examples: create an example for enabling and configuring retry

examples/features/retry/README.md

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+# Retry
+
+This example shows how to enable and configure retry on gRPC clients.
+
+## Documentation
+
+[gRFC for client-side retry support](https://github.com/grpc/proposal/blob/master/A6-client-retries.md)
+
+## Try it
+
+This example includes a service implementation that fails requests three times with status
+code `Unavailable`, then passes the fourth.  The client is configured to make four retry attempts
+when receiving an `Unavailable` status code.
+
+First start the server:
+
+```bash
+go run server/main.go
+```
+
+Then run the client.  Note that when running the client, `GRPC_GO_RETRY=on` must be set in
+your environment:
+
+```bash
+GRPC_GO_RETRY=on go run client/main.go
+```
+
+## Usage
+
+### Define your retry policy
+
+Retry is enabled via the service config, which can be provided by the name resolver or
+a DialOption (described below).  In the below config, we set retry policy for the
+"grpc.example.echo.Echo" method.
+
+MaxAttempts: how many times to attempt the RPC before failing.
+InitialBackoff, MaxBackoff, BackoffMultiplier: configures delay between attempts.
+RetryableStatusCodes: Retry only when receiving these status codes.
+
+```go
+        var retryPolicy = `{
+            "methodConfig": [{
+                // config per method or all methods under service
+                "name": [{"service": "grpc.examples.echo.Echo"}],
+                "waitForReady": true,
+
+                "retryPolicy": {
+                    "MaxAttempts": 4,
+                    "InitialBackoff": ".01s",
+                    "MaxBackoff": ".01s",
+                    "BackoffMultiplier": 1.0,
+                    // this value is grpc code
+                    "RetryableStatusCodes": [ "UNAVAILABLE" ]
+                }
+            }]
+        }`
+```
+
+### Providing the retry policy as a DialOption
+
+To use the above service config, pass it with `grpc.WithDefaultServiceConfig` to
+`grpc.Dial`.
+
+```go
+conn, err := grpc.Dial(ctx,grpc.WithInsecure(), grpc.WithDefaultServiceConfig(retryPolicy))
+```

examples/features/retry/client/main.go

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+/*
+ *
+ * Copyright 2019 gRPC authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ */
+
+// Binary client is an example client.
+package main
+
+import (
+	"context"
+	"flag"
+	"log"
+	"time"
+
+	"google.golang.org/grpc"
+	pb "google.golang.org/grpc/examples/features/proto/echo"
+)
+
+var (
+	addr = flag.String("addr", "localhost:50052", "the address to connect to")
+	// see https://github.com/grpc/grpc/blob/master/doc/service_config.md to know more about service config
+	retryPolicy = `{
+		"methodConfig": [{
+		  "name": [{"service": "grpc.examples.echo.Echo"}],
+		  "waitForReady": true,
+		  "retryPolicy": {
+			  "MaxAttempts": 4,
+			  "InitialBackoff": ".01s",
+			  "MaxBackoff": ".01s",
+			  "BackoffMultiplier": 1.0,
+			  "RetryableStatusCodes": [ "UNAVAILABLE" ]
+		  }
+		}]}`
+)
+
+// use grpc.WithDefaultServiceConfig() to set service config
+func retryDial() (*grpc.ClientConn, error) {
+	return grpc.Dial(*addr, grpc.WithInsecure(), grpc.WithDefaultServiceConfig(retryPolicy))
+}
+
+func main() {
+	flag.Parse()
+
+	// Set up a connection to the server.
+	conn, err := retryDial()
+	if err != nil {
+		log.Fatalf("did not connect: %v", err)
+	}
+	defer func() {
+		if e := conn.Close(); e != nil {
+			log.Printf("failed to close connection: %s", e)
+		}
+	}()
+
+	c := pb.NewEchoClient(conn)
+
+	ctx, cancel := context.WithTimeout(context.Background(), 1*time.Second)
+	defer cancel()
+
+	reply, err := c.UnaryEcho(ctx, &pb.EchoRequest{Message: "Try and Success"})
+	if err != nil {
+		log.Fatalf("UnaryEcho error: %v", err)
+	}
+	log.Printf("UnaryEcho reply: %v", reply)
+}

examples/features/retry/server/main.go

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+/*
+ *
+ * Copyright 2019 gRPC authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ */
+
+// Binary server is an example server.
+package main
+
+import (
+	"context"
+	"flag"
+	"fmt"
+	"log"
+	"net"
+	"sync"
+
+	"google.golang.org/grpc"
+	"google.golang.org/grpc/codes"
+	pb "google.golang.org/grpc/examples/features/proto/echo"
+	"google.golang.org/grpc/status"
+)
+
+var port = flag.Int("port", 50052, "port number")
+
+type failingServer struct {
+	mu sync.Mutex
+
+	reqCounter uint
+	reqModulo  uint
+}
+
+// this method will fail reqModulo - 1 times RPCs and return status code Unavailable,
+// and succeeded RPC on reqModulo times.
+func (s *failingServer) maybeFailRequest() error {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	s.reqCounter++
+	if (s.reqModulo > 0) && (s.reqCounter%s.reqModulo == 0) {
+		return nil
+	}
+
+	return status.Errorf(codes.Unavailable, "maybeFailRequest: failing it")
+}
+
+func (s *failingServer) UnaryEcho(ctx context.Context, req *pb.EchoRequest) (*pb.EchoResponse, error) {
+	if err := s.maybeFailRequest(); err != nil {
+		log.Println("request failed count:", s.reqCounter)
+		return nil, err
+	}
+
+	log.Println("request succeeded count:", s.reqCounter)
+	return &pb.EchoResponse{Message: req.Message}, nil
+}
+
+func (s *failingServer) ServerStreamingEcho(req *pb.EchoRequest, stream pb.Echo_ServerStreamingEchoServer) error {
+	return status.Error(codes.Unimplemented, "RPC unimplemented")
+}
+
+func (s *failingServer) ClientStreamingEcho(stream pb.Echo_ClientStreamingEchoServer) error {
+	return status.Error(codes.Unimplemented, "RPC unimplemented")
+}
+
+func (s *failingServer) BidirectionalStreamingEcho(stream pb.Echo_BidirectionalStreamingEchoServer) error {
+	return status.Error(codes.Unimplemented, "RPC unimplemented")
+}
+
+func main() {
+	flag.Parse()
+
+	address := fmt.Sprintf(":%v", *port)
+	lis, err := net.Listen("tcp", address)
+	if err != nil {
+		log.Fatalf("failed to listen: %v", err)
+	}
+	fmt.Println("listen on address", address)
+
+	s := grpc.NewServer()
+
+	// Configure server to pass every fourth RPC;
+	// client is configured to make four attempts.
+	failingservice := &failingServer{
+		reqCounter: 0,
+		reqModulo:  4,
+	}
+
+	pb.RegisterEchoServer(s, failingservice)
+	if err := s.Serve(lis); err != nil {
+		log.Fatalf("failed to serve: %v", err)
+	}
+}