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RPC Server/Client Implementation based on Netty and Protocol Buffer
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/**
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you 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 org.apache.tajo.rpc;

import com.google.protobuf.BlockingService;
import com.google.protobuf.Descriptors.MethodDescriptor;
import com.google.protobuf.Message;
import com.google.protobuf.RpcController;
import io.netty.channel.*;
import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
import org.apache.tajo.rpc.RpcProtos.RpcRequest;
import org.apache.tajo.rpc.RpcProtos.RpcResponse;

import java.lang.reflect.Method;
import java.net.InetSocketAddress;

public class BlockingRpcServer extends NettyServerBase {
  private static Log LOG = LogFactory.getLog(BlockingRpcServer.class);
  private final BlockingService service;
  private final ChannelInitializer initializer;

  public BlockingRpcServer(final Class protocol,
                           final Object instance,
                           final InetSocketAddress bindAddress,
                           final int workerNum)
      throws Exception {

    super(protocol.getSimpleName(), bindAddress);

    String serviceClassName = protocol.getName() + "$" +
        protocol.getSimpleName() + "Service";
    Class serviceClass = Class.forName(serviceClassName);
    Class interfaceClass = Class.forName(serviceClassName +
        "$BlockingInterface");
    Method method = serviceClass.getMethod(
        "newReflectiveBlockingService", interfaceClass);

    this.service = (BlockingService) method.invoke(null, instance);
    this.initializer = new ProtoChannelInitializer(new ServerHandler(), RpcRequest.getDefaultInstance());

    super.init(this.initializer, workerNum);
  }

  @ChannelHandler.Sharable
  private class ServerHandler extends SimpleChannelInboundHandler {

    @Override
    public void channelRegistered(ChannelHandlerContext ctx) throws Exception {
      accepted.add(ctx.channel());
      if(LOG.isDebugEnabled()){
        LOG.debug(String.format(serviceName + " accepted number of connections (%d)", accepted.size()));
      }
      super.channelRegistered(ctx);
    }

    @Override
    public void channelUnregistered(ChannelHandlerContext ctx) throws Exception {
      accepted.remove(ctx.channel());
      if (LOG.isDebugEnabled()) {
        LOG.debug(serviceName + " closes a connection. The number of current connections are " + accepted.size());
      }
      super.channelUnregistered(ctx);
    }

    @Override
    protected void channelRead0(ChannelHandlerContext ctx, RpcRequest request) throws Exception {

      String methodName = request.getMethodName();
      MethodDescriptor methodDescriptor = service.getDescriptorForType().findMethodByName(methodName);

      if (methodDescriptor == null) {
        throw new RemoteCallException(request.getId(), new NoSuchMethodException(methodName));
      }
      Message paramProto = null;
      if (request.hasRequestMessage()) {
        try {
          paramProto = service.getRequestPrototype(methodDescriptor).newBuilderForType()
              .mergeFrom(request.getRequestMessage()).build();

        } catch (Throwable t) {
          throw new RemoteCallException(request.getId(), methodDescriptor, t);
        }
      }
      Message returnValue;
      RpcController controller = new NettyRpcController();

      try {
        returnValue = service.callBlockingMethod(methodDescriptor, controller, paramProto);
      } catch (Throwable t) {
        throw new RemoteCallException(request.getId(), methodDescriptor, t);
      }

      RpcResponse.Builder builder = RpcResponse.newBuilder().setId(request.getId());

      if (returnValue != null) {
        builder.setResponseMessage(returnValue.toByteString());
      }

      if (controller.failed()) {
        builder.setErrorMessage(controller.errorText());
      }
      ctx.writeAndFlush(builder.build());
    }

    @Override
    public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) {
      if (cause instanceof RemoteCallException) {
        RemoteCallException callException = (RemoteCallException) cause;
        ctx.writeAndFlush(callException.getResponse());
      }
    }
  }
}