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; }