org/grpc/examples/features/proto/echo"
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"google.golang.org/grpc/resolver"
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)
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const (
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exampleScheme = "example"
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exampleServiceName = "lb.example.grpc.io"
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)
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var addrs = []string{"localhost:50051", "localhost:50052"}
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func callUnaryEcho(c ecpb.EchoClient, message string) {
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ctx, cancel := context.WithTimeout(context.Background(), time.Second)
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defer cancel()
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r, err := c.UnaryEcho(ctx, &ecpb.EchoRequest{Message: message})
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if err != nil {
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log.Fatalf("could not greet: %v", err)
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}
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fmt.Println(r.Message)
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}
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func makeRPCs(cc *grpc.ClientConn, n int) {
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hwc := ecpb.NewEchoClient(cc)
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for i := 0; i < n; i++ {
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callUnaryEcho(hwc, "this is examples/load_balancing")
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}
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}
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func main() {
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pickfirstConn, err := grpc.Dial(
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fmt.Sprintf("%s:///%s", exampleScheme, exampleServiceName),
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// grpc.WithBalancerName("pick_first"), // "pick_first" is the default, so this DialOption is not necessary.
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grpc.WithInsecure(),
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)
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if err != nil {
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log.Fatalf("did not connect: %v", err)
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}
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defer pickfirstConn.Close()
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fmt.Println("--- calling helloworld.Greeter/SayHello with pick_first ---")
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makeRPCs(pickfirstConn, 10)
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fmt.Println()
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// Make another ClientConn with round_robin policy.
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roundrobinConn, err := grpc.Dial(
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fmt.Sprintf("%s:///%s", exampleScheme, exampleServiceName),
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grpc.WithBalancerName("round_robin"), // This sets the initial balancing policy.
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grpc.WithInsecure(),
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)
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if err != nil {
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log.Fatalf("did not connect: %v", err)
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}
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defer roundrobinConn.Close()
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fmt.Println("--- calling helloworld.Greeter/SayHello with round_robin ---")
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makeRPCs(roundrobinConn, 10)
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}
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// Following is an example name resolver implementation. Read the name
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// resolution example to learn more about it.
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type exampleResolverBuilder struct{}
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func (*exampleResolverBuilder) Build(target resolver.Target, cc resolver.ClientConn, opts resolver.BuildOption) (resolver.Resolver, error) {
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r := &exampleResolver{
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target: target,
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cc: cc,
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addrsStore: map[string][]string{
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exampleServiceName: addrs,
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},
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}
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r.start()
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return r, nil
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}
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func (*exampleResolverBuilder) Scheme() string { return exampleScheme }
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type exampleResolver struct {
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target resolver.Target
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cc resolver.ClientConn
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addrsStore map[string][]string
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}
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func (r *exampleResolver) start() {
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addrStrs := r.addrsStore[r.target.Endpoint]
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addrs := make([]resolver.Address, len(addrStrs), len(addrStrs))
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for i, s := range addrStrs {
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addrs[i] = resolver.Address{Addr: s}
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}
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r.cc.NewAddress(addrs)
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}
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func (*exampleResolver) ResolveNow(o resolver.ResolveNowOption) {}
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func (*exampleResolver) Close() {}
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func init() {
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resolver.Register(&exampleResolverBuilder{})
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}

Diff for: ‎examples/features/load_balancing/server/main.go

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/*
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*
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* Copyright 2018 gRPC authors.
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*
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*/
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// Binary server is an example server.
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package main
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import (
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"context"
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"fmt"
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"log"
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"net"
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"sync"
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"google.golang.org/grpc"
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"google.golang.org/grpc/codes"
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ecpb "google.golang.org/grpc/examples/features/proto/echo"
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"google.golang.org/grpc/status"
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)
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var (
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addrs = []string{":50051", ":50052"}
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)
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type ecServer struct {
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addr string
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}
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func (s *ecServer) UnaryEcho(ctx context.Context, req *ecpb.EchoRequest) (*ecpb.EchoResponse, error) {
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return &ecpb.EchoResponse{Message: fmt.Sprintf("%s (from %s)", req.Message, s.addr)}, nil
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}
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func (s *ecServer) ServerStreamingEcho(*ecpb.EchoRequest, ecpb.Echo_ServerStreamingEchoServer) error {
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return status.Errorf(codes.Unimplemented, "not implemented")
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}
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func (s *ecServer) ClientStreamingEcho(ecpb.Echo_ClientStreamingEchoServer) error {
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return status.Errorf(codes.Unimplemented, "not implemented")
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}
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func (s *ecServer) BidirectionalStreamingEcho(ecpb.Echo_BidirectionalStreamingEchoServer) error {
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return status.Errorf(codes.Unimplemented, "not implemented")
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}
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func startServer(addr string) {
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lis, err := net.Listen("tcp", addr)
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if err != nil {
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log.Fatalf("failed to listen: %v", err)
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}
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s := grpc.NewServer()
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ecpb.RegisterEchoServer(s, &ecServer{addr: addr})
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log.Printf("serving on %s\n", addr)
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if err := s.Serve(lis); err != nil {
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log.Fatalf("failed to serve: %v", err)
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}
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}
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func main() {
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var wg sync.WaitGroup
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for _, addr := range addrs {
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wg.Add(1)
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go func(addr string) {
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defer wg.Done()
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startServer(addr)
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}(addr)
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}
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wg.Wait()
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}

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