RocketMQ获取指定消息的实现方法（源码）

概要

消息查询是什么？

消息查询就是根据用户提供的msgId从MQ中取出该消息

RocketMQ如果有多个节点如何查询？

问题：RocketMQ分布式结构中，数据分散在各个节点，即便是同一Topic的数据，也未必都在一个broker上。客户端怎么知道数据该去哪个节点上查？

猜想1：逐个访问broker节点查询数据

猜想2：有某种数据中心存在，该中心知道所有消息存储的位置，只要向该中心查询即可得到消息具体位置，进而取得消息内容

实际：

1.消息Id中含有消息所在的broker的地址信息（IP\Port）以及该消息在CommitLog中的偏移量。

2.客户端实现会从msgId字符串中解析出broker地址，向指定broker节查询消息。

问题：CommitLog文件有多个，只有偏移量估计不能确定在哪个文件吧？

实际：单个Broker节点内offset是全局唯一的，不是每个CommitLog文件的偏移量都是从0开始的。单个节点内所有CommitLog文件共用一套偏移量，每个文件的文件名为其第一个消息的偏移量。所以可以根据偏移量和文件名确定CommitLog文件。

源码阅读

0.使用方式

MessageExt  msg = consumer.viewMessage(msgId);

1.消息ID解析

这个了解下就可以了

public class MessageId {
 private SocketAddress address;
 private long offset;

 public MessageId(SocketAddress address, long offset) {
  this.address = address;
  this.offset = offset;
 }

 //get-set
}

//from MQAdminImpl.java
public MessageExt viewMessage(
 String msgId) throws RemotingException, MQBrokerException, InterruptedException, MQClientException {

 MessageId messageId = null;
 try {
  //从msgId字符串中解析出address和offset
  //address = ip:port
  //offset为消息在CommitLog文件中的偏移量
  messageId = MessageDecoder.decodeMessageId(msgId);
 } catch (Exception e) {
  throw new MQClientException(ResponseCode.NO_MESSAGE, "query message by id finished, but no message.");
 }
 return this.mQClientFactory.getMQClientAPIImpl().viewMessage(RemotingUtil.socketAddress2String(messageId.getAddress()),
  messageId.getOffset(), timeoutMillis);
}

//from MessageDecoder.java
public static MessageId decodeMessageId(final String msgId) throws UnknownHostException {
 SocketAddress address;
 long offset;
 //ipv4和ipv6的区别
 //如果msgId总长度超过32字符，则为ipv6
 int ipLength = msgId.length() == 32 ? 4 * 2 : 16 * 2;

 byte[] ip = UtilAll.string2bytes(msgId.substring(0, ipLength));
 byte[] port = UtilAll.string2bytes(msgId.substring(ipLength, ipLength + 8));
 ByteBuffer bb = ByteBuffer.wrap(port);
 int portInt = bb.getInt(0);
 address = new InetSocketAddress(InetAddress.getByAddress(ip), portInt);

 // offset
 byte[] data = UtilAll.string2bytes(msgId.substring(ipLength + 8, ipLength + 8 + 16));
 bb = ByteBuffer.wrap(data);
 offset = bb.getLong(0);

 return new MessageId(address, offset);
}

2.长连接客户端RPC实现

要发请求首先得先建立连接，这里方法可以看到创建连接相关的操作。值得注意的是，第一次访问的时候可能连接还没建立，建立连接需要消耗一段时间。代码中对这个时间也做了判断，如果连接建立完成后，发现已经超时，则不再发出请求。目的应该是尽可能减少请求线程的阻塞时间。

//from NettyRemotingClient.java
@Override
public RemotingCommand invokeSync(String addr, final RemotingCommand request, long timeoutMillis)
 throws InterruptedException, RemotingConnectException, RemotingSendRequestException, RemotingTimeoutException {
 long beginStartTime = System.currentTimeMillis();
 //这里会先检查有无该地址的通道，有则返回，无则创建
 final Channel channel = this.getAndCreateChannel(addr);
 if (channel != null && channel.isActive()) {
  try {
   //前置钩子
   doBeforeRpcHooks(addr, request);
   //判断通道建立完成时是否已到达超时时间，如果超时直接抛出异常。不发请求
   long costTime = System.currentTimeMillis() - beginStartTime;
   if (timeoutMillis < costTime) {
    throw new RemotingTimeoutException("invokeSync call timeout");
   }
   //同步调用
   RemotingCommand response = this.invokeSyncImpl(channel, request, timeoutMillis - costTime);
   //后置钩子
   doAfterRpcHooks(RemotingHelper.parseChannelRemoteAddr(channel), request, response); //后置钩子
   return response;
  } catch (RemotingSendRequestException e) {
   log.warn("invokeSync: send request exception, so close the channel[{}]", addr);
   this.closeChannel(addr, channel);
   throw e;
  } catch (RemotingTimeoutException e) {
   if (nettyClientConfig.isClientCloseSocketIfTimeout()) {
    this.closeChannel(addr, channel);
    log.warn("invokeSync: close socket because of timeout, {}ms, {}", timeoutMillis, addr);
   }
   log.warn("invokeSync: wait response timeout exception, the channel[{}]", addr);
   throw e;
  }
 } else {
  this.closeChannel(addr, channel);
  throw new RemotingConnectException(addr);
 }
}

下一步看看它的同步调用做了什么处理。注意到它会构建一个Future对象加入待响应池，发出请求报文后就挂起线程，然后等待唤醒（waitResponse内部使用CountDownLatch等待）。

//from NettyRemotingAbstract.javapublic RemotingCommand invokeSyncImpl(final Channel channel, final RemotingCommand request,
 final long timeoutMillis)
 throws InterruptedException, RemotingSendRequestException, RemotingTimeoutException {
 //请求id
 final int opaque = request.getOpaque();

 try {
  //请求存根
  final ResponseFuture responseFuture = new ResponseFuture(channel, opaque, timeoutMillis, null, null);
  //加入待响应的请求池
  this.responseTable.put(opaque, responseFuture);
  final SocketAddress addr = channel.remoteAddress();
  //将请求发出，成功发出时更新状态
  channel.writeAndFlush(request).addListener(new ChannelFutureListener() {
   @Override
   public void operationComplete(ChannelFuture f) throws Exception {
    if (f.isSuccess()) { //若成功发出，更新请求状态为“已发出”
     responseFuture.setSendRequestOK(true);
     return;
    } else {
     responseFuture.setSendRequestOK(false);
    }

    //若发出失败，则从池中移除（没用了，释放资源）
    responseTable.remove(opaque);
    responseFuture.setCause(f.cause());
    //putResponse的时候会唤醒等待的线程
    responseFuture.putResponse(null);
    log.warn("send a request command to channel <" + addr + "> failed.");
   }
  });

  //只等待一段时间，不会一直等下去
  //若正常响应，则收到响应后，此线程会被唤醒，继续执行下去
  //若超时，则到达该时间后线程苏醒，继续执行
  RemotingCommand responseCommand = responseFuture.waitResponse(timeoutMillis);
  if (null == responseCommand) {
   if (responseFuture.isSendRequestOK()) {
    throw new RemotingTimeoutException(RemotingHelper.parseSocketAddressAddr(addr), timeoutMillis,
     responseFuture.getCause());
   } else {
    throw new RemotingSendRequestException(RemotingHelper.parseSocketAddressAddr(addr), responseFuture.getCause());
   }
  }

  return responseCommand;
 } finally {
  //正常响应完成时，将future释放（正常逻辑）
  //超时时，将future释放。这个请求已经作废了，后面如果再收到响应，就可以直接丢弃了（由于找不到相关的响应钩子，就不处理了）
  this.responseTable.remove(opaque);
 }
}

好，我们再来看看收到报文的时候是怎么处理的。我们都了解JDK中的Future的原理，大概就是将这个任务提交给其他线程处理，该线程处理完毕后会将结果写入到Future对象中，写入时如果有线程在等待该结果，则唤醒这些线程。这里也差不多，只不过执行线程在服务端，服务执行完毕后会将结果通过长连接发送给客户端，客户端收到后根据报文中的ID信息从待响应池中找到Future对象，然后就是类似的处理了。

class NettyClientHandler extends SimpleChannelInboundHandler<RemotingCommand> {

 //底层解码完毕得到RemotingCommand的报文
 @Override
 protected void channelRead0(ChannelHandlerContext ctx, RemotingCommand msg) throws Exception {
  processMessageReceived(ctx, msg);
 }
}

public void processMessageReceived(ChannelHandlerContext ctx, RemotingCommand msg) throws Exception {
 final RemotingCommand cmd = msg;
 if (cmd != null) {
  //判断类型
  switch (cmd.getType()) {
   case REQUEST_COMMAND:
    processRequestCommand(ctx, cmd);
    break;
   case RESPONSE_COMMAND:
    processResponseCommand(ctx, cmd);
    break;
   default:
    break;
  }
 }
}

public void processResponseCommand(ChannelHandlerContext ctx, RemotingCommand cmd) {
 //取得消息id
 final int opaque = cmd.getOpaque();
 //从待响应池中取得对应请求
 final ResponseFuture responseFuture = responseTable.get(opaque);
 if (responseFuture != null) {
  //将响应值注入到ResponseFuture对象中，等待线程可从这个对象获取结果
  responseFuture.setResponseCommand(cmd);
  //请求已处理完毕，释放该请求
  responseTable.remove(opaque);

  //如果有回调函数的话则回调（由当前线程处理）
  if (responseFuture.getInvokeCallback() != null) {
   executeInvokeCallback(responseFuture);
  } else {
   //没有的话，则唤醒等待线程（由等待线程做处理）
   responseFuture.putResponse(cmd);
   responseFuture.release();
  }
 } else {
  log.warn("receive response, but not matched any request, " + RemotingHelper.parseChannelRemoteAddr(ctx.channel()));
  log.warn(cmd.toString());
 }
}

总结一下，客户端的处理时序大概是这样的：

结构大概是这样的：

3.服务端的处理

//todo 服务端待补充CommitLog文件映射相关内容

class NettyServerHandler extends SimpleChannelInboundHandler<RemotingCommand> {

  @Override
  protected void channelRead0(ChannelHandlerContext ctx, RemotingCommand msg) throws Exception {
    processMessageReceived(ctx, msg);
  }
}

//from NettyRemotingAbscract.java
public void processMessageReceived(ChannelHandlerContext ctx, RemotingCommand msg) throws Exception {
  final RemotingCommand cmd = msg;
  if (cmd != null) {
    switch (cmd.getType()) {
      case REQUEST_COMMAND: //服务端走这里
        processRequestCommand(ctx, cmd);
        break;
      case RESPONSE_COMMAND:
        processResponseCommand(ctx, cmd);
        break;
      default:
        break;
    }
  }
}

//from NettyRemotingAbscract.java
public void processRequestCommand(final ChannelHandlerContext ctx, final RemotingCommand cmd) {
  //查看有无该请求code相关的处理器
  final Pair<NettyRequestProcessor, ExecutorService> matched = this.processorTable.get(cmd.getCode());
  //如果没有，则使用默认处理器（可能没有默认处理器）
  final Pair<NettyRequestProcessor, ExecutorService> pair = null == matched ? this.defaultRequestProcessor : matched;
  final int opaque = cmd.getOpaque();

  if (pair != null) {
    Runnable run = new Runnable() {
      @Override
      public void run() {
        try {
          doBeforeRpcHooks(RemotingHelper.parseChannelRemoteAddr(ctx.channel()), cmd);
          final RemotingResponseCallback callback = new RemotingResponseCallback() {
            @Override
            public void callback(RemotingCommand response) {
              doAfterRpcHooks(RemotingHelper.parseChannelRemoteAddr(ctx.channel()), cmd, response);
              if (!cmd.isOnewayRPC()) {
                if (response != null) { //不为null，则由本类将响应值写会给请求方
                  response.setOpaque(opaque);
                  response.markResponseType();
                  try {
                    ctx.writeAndFlush(response);
                  } catch (Throwable e) {
                    log.error("process request over, but response failed", e);
                    log.error(cmd.toString());
                    log.error(response.toString());
                  }
                } else { //为null，意味着processor内部已经将响应处理了，这里无需再处理。
                }
              }
            }
          };
          if (pair.getObject1() instanceof AsyncNettyRequestProcessor) {//QueryMessageProcessor为异步处理器
            AsyncNettyRequestProcessor processor = (AsyncNettyRequestProcessor)pair.getObject1();
            processor.asyncProcessRequest(ctx, cmd, callback);
          } else {
            NettyRequestProcessor processor = pair.getObject1();
            RemotingCommand response = processor.processRequest(ctx, cmd);
            doAfterRpcHooks(RemotingHelper.parseChannelRemoteAddr(ctx.channel()), cmd, response);
            callback.callback(response);
          }
        } catch (Throwable e) {
          log.error("process request exception", e);
          log.error(cmd.toString());

          if (!cmd.isOnewayRPC()) {
            final RemotingCommand response = RemotingCommand.createResponseCommand(RemotingSysResponseCode.SYSTEM_ERROR,
              RemotingHelper.exceptionSimpleDesc(e));
            response.setOpaque(opaque);
            ctx.writeAndFlush(response);
          }
        }
      }
    };

    if (pair.getObject1().rejectRequest()) {
      final RemotingCommand response = RemotingCommand.createResponseCommand(RemotingSysResponseCode.SYSTEM_BUSY,
        "[REJECTREQUEST]system busy, start flow control for a while");
      response.setOpaque(opaque);
      ctx.writeAndFlush(response);
      return;
    }

    try {
      final RequestTask requestTask = new RequestTask(run, ctx.channel(), cmd);
      pair.getObject2().submit(requestTask);
    } catch (RejectedExecutionException e) {
      if ((System.currentTimeMillis() % 10000) == 0) {
        log.warn(RemotingHelper.parseChannelRemoteAddr(ctx.channel())
          + ", too many requests and system thread pool busy, RejectedExecutionException "
          + pair.getObject2().toString()
          + " request code: " + cmd.getCode());
      }

      if (!cmd.isOnewayRPC()) {
        final RemotingCommand response = RemotingCommand.createResponseCommand(RemotingSysResponseCode.SYSTEM_BUSY,
          "[OVERLOAD]system busy, start flow control for a while");
        response.setOpaque(opaque);
        ctx.writeAndFlush(response);
      }
    }
  } else {
    String error = " request type " + cmd.getCode() + " not supported";
    final RemotingCommand response =
      RemotingCommand.createResponseCommand(RemotingSysResponseCode.REQUEST_CODE_NOT_SUPPORTED, error);
    response.setOpaque(opaque);
    ctx.writeAndFlush(response);
    log.error(RemotingHelper.parseChannelRemoteAddr(ctx.channel()) + error);
  }
}

//from QueryMessageProcesor.java
@Override
public RemotingCommand processRequest(ChannelHandlerContext ctx, RemotingCommand request)
  throws RemotingCommandException {
  switch (request.getCode()) {
    case RequestCode.QUERY_MESSAGE:
      return this.queryMessage(ctx, request);
    case RequestCode.VIEW_MESSAGE_BY_ID: //通过msgId查询消息
      return this.viewMessageById(ctx, request);
    default:
      break;
  }

  return null;
}

public RemotingCommand viewMessageById(ChannelHandlerContext ctx, RemotingCommand request)
  throws RemotingCommandException {
  final RemotingCommand response = RemotingCommand.createResponseCommand(null);
  final ViewMessageRequestHeader requestHeader =
    (ViewMessageRequestHeader) request.decodeCommandCustomHeader(ViewMessageRequestHeader.class);

  response.setOpaque(request.getOpaque());

  //getMessagetStore得到当前映射到内存中的CommitLog文件，然后根据偏移量取得数据
  final SelectMappedBufferResult selectMappedBufferResult =
    this.brokerController.getMessageStore().selectOneMessageByOffset(requestHeader.getOffset());
  if (selectMappedBufferResult != null) {
    response.setCode(ResponseCode.SUCCESS);
    response.setRemark(null);

    //将响应通过socket写回给客户端
    try {
      //response对象的数据作为header
      //消息内容作为body
      FileRegion fileRegion =
        new OneMessageTransfer(response.encodeHeader(selectMappedBufferResult.getSize()),
          selectMappedBufferResult);
      ctx.channel().writeAndFlush(fileRegion).addListener(new ChannelFutureListener() {
        @Override
        public void operationComplete(ChannelFuture future) throws Exception {
          selectMappedBufferResult.release();
          if (!future.isSuccess()) {
            log.error("Transfer one message from page cache failed, ", future.cause());
          }
        }
      });
    } catch (Throwable e) {
      log.error("", e);
      selectMappedBufferResult.release();
    }

    return null; //如果有值，则直接写回给请求方。这里返回null是不需要由外层处理响应。
  } else {
    response.setCode(ResponseCode.SYSTEM_ERROR);
    response.setRemark("can not find message by the offset, " + requestHeader.getOffset());
  }

  return response;
}

总结

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