【源码】RocketMQ如何实现获取指定消息

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.java
public 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;
}