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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