RocketMQ之连接以及连接缓存

 发现rabbitmq有一个ConnectionFactory。发现rocketmq好像没这个东西。按道理来说如果每次发送消息都新建一条连接肯定是不可能的。

ps:其实之所以是有上面的疑问是因为数据库连接池那个地方来的，因为数据库连接connection并没有说是线程安全的，所以为了线程安全会为每个事物单独分配一个连接。但是rocketmq用的是netty的长连接channel，Java 上关于SocketChannel 的注释 说明是安全的。

《Netty : writeAndFlush的线程安全及并发问题》

可见，writeAndFlush如果在Netty线程池内执行，则是直接write；否则，将作为一个task插入到Netty线程池执行。

这个问题归根到底是netty的问题。既然是线程安全的，那么整个系统就可以只用一个连接了。

所以我们从源码角度来分析一下rocketmq是如何做得。

下面我们追踪一条消息发送的过程，下面是发送消息时涉及io的模块。

rocketmq就是在channelTables里面维护的连接。

    private Channel getAndCreateNameserverChannel() throws InterruptedException {
        String addr = this.namesrvAddrChoosed.get();
        if (addr != null) {
            ChannelWrapper cw = this.channelTables.get(addr);
            if (cw != null && cw.isOK()) {
                return cw.getChannel();
            }
        }

        final List<String> addrList = this.namesrvAddrList.get();
        if (this.lockNamesrvChannel.tryLock(LockTimeoutMillis, TimeUnit.MILLISECONDS)) {
            try {
                addr = this.namesrvAddrChoosed.get();
                if (addr != null) {
                    ChannelWrapper cw = this.channelTables.get(addr);
                    if (cw != null && cw.isOK()) {
                        return cw.getChannel();
                    }
                }

                if (addrList != null && !addrList.isEmpty()) {
                    for (int i = 0; i < addrList.size(); i++) {
                        int index = this.namesrvIndex.incrementAndGet();
                        index = Math.abs(index);
                        index = index % addrList.size();
                        String newAddr = addrList.get(index);

                        this.namesrvAddrChoosed.set(newAddr);
                        Channel channelNew = this.createChannel(newAddr);
                        if (channelNew != null)
                            return channelNew;
                    }
                }
            } catch (Exception e) {
                log.error("getAndCreateNameserverChannel: create name server channel exception", e);
            } finally {
                this.lockNamesrvChannel.unlock();
            }
        } else {
            log.warn("getAndCreateNameserverChannel: try to lock name server, but timeout, {}ms", LockTimeoutMillis);
        }

        return null;
    }

    private Channel createChannel(final String addr) throws InterruptedException {
        ChannelWrapper cw = this.channelTables.get(addr);
        if (cw != null && cw.isOK()) {
            return cw.getChannel();
        }

        //lockChannelTables是ReentrantLock锁。tryLock()  尝试获取锁，不管成功失败，都立即返回true、false
        if (this.lockChannelTables.tryLock(LockTimeoutMillis, TimeUnit.MILLISECONDS)) {
            try {
                boolean createNewConnection = false;
                cw = this.channelTables.get(addr);
                if (cw != null) {

                    if (cw.isOK()) {
                        return cw.getChannel();
                    }

                    else if (!cw.getChannelFuture().isDone()) {
                        createNewConnection = false;
                    }

                    else {
                        this.channelTables.remove(addr);
                        createNewConnection = true;
                    }
                }

                else {
                    createNewConnection = true;
                }

                if (createNewConnection) {
                    ChannelFuture channelFuture = this.bootstrap.connect(RemotingHelper.string2SocketAddress(addr));
                    log.info("createChannel: begin to connect remote host[{}] asynchronously", addr);
                    cw = new ChannelWrapper(channelFuture);
　　　　　　　　　　　　//连接被保存到channelTables中。
                    this.channelTables.put(addr, cw);
                }
            } catch (Exception e) {
                log.error("createChannel: create channel exception", e);
            } finally {
                this.lockChannelTables.unlock();
            }
        } else {
            log.warn("createChannel: try to lock channel table, but timeout, {}ms", LockTimeoutMillis);
        }

        if (cw != null) {
            ChannelFuture channelFuture = cw.getChannelFuture();
            if (channelFuture.awaitUninterruptibly(this.nettyClientConfig.getConnectTimeoutMillis())) {
                if (cw.isOK()) {
                    log.info("createChannel: connect remote host[{}] success, {}", addr, channelFuture.toString());
                    return cw.getChannel();
                } else {
                    log.warn("createChannel: connect remote host[" + addr + "] failed, " + channelFuture.toString(), channelFuture.cause());
                }
            } else {
                log.warn("createChannel: connect remote host[{}] timeout {}ms, {}", addr, this.nettyClientConfig.getConnectTimeoutMillis(),
                        channelFuture.toString());
            }
        }

        return null;
    }

    /**
     * @see {@link #connect()}
     */
    private ChannelFuture doConnect(final SocketAddress remoteAddress, final SocketAddress localAddress) {
        final ChannelFuture regFuture = initAndRegister();
        final Channel channel = regFuture.channel();
        if (regFuture.cause() != null) {
            return regFuture;
        }

        final ChannelPromise promise = channel.newPromise();
        if (regFuture.isDone()) {
            doConnect0(regFuture, channel, remoteAddress, localAddress, promise);
        } else {
            regFuture.addListener(new ChannelFutureListener() {
                @Override
                public void operationComplete(ChannelFuture future) throws Exception {
                    doConnect0(regFuture, channel, remoteAddress, localAddress, promise);
                }
            });
        }

        return promise;
    }

    private static void doConnect0(
            final ChannelFuture regFuture, final Channel channel,
            final SocketAddress remoteAddress, final SocketAddress localAddress, final ChannelPromise promise) {

        // This method is invoked before channelRegistered() is triggered.  Give user handlers a chance to set up
        // the pipeline in its channelRegistered() implementation.
        channel.eventLoop().execute(new OneTimeTask() {
            @Override
            public void run() {
                if (regFuture.isSuccess()) {
                    if (localAddress == null) {
　　　　　　　　　　　　　　//io.netty.channel.connect(remoteAddress,promise);
                        channel.connect(remoteAddress, promise);
                    } else {
                        channel.connect(remoteAddress, localAddress, promise);
                    }
                    promise.addListener(ChannelFutureListener.CLOSE_ON_FAILURE);
                } else {
                    promise.setFailure(regFuture.cause());
                }
            }
        });
    }

连接这块就点到netty为止，进一步了解可以参考netty这方面的资料。

下面分析rocketmq对channelTables的维护。

 1，关闭系统时清空：

2.关闭channel时