rocketmq broker-消息保存流程分析

文章基於rocket-mq4.0 代碼分析

在Broker啓動類BrokerStartup啓動過程中調用BrokerController的initialize()方法

在該方法執行過程中會給不同的請求註冊不同的處理器

具體代碼：

SendMessageProcessor這個類就是消息保存的實際處理器，處理方法是 processRequest

start流程啓動後開啓調用監聽

org.apache.rocketmq.broker.BrokerController#start
↓
org.apache.rocketmq.remoting.netty.NettyRemotingServer#start
↓
//注入netty nio監聽處理器
       ServerBootstrap childHandler =
            this.serverBootstrap.group(this.eventLoopGroupBoss, this.eventLoopGroupSelector)
                .channel(useEpoll() ? EpollServerSocketChannel.class : NioServerSocketChannel.class)
                .option(ChannelOption.SO_BACKLOG, 1024)
                .option(ChannelOption.SO_REUSEADDR, true)
                .option(ChannelOption.SO_KEEPALIVE, false)
                .childOption(ChannelOption.TCP_NODELAY, true)
                .childOption(ChannelOption.SO_SNDBUF, nettyServerConfig.getServerSocketSndBufSize())
                .childOption(ChannelOption.SO_RCVBUF, nettyServerConfig.getServerSocketRcvBufSize())
                .localAddress(new InetSocketAddress(this.nettyServerConfig.getListenPort()))
                .childHandler(new ChannelInitializer<SocketChannel>() {
                    @Override
                    public void initChannel(SocketChannel ch) throws Exception {
                        ch.pipeline()
                            .addLast(defaultEventExecutorGroup, HANDSHAKE_HANDLER_NAME,
                                new HandshakeHandler(TlsSystemConfig.tlsMode))
                            .addLast(defaultEventExecutorGroup,
                                new NettyEncoder(),
                                new NettyDecoder(),
                                new IdleStateHandler(0, 0, nettyServerConfig.getServerChannelMaxIdleTimeSeconds()),
                                new NettyConnectManageHandler(),
                                new NettyServerHandler()
                            );
                    }
                });

class NettyServerHandler extends SimpleChannelInboundHandler<RemotingCommand>
 是處理繼承了消息接入的處理器，最終會把 REQUEST_COMMAND 類型的消息交給 SendMessageProcessor 處理

***********其實這裏rocketmq爲什麼會把這個Processor取名爲SendMessageProcessor我完全不能理解（爲什麼名字裏是Send?），對於broker來說這明明是一個接收消息的處理器不是發消息，難道取名字的時候是站在client角度的？不知道

主流程調用會到

    private RemotingCommand sendMessage(final ChannelHandlerContext ctx,
                                        final RemotingCommand request,
                                        final SendMessageContext sendMessageContext,
                                        final SendMessageRequestHeader requestHeader) throws RemotingCommandException {

這個方法會判斷是否事務消息而做不同的處理

普通消息和事務消息都會被封裝成  MessageExtBrokerInner 對象，最終都是調用 DefaultMessageStore的 putMessage方法處理消息；

public PutMessageResult putMessage(MessageExtBrokerInner msg) 

DefaultMessageStore又會調用CommitLog的

public PutMessageResult putMessage(final MessageExtBrokerInner msg)

方法保存消息；CommitLog裏維護了commitlog文件的引用隊列，程序會拿到最後一個commitlog文件然後向文件裏寫入數據

上圖中的 

result = mappedFile.appendMessage(msg, this.appendMessageCallback);

是一個將數據append到LogFile真實文件的內存映射中（MappedFile會回調CommitLog內部類DefaultAppendMessageCallback處理）並將結果包裝成一個 AppendMessageResult 對象

向文件映射中put

如果包裝成功會調用我們經常聽到的刷盤服務進行真正的落盤

以  GroupCommitService 爲例，核心代碼跟蹤到 

CommitLog.this.mappedFileQueue.flush(0);

MappedFileQueue

    public boolean flush(final int flushLeastPages) {
        boolean result = true;
        MappedFile mappedFile = this.findMappedFileByOffset(this.flushedWhere, false);
        if (mappedFile != null) {
            long tmpTimeStamp = mappedFile.getStoreTimestamp();
            int offset = mappedFile.flush(flushLeastPages);
            long where = mappedFile.getFileFromOffset() + offset;
            result = where == this.flushedWhere;
            this.flushedWhere = where;
            if (0 == flushLeastPages) {
                this.storeTimestamp = tmpTimeStamp;
            }
        }

        return result;
    }

然後調用 MappedFile

    public int flush(final int flushLeastPages) {
        if (this.isAbleToFlush(flushLeastPages)) {
            if (this.hold()) {
                int value = getReadPosition();

                try {
                    //We only append data to fileChannel or mappedByteBuffer, never both.
                    if (writeBuffer != null || this.fileChannel.position() != 0) {
                        this.fileChannel.force(false);
                    } else {
                        this.mappedByteBuffer.force();
                    }
                } catch (Throwable e) {
                    log.error("Error occurred when force data to disk.", e);
                }

                this.flushedPosition.set(value);
                this.release();
            } else {
                log.warn("in flush, hold failed, flush offset = " + this.flushedPosition.get());
                this.flushedPosition.set(getReadPosition());
            }
        }
        return this.getFlushedPosition();
    }

最終調用的是 FileChannel.force() 方法

該方法將通道里尚未寫入磁盤的數據強制寫到磁盤上，至此broker將接收到的消息寫到文件中流程結束