examples: Add pre-serialized-message example (#10112)

This came out of the question #9707, and could be useful to others.

5 files changed, 319 insertions, 0 deletions

diff --git a/examples/README.md b/examples/README.md
index ae849fa7d..9664942b5 100644
--- a/examples/README.md
+++ b/examples/README.md
@@ -205,6 +205,8 @@ $ bazel-bin/hello-world-client
 
 - [JWT-based Authentication](example-jwt-auth)
 
+- [Pre-serialized messages](src/main/java/io/grpc/examples/preserialized)
+
 ## Unit test examples
 
 Examples for unit testing gRPC clients and servers are located in [examples/src/test](src/test).
diff --git a/examples/build.gradle b/examples/build.gradle
index fb799b42e..7322ae290 100644
--- a/examples/build.gradle
+++ b/examples/build.gradle
@@ -110,6 +110,8 @@ createStartScripts('io.grpc.examples.multiplex.MultiplexingServer')
 createStartScripts('io.grpc.examples.multiplex.SharingClient')
 createStartScripts('io.grpc.examples.nameresolve.NameResolveClient')
 createStartScripts('io.grpc.examples.nameresolve.NameResolveServer')
+createStartScripts('io.grpc.examples.preserialized.PreSerializedClient')
+createStartScripts('io.grpc.examples.preserialized.PreSerializedServer')
 createStartScripts('io.grpc.examples.retrying.RetryingHelloWorldClient')
 createStartScripts('io.grpc.examples.retrying.RetryingHelloWorldServer')
 createStartScripts('io.grpc.examples.routeguide.RouteGuideClient')
diff --git a/examples/src/main/java/io/grpc/examples/preserialized/ByteArrayMarshaller.java b/examples/src/main/java/io/grpc/examples/preserialized/ByteArrayMarshaller.java
new file mode 100644
index 000000000..c6f099280
--- /dev/null
+++ b/examples/src/main/java/io/grpc/examples/preserialized/ByteArrayMarshaller.java
@@ -0,0 +1,43 @@
+/*
+ * Copyright 2023 The 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.
+ */
+
+package io.grpc.examples.preserialized;
+
+import com.google.common.io.ByteStreams;
+import io.grpc.MethodDescriptor;
+import java.io.ByteArrayInputStream;
+import java.io.IOException;
+import java.io.InputStream;
+
+/**
+ * A marshaller that produces a byte[] instead of decoding into typical POJOs. It can be used for
+ * any message type.
+ */
+final class ByteArrayMarshaller implements MethodDescriptor.Marshaller<byte[]> {
+  @Override
+  public byte[] parse(InputStream stream) {
+    try {
+      return ByteStreams.toByteArray(stream);
+    } catch (IOException ex) {
+      throw new RuntimeException(ex);
+    }
+  }
+
+  @Override
+  public InputStream stream(byte[] b) {
+    return new ByteArrayInputStream(b);
+  }
+}
diff --git a/examples/src/main/java/io/grpc/examples/preserialized/PreSerializedClient.java b/examples/src/main/java/io/grpc/examples/preserialized/PreSerializedClient.java
new file mode 100644
index 000000000..511e8a177
--- /dev/null
+++ b/examples/src/main/java/io/grpc/examples/preserialized/PreSerializedClient.java
@@ -0,0 +1,108 @@
+/*
+ * Copyright 2023 The 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.
+ */
+
+package io.grpc.examples.preserialized;
+
+import io.grpc.CallOptions;
+import io.grpc.Channel;
+import io.grpc.Grpc;
+import io.grpc.InsecureChannelCredentials;
+import io.grpc.ManagedChannel;
+import io.grpc.MethodDescriptor;
+import io.grpc.StatusRuntimeException;
+import io.grpc.examples.helloworld.GreeterGrpc;
+import io.grpc.examples.helloworld.HelloReply;
+import io.grpc.examples.helloworld.HelloRequest;
+import io.grpc.stub.ClientCalls;
+import java.util.concurrent.TimeUnit;
+import java.util.logging.Level;
+import java.util.logging.Logger;
+
+/**
+ * A client that requests a greeting from a hello-world server, but using a pre-serialized request.
+ * This is a performance optimization that can be useful if you read the request from on-disk or a
+ * database where it is already serialized, or if you need to send the same complicated message to
+ * many servers. The same approach can avoid deserializing responses, to be stored in a database.
+ * This adjustment is client-side only; the server is unable to detect the difference, so this
+ * client is fully-compatible with the normal {@link HelloWorldServer}.
+ */
+public class PreSerializedClient {
+  private static final Logger logger = Logger.getLogger(PreSerializedClient.class.getName());
+
+  /**
+   * Modified sayHello() descriptor with bytes as the request, instead of HelloRequest. By adjusting
+   * toBuilder() you can choose which of the request and response are bytes.
+   */
+  private static final MethodDescriptor<byte[], HelloReply> SAY_HELLO
+      = GreeterGrpc.getSayHelloMethod()
+      .toBuilder(new ByteArrayMarshaller(), GreeterGrpc.getSayHelloMethod().getResponseMarshaller())
+      .build();
+
+  private final Channel channel;
+
+  /** Construct client for accessing hello-world server using the existing channel. */
+  public PreSerializedClient(Channel channel) {
+    this.channel = channel;
+  }
+
+  /** Say hello to server. */
+  public void greet(String name) {
+    logger.info("Will try to greet " + name + " ...");
+    byte[] request = HelloRequest.newBuilder().setName(name).build().toByteArray();
+    HelloReply response;
+    try {
+      // Stubs use ClientCalls to send RPCs. Since the generated stub won't have byte[] in its
+      // method signature, this uses ClientCalls directly. It isn't as convenient, but it behaves
+      // the same as a normal stub.
+      response = ClientCalls.blockingUnaryCall(channel, SAY_HELLO, CallOptions.DEFAULT, request);
+    } catch (StatusRuntimeException e) {
+      logger.log(Level.WARNING, "RPC failed: {0}", e.getStatus());
+      return;
+    }
+    logger.info("Greeting: " + response.getMessage());
+  }
+
+  /**
+   * Greet server. If provided, the first element of {@code args} is the name to use in the
+   * greeting. The second argument is the target server.
+   */
+  public static void main(String[] args) throws Exception {
+    String user = "world";
+    String target = "localhost:50051";
+    if (args.length > 0) {
+      if ("--help".equals(args[0])) {
+        System.err.println("Usage: [name [target]]");
+        System.err.println("");
+        System.err.println("  name    The name you wish to be greeted by. Defaults to " + user);
+        System.err.println("  target  The server to connect to. Defaults to " + target);
+        System.exit(1);
+      }
+      user = args[0];
+    }
+    if (args.length > 1) {
+      target = args[1];
+    }
+
+    ManagedChannel channel = Grpc.newChannelBuilder(target, InsecureChannelCredentials.create())
+        .build();
+    try {
+      PreSerializedClient client = new PreSerializedClient(channel);
+      client.greet(user);
+    } finally {
+      channel.shutdownNow().awaitTermination(5, TimeUnit.SECONDS);
+    }
+  }
+}
diff --git a/examples/src/main/java/io/grpc/examples/preserialized/PreSerializedServer.java b/examples/src/main/java/io/grpc/examples/preserialized/PreSerializedServer.java
new file mode 100644
index 000000000..51beca573
--- /dev/null
+++ b/examples/src/main/java/io/grpc/examples/preserialized/PreSerializedServer.java
@@ -0,0 +1,164 @@
+/*
+ * Copyright 2023 The 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.
+ */
+
+package io.grpc.examples.preserialized;
+
+import io.grpc.BindableService;
+import io.grpc.Grpc;
+import io.grpc.InsecureServerCredentials;
+import io.grpc.MethodDescriptor;
+import io.grpc.Server;
+import io.grpc.ServerCallHandler;
+import io.grpc.ServerMethodDefinition;
+import io.grpc.ServerServiceDefinition;
+import io.grpc.ServiceDescriptor;
+import io.grpc.examples.helloworld.GreeterGrpc;
+import io.grpc.examples.helloworld.HelloReply;
+import io.grpc.examples.helloworld.HelloRequest;
+import io.grpc.stub.ServerCalls;
+import io.grpc.stub.StreamObserver;
+import java.io.IOException;
+import java.util.concurrent.TimeUnit;
+import java.util.logging.Logger;
+
+/**
+ * Server that provides a {@code Greeter} service, but that uses a pre-serialized response. This is
+ * a performance optimization that can be useful if you read the response from on-disk or a database
+ * where it is already serialized, or if you need to send the same complicated message to many
+ * clients. The same approach can avoid deserializing requests, to be stored in a database. This
+ * adjustment is server-side only; the client is unable to detect the differences, so this server is
+ * fully-compatible with the normal {@link HelloWorldClient}.
+ */
+public class PreSerializedServer {
+  private static final Logger logger = Logger.getLogger(PreSerializedServer.class.getName());
+
+  private Server server;
+
+  private void start() throws IOException {
+    int port = 50051;
+    server = Grpc.newServerBuilderForPort(port, InsecureServerCredentials.create())
+        .addService(new GreeterImpl())
+        .build()
+        .start();
+    logger.info("Server started, listening on " + port);
+    Runtime.getRuntime().addShutdownHook(new Thread() {
+      @Override
+      public void run() {
+        // Use stderr here since the logger may have been reset by its JVM shutdown hook.
+        System.err.println("*** shutting down gRPC server since JVM is shutting down");
+        try {
+          PreSerializedServer.this.stop();
+        } catch (InterruptedException e) {
+          e.printStackTrace(System.err);
+        }
+        System.err.println("*** server shut down");
+      }
+    });
+  }
+
+  private void stop() throws InterruptedException {
+    if (server != null) {
+      server.shutdown().awaitTermination(30, TimeUnit.SECONDS);
+    }
+  }
+
+  /**
+   * Await termination on the main thread since the grpc library uses daemon threads.
+   */
+  private void blockUntilShutdown() throws InterruptedException {
+    if (server != null) {
+      server.awaitTermination();
+    }
+  }
+
+  /**
+   * Main launches the server from the command line.
+   */
+  public static void main(String[] args) throws IOException, InterruptedException {
+    final PreSerializedServer server = new PreSerializedServer();
+    server.start();
+    server.blockUntilShutdown();
+  }
+
+  static class GreeterImpl implements GreeterGrpc.AsyncService, BindableService {
+
+    public void byteSayHello(HelloRequest req, StreamObserver<byte[]> responseObserver) {
+      HelloReply reply = HelloReply.newBuilder().setMessage("Hello " + req.getName()).build();
+      responseObserver.onNext(reply.toByteArray());
+      responseObserver.onCompleted();
+    }
+
+    @Override
+    public ServerServiceDefinition bindService() {
+      MethodDescriptor<HelloRequest, HelloReply> sayHello = GreeterGrpc.getSayHelloMethod();
+      // Modifying the method descriptor to use bytes as the response, instead of HelloReply. By
+      // adjusting toBuilder() you can choose which of the request and response are bytes.
+      MethodDescriptor<HelloRequest, byte[]> byteSayHello = sayHello
+          .toBuilder(sayHello.getRequestMarshaller(), new ByteArrayMarshaller())
+          .build();
+      // GreeterGrpc.bindService() will bind every service method, including sayHello(). (Although
+      // Greeter only has one method, this approach would work for any service.) AsyncService
+      // provides a default implementation of sayHello() that returns UNIMPLEMENTED, and that
+      // implementation will be used by bindService(). replaceMethod() will rewrite that method to
+      // use our byte-based method instead.
+      //
+      // The generated bindService() uses ServerCalls to make RPC handlers. Since the generated
+      // bindService() won't expect byte[] in the AsyncService, this uses ServerCalls directly. It
+      // isn't as convenient, but it behaves the same as a normal RPC handler.
+      return replaceMethod(
+          GreeterGrpc.bindService(this),
+          byteSayHello,
+          ServerCalls.asyncUnaryCall(this::byteSayHello));
+    }
+
+    /** Rewrites the ServerServiceDefinition replacing one method's definition. */
+    private static <ReqT, RespT> ServerServiceDefinition replaceMethod(
+        ServerServiceDefinition def,
+        MethodDescriptor<ReqT, RespT> newDesc,
+        ServerCallHandler<ReqT, RespT> newHandler) {
+      // There are two data structures involved. The first is the "descriptor" which describes the
+      // service and methods as a schema. This is the same on client and server. The second is the
+      // "definition" which includes the handlers to execute methods. This is specific to the server
+      // and is generated by "bind." This adjusts both the descriptor and definition.
+
+      // Descriptor
+      ServiceDescriptor desc = def.getServiceDescriptor();
+      ServiceDescriptor.Builder descBuilder = ServiceDescriptor.newBuilder(desc.getName())
+          .setSchemaDescriptor(desc.getSchemaDescriptor())
+          .addMethod(newDesc); // Add the modified method
+      // Copy methods other than the modified one
+      for (MethodDescriptor<?,?> md : desc.getMethods()) {
+        if (newDesc.getFullMethodName().equals(md.getFullMethodName())) {
+          continue;
+        }
+        descBuilder.addMethod(md);
+      }
+
+      // Definition
+      ServerServiceDefinition.Builder defBuilder =
+          ServerServiceDefinition.builder(descBuilder.build())
+          .addMethod(newDesc, newHandler); // Add the modified method
+      // Copy methods other than the modified one
+      for (ServerMethodDefinition<?,?> smd : def.getMethods()) {
+        if (newDesc.getFullMethodName().equals(smd.getMethodDescriptor().getFullMethodName())) {
+          continue;
+        }
+        defBuilder.addMethod(smd);
+      }
+      return defBuilder.build();
+    }
+  }
+}