Rocketmq源码解读之消息拉取

最近阅读了Rocketmq关于pullmessage的实现方式，分享出来

众所周知，Rocketmq在consumer端是拉取消息的方式，它会在客户端维护一个PullRequestQueue，这个是一个阻塞队列（LinkedBlockingQueue），内部的节点是PullRequest，每一个PullRequest代表了一个消费的分组单元

PullRequest会记录一个topic对应的consumerGroup的拉取进度，包括MessageQueue和PorcessQueue，还有拉取的offset

（代码片段一）
public class PullRequest {
    private String consumerGroup;
    private MessageQueue messageQueue;
    private ProcessQueue processQueue;
    private long nextOffset;
    private boolean lockedFirst = false;
}

其中MessageQueue记录元信息：

（代码片段二）
public class MessageQueue implements Comparable<MessageQueue>, Serializable {
    private String topic;
    private String brokerName;
    private int queueId;
}

PorcessQueue记录一次拉取之后实际消息体和拉取相关操作记录的快照

（代码片段三）
public class ProcessQueue {
    private final ReadWriteLock lockTreeMap = new ReentrantReadWriteLock();
    private final TreeMap<Long, MessageExt> msgTreeMap = new TreeMap<Long, MessageExt>();
    private final AtomicLong msgCount = new AtomicLong();
    private final AtomicLong msgSize = new AtomicLong();
    private final Lock lockConsume = new ReentrantLock();
    private final TreeMap<Long, MessageExt> consumingMsgOrderlyTreeMap = new TreeMap<Long, MessageExt>();
    private final AtomicLong tryUnlockTimes = new AtomicLong(0);
    private volatile long queueOffsetMax = 0L;
    private volatile boolean dropped = false;
    private volatile long lastPullTimestamp = System.currentTimeMillis();
    private volatile long lastConsumeTimestamp = System.currentTimeMillis();
    private volatile boolean locked = false;
    private volatile long lastLockTimestamp = System.currentTimeMillis();
    private volatile boolean consuming = false;
    private volatile long msgAccCnt = 0;
}

PullMessageService负责轮询PullRequestQueue，并进行消息元的拉取

（代码片段四）
public class PullMessageService extends ServiceThread {
    private final InternalLogger log = ClientLogger.getLog();
    private final LinkedBlockingQueue<PullRequest> pullRequestQueue = new LinkedBlockingQueue<PullRequest>();
    private final MQClientInstance mQClientFactory;
    @Override
    public void run() {
        while (!this.isStopped()) {
            try {
                PullRequest pullRequest = this.pullRequestQueue.take();
                this.pullMessage(pullRequest);
            } catch (InterruptedException ignored) {
            } catch (Exception e) {
                log.error("Pull Message Service Run Method exception", e);
            }
        }
    }
}

在发送的时候会维护一个PullCallback，这是拉取收到消息后的回调处理

（代码片段五）
public interface PullCallback {
    void onSuccess(final PullResult pullResult);
    void onException(final Throwable e);
}

这里的实现逻辑就不贴了，本质上就是把消息丢给消费线程池来处理

pullMessage分为同步拉取和异步拉取两种模式，先解读异步拉取，然后再解读同步拉取，再说明两者的区别

其实从这里已经大概可以看出来，异步的方式，这个方法返回值是null，同步的方式必须要返回PullResult，后续说明区别

（代码片段六）
public PullResult pullMessage(
        final String addr,
        final PullMessageRequestHeader requestHeader,
        final long timeoutMillis,
        final CommunicationMode communicationMode,
        final PullCallback pullCallback
    ) throws RemotingException, MQBrokerException, InterruptedException {
        RemotingCommand request = RemotingCommand.createRequestCommand(RequestCode.PULL_MESSAGE, requestHeader);
        switch (communicationMode) {
            case ONEWAY:
                assert false;
                return null;
            case ASYNC:
                this.pullMessageAsync(addr, request, timeoutMillis, pullCallback);
                return null;
            case SYNC:
                return this.pullMessageSync(addr, request, timeoutMillis);
            default:
                assert false;
                break;
        }

        return null;
    }

先介绍异步拉取

可以看到，把PullCallback传进去，并封装了InvokeCallback，

（代码片段七）
private void pullMessageAsync(
        final String addr,
        final RemotingCommand request,
        final long timeoutMillis,
        final PullCallback pullCallback
    ) throws RemotingException, InterruptedException {
        this.remotingClient.invokeAsync(addr, request, timeoutMillis, new InvokeCallback() {
            @Override
            public void operationComplete(ResponseFuture responseFuture) {
                RemotingCommand response = responseFuture.getResponseCommand();
                if (response != null) {
                    try {
                        PullResult pullResult = MQClientAPIImpl.this.processPullResponse(response);
                        assert pullResult != null;
                        pullCallback.onSuccess(pullResult);
                    } catch (Exception e) {
                        pullCallback.onException(e);
                    }
                } else {
                    if (!responseFuture.isSendRequestOK()) {
                        pullCallback.onException(new MQClientException("send request failed to " + addr + ". Request: " + request, responseFuture.getCause()));
                    } else if (responseFuture.isTimeout()) {
                        pullCallback.onException(new MQClientException("wait response from " + addr + " timeout :" + responseFuture.getTimeoutMillis() + "ms" + ". Request: " + request,
                                responseFuture.getCause()));
                    } else {
                        pullCallback.onException(new MQClientException("unknown reason. addr: " + addr + ", timeoutMillis: " + timeoutMillis + ". Request: " + request, responseFuture.getCause()));
                    }
                }
            }
        });
    }

接下来进入NettyRemotingAbstract这个类中，使用netty的Chanle发送

（代码片段八）
public void invokeAsyncImpl(final Channel channel, final RemotingCommand request, final long timeoutMillis,
        final InvokeCallback invokeCallback)
        throws InterruptedException, RemotingTooMuchRequestException, RemotingTimeoutException, RemotingSendRequestException {
        final int opaque = request.getOpaque();
        boolean acquired = this.semaphoreAsync.tryAcquire(timeoutMillis, TimeUnit.MILLISECONDS);
        if (acquired) {
            final SemaphoreReleaseOnlyOnce once = new SemaphoreReleaseOnlyOnce(this.semaphoreAsync);

            final ResponseFuture responseFuture = new ResponseFuture(opaque, timeoutMillis, invokeCallback, once);
            this.responseTable.put(opaque, responseFuture);
            try {
                channel.writeAndFlush(request).addListener(new ChannelFutureListener() {
                    @Override
                    public void operationComplete(ChannelFuture f) throws Exception {
                        if (f.isSuccess()) {
                            responseFuture.setSendRequestOK(true);
                            return;
                        } else {
                            responseFuture.setSendRequestOK(false);
                        }

                        responseFuture.putResponse(null);
                        responseTable.remove(opaque);
                        try {
                            executeInvokeCallback(responseFuture);
                        } catch (Throwable e) {
                            log.warn("excute callback in writeAndFlush addListener, and callback throw", e);
                        } finally {
                            responseFuture.release();
                        }

                        log.warn("send a request command to channel <{}> failed.", RemotingHelper.parseChannelRemoteAddr(channel));
                    }
                });
            } catch (Exception e) {
                responseFuture.release();
                log.warn("send a request command to channel <" + RemotingHelper.parseChannelRemoteAddr(channel) + "> Exception", e);
                throw new RemotingSendRequestException(RemotingHelper.parseChannelRemoteAddr(channel), e);
            }
        } else {
            if (timeoutMillis <= 0) {
                throw new RemotingTooMuchRequestException("invokeAsyncImpl invoke too fast");
            } else {
                String info =
                    String.format("invokeAsyncImpl tryAcquire semaphore timeout, %dms, waiting thread nums: %d semaphoreAsyncValue: %d",
                        timeoutMillis,
                        this.semaphoreAsync.getQueueLength(),
                        this.semaphoreAsync.availablePermits()
                    );
                log.warn(info);
                throw new RemotingTimeoutException(info);
            }
        }
    }

这里需要详细解读一下：

RemotingCommand是request和response的载体，先从request中取出opaque，这是一个自增的操作id，然后将传进来的opaque和invokeCallback封装成一个ResponseFuture，再put到一个叫

responseTable的map中，这个map是一个核心的map，维护着opaque和对应的ResponseFuture

    /**
     * This map caches all on-going requests.
     */
    protected final ConcurrentMap<Integer /* opaque */, ResponseFuture> responseTable =
        new ConcurrentHashMap<Integer, ResponseFuture>(256);

从注释中可以看出，它缓存着正在执行的request

再回到刚刚的（代码片段八）中，channel.writeAndFlush(request).addListener(new ChannelFutureListener(){...})，netty在writeAndFlush发送完之后会回调我们ChannelFutureListener的operationComplete方法：如果发送成功则responseFuture.setSendRequestOK(true); 并且就return了；如果发送失败，则从responseTable中移除，并且起一个异步线程执行responseFuture中的InvokeCallback，在（代码片段七）中，可以看到当responseFuture.isSendRequestOK()是false的时候，执行了onException，这里就不多介绍了。

那么此时发送的逻辑就全部结束了，整个过程没有任何的阻塞，当Broker收到拉取请求后，会按照queueOffset等信息封装好返回consumer端，

会经过NettyRemotingServer上注册的NettyServerHandler

（代码片段九）
class NettyServerHandler extends SimpleChannelInboundHandler<RemotingCommand> {
        @Override
        protected void channelRead0(ChannelHandlerContext ctx, RemotingCommand msg) throws Exception {
            processTunnelId(ctx, msg);
            processMessageReceived(ctx, msg);
        }

        public void processTunnelId(ChannelHandlerContext ctx, RemotingCommand msg) {
            if (nettyServerConfig.isValidateTunnelIdFromVtoaEnable()) {
                if (null != msg && msg.getType() == RemotingCommandType.REQUEST_COMMAND) {
                    Vtoa vtoa = tunnelTable.get(ctx.channel());
                    if (null == vtoa) {
                        vtoa = VpcTunnelUtils.getInstance().getTunnelID(ctx);
                        tunnelTable.put(ctx.channel(), vtoa);
                    }
                    msg.addExtField(VpcTunnelUtils.PROPERTY_VTOA_TUNNEL_ID, String.valueOf(vtoa.getVid()));
                }
            }
        }
    }

最终会调用到NettyRemotingAbstract的processResponseCommand，RemotingCommand中根据opaque从responseTable中获取ResponseFuture，然后同样也是执行callback，这样，就实现了整个pullmessage的异步模式

(代码片段十)
public void processResponseCommand(ChannelHandlerContext ctx, RemotingCommand cmd) {
        final int opaque = cmd.getOpaque();
        final ResponseFuture responseFuture = responseTable.get(opaque);
        if (responseFuture != null) {
            responseFuture.setResponseCommand(cmd);

            responseTable.remove(opaque);

            if (responseFuture.getInvokeCallback() != null) { //异步
                executeInvokeCallback(responseFuture); //执行回调
            } else { //同步
                responseFuture.putResponse(cmd); //为了解除阻塞
                responseFuture.release();
            }
        } else {
            log.warn("receive response, but not matched any request, " + RemotingHelper.parseChannelRemoteAddr(ctx.channel()));
            log.warn(cmd.toString());
        }
    }

我们再看下同步的方式是如何实现的

回顾下代码片段六，同步的方式是需要返回PullResult的，换句话说，这种方式是需要在发送的线程中来处理返回结果的

我们从代码片段六跟下去，跟到NettyRemotingAbstract的invokeSyncImpl

（代码片段十一）
public RemotingCommand invokeSyncImpl(final Channel channel, final RemotingCommand request,
        final long timeoutMillis)
        throws InterruptedException, RemotingSendRequestException, RemotingTimeoutException {
        final int opaque = request.getOpaque();

        try {
            final ResponseFuture responseFuture = new ResponseFuture(opaque, timeoutMillis, null, null);
            this.responseTable.put(opaque, responseFuture);
            final SocketAddress addr = channel.remoteAddress();
            channel.writeAndFlush(request).addListener(new ChannelFutureListener() {
                @Override
                public void operationComplete(ChannelFuture f) throws Exception {
                    if (f.isSuccess()) {
                        responseFuture.setSendRequestOK(true);
                        return;
                    } else {
                        responseFuture.setSendRequestOK(false);
                    }

                    responseTable.remove(opaque);
                    responseFuture.setCause(f.cause());
                    responseFuture.putResponse(null);
                    log.warn("send a request command to channel <" + addr + "> failed.");
                }
            });

            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 {
            this.responseTable.remove(opaque);
        }
    }

和异步发送的代码片段八对比一下，可以看到，同步方式也要放到responseTable中，这里就有个疑惑了，既然都同步了，还要放到responseTable中干什么呢，继续往下看，

ChannelFutureListener都是一样的，如果发送成功就返回了，然后到了最关键的一行：

RemotingCommand responseCommand = responseFuture.waitResponse(timeoutMillis);

这一行顾名思义就是阻塞了，但是也不能一直阻塞住（因为PullMessageService是单线程的，如果因为一个异常就阻塞那就跪了），所以是一个设置了超时时间的阻塞，看下是如何阻塞的

 

ResponseFuture中有这两个方法，当putResponse的时候，把RemotingCommand赋值，并且countDownLatch.countDown，而在waitResponse的时候countDownLatch.await

（代码片段十二）
public RemotingCommand waitResponse(final long timeoutMillis) throws InterruptedException {
        this.countDownLatch.await(timeoutMillis, TimeUnit.MILLISECONDS);
        return this.responseCommand;
}

public void putResponse(final RemotingCommand responseCommand) {
        this.responseCommand = responseCommand;
        this.countDownLatch.countDown();
}

这样就清晰多了，剩下的疑问就是，在什么时候putResponse的，有两个地方：

第一个地方，当拉取消息回来的时候，回顾下代码片段十，有一句是（responseFuture.getInvokeCallback() != null），通过刚刚的流程已经知道，只有异步的时候invokeCallback才不为null，因此走到else，看到在这个时候responseFuture.putResponse(cmd)和responseFuture.release()，也就是说同步方式也是通过responseTable存储的方式，来获取结果，并且通过CountDownLatch来阻塞发送的线程，当收到消息之后再countDown，发送端最终返回PullResult来处理消息

第二个地方，回顾下代码片段十一，在ChannelFutureListener中当发送失败了以后，也会put一个null值：responseFuture.putResponse(null)，这里只是为了将阻塞放开

至此，Rockmq关于pullmessage的同步和异步方式就已经说明白了，总结一下，同步和异步本质上都是“异步”的，因为netty就是一个异步的框架，Rockmq只是利用了CountDownLatch来阻塞住发送端线程来实现了“同步”的效果，

通过一个responseTable来缓存住发送出去的请求，等收到的时候从这个缓存里按对应关系取出来，再去做对应的consumer线程的消息处理