[网络协议]Dubbo源码解析-服务提供者线程模型分析

前言：

通过之前对provider启动过程的学习，我们知道，提供者默认是以Netty来启动对应协议端口来提供服务的。

Netty的标准启动模式下有两个线程组：boss和work线程组。

在接收到具体的请求后，如果服务提供者对该请求处理时间比较短，那么直接在work线程上处理即可；

如果服务提供者对该请求处理时间比较长，那么如果还在work线程上处理，则会阻塞其他请求的处理，降低了整个provider的处理能力。

如何解决这种问题呢？

可以通过自定义一个线程池，将work线程接收到的请求，交由该线程池来处理，这样就避免了请求都阻塞在work线程上。

基于这种解决方案，Dubbo提供了多种线程模式。我们一起来看下。

1.创建线程模型入口

先来分析下线程模型创建的入口。

还是从NettyServer（Netty4下的）的创建开始

public class NettyServer extends AbstractServer implements RemotingServer {
    public NettyServer(URL url, ChannelHandler handler) throws RemotingException {
        // 具体创建在ChannelHandlers.wrap()中
        super(ExecutorUtil.setThreadName(url, SERVER_THREAD_POOL_NAME), ChannelHandlers.wrap(handler, url));
    }
}

1.1 ChannelHandlers.wrap()

public class ChannelHandlers {
    private static ChannelHandlers INSTANCE = new ChannelHandlers();

    public static ChannelHandler wrap(ChannelHandler handler, URL url) {
        return ChannelHandlers.getInstance().wrapInternal(handler, url);
    }

    protected ChannelHandler wrapInternal(ChannelHandler handler, URL url) {
        // 本质上最终还是调用到这
        return new MultiMessageHandler(new HeartbeatHandler(ExtensionLoader.getExtensionLoader(Dispatcher.class)
                .getAdaptiveExtension().dispatch(handler, url)));
    }
}

从NettyServer的构造方法中可以看到，传入的Handler通过不断的复合，最终以MultiMessageHandler --> HeartbeatHandler --> （Dispatcher生成的Handler） 这种顺序来处理请求。

所以，线程模式的创建是在NettyServer创建时就已经完成的。

2.任务执行线程模型

线程模式本身通过SPI的方式来获取Dispatcher接口的实现类。具体有以下实现

下面具体来分析下每种Dispatcher的不同之处。

2.1 AllDispatcher（默认）

当前类型Dispatcher，则会把接收到的所有请求（请求、响应、连接心跳等事件）都交由自定义的线程池来处理。

public class AllDispatcher implements Dispatcher {
    public static final String NAME = "all";
    @Override
    public ChannelHandler dispatch(ChannelHandler handler, URL url) {
        return new AllChannelHandler(handler, url);
    }
}

public class AllChannelHandler extends WrappedChannelHandler {
    public AllChannelHandler(ChannelHandler handler, URL url) {
        super(handler, url);
    }

    @Override
    public void connected(Channel channel) throws RemotingException {
        ExecutorService executor = getExecutorService();
        try {
            executor.execute(new ChannelEventRunnable(channel, handler, ChannelState.CONNECTED));
        } catch (Throwable t) {
            throw new ExecutionException("connect event", channel, getClass() + " error when process connected event .", t);
        }
    }

    @Override
    public void disconnected(Channel channel) throws RemotingException {
        ExecutorService executor = getExecutorService();
        try {
            executor.execute(new ChannelEventRunnable(channel, handler, ChannelState.DISCONNECTED));
        } catch (Throwable t) {
            throw new ExecutionException("disconnect event", channel, getClass() + " error when process disconnected event .", t);
        }
    }

    @Override
    public void received(Channel channel, Object message) throws RemotingException {
        ExecutorService executor = getPreferredExecutorService(message);
        try {
            // 在接收到请求时，直接交由executor线程池处理
            executor.execute(new ChannelEventRunnable(channel, handler, ChannelState.RECEIVED, message));
        } catch (Throwable t) {
        	if(message instanceof Request && t instanceof RejectedExecutionException){
                sendFeedback(channel, (Request) message, t);
                return;
        	}
            throw new ExecutionException(message, channel, getClass() + " error when process received event .", t);
        }
    }

    @Override
    public void caught(Channel channel, Throwable exception) throws RemotingException {
        ExecutorService executor = getExecutorService();
        try {
            executor.execute(new ChannelEventRunnable(channel, handler, ChannelState.CAUGHT, exception));
        } catch (Throwable t) {
            throw new ExecutionException("caught event", channel, getClass() + " error when process caught event .", t);
        }
    }
}

通过源码可知：针对以上覆写的几种请求类型，全部交由自定义线程池来执行。

2.2 DirectDispatcher

所有的请求都直接在work线程上执行，不交由自定义线程池来处理。

public class DirectDispatcher implements Dispatcher {
    public static final String NAME = "direct";
    @Override
    public ChannelHandler dispatch(ChannelHandler handler, URL url) {
        return new DirectChannelHandler(handler, url);
    }
}

public class DirectChannelHandler extends WrappedChannelHandler {

    public DirectChannelHandler(ChannelHandler handler, URL url) {
        super(handler, url);
    }

    @Override
    public void received(Channel channel, Object message) throws RemotingException {
        ExecutorService executor = getPreferredExecutorService(message);
        if (executor instanceof ThreadlessExecutor) {
            try {
                executor.execute(new ChannelEventRunnable(channel, handler, ChannelState.RECEIVED, message));
            } catch (Throwable t) {
                throw new ExecutionException(message, channel, getClass() + " error when process received event .", t);
            }
            // 直接在当前线程执行handler操作
        } else {
            handler.received(channel, message);
        }
    }
}

2.3 MessageOnlyDispatcher

只有请求响应消息派发到自定义线程池，其他的连接、心跳等直接在work线程上执行

public class MessageOnlyDispatcher implements Dispatcher {
    public static final String NAME = "message";
    @Override
    public ChannelHandler dispatch(ChannelHandler handler, URL url) {
        return new MessageOnlyChannelHandler(handler, url);
    }
}

public class MessageOnlyChannelHandler extends WrappedChannelHandler {
    @Override
    public void received(Channel channel, Object message) throws RemotingException {
        ExecutorService executor = getPreferredExecutorService(message);
        try {
            executor.execute(new ChannelEventRunnable(channel, handler, ChannelState.RECEIVED, message));
        } catch (Throwable t) {
            if(message instanceof Request && t instanceof RejectedExecutionException){
                sendFeedback(channel, (Request) message, t);
                return;
            }
            throw new ExecutionException(message, channel, getClass() + " error when process received event .", t);
        }
    }

}

只重写了received方法，其他的connect等方法直接在当前work线程上执行Handler相关方法

2.4 ExecutionDispatcher

只有请求类型消息交由自定义线程池执行，其他类型消息都在work线程上执行

public class ExecutionDispatcher implements Dispatcher {
    public static final String NAME = "execution";
    @Override
    public ChannelHandler dispatch(ChannelHandler handler, URL url) {
        return new ExecutionChannelHandler(handler, url);
    }
}

public class ExecutionChannelHandler extends WrappedChannelHandler {

    public ExecutionChannelHandler(ChannelHandler handler, URL url) {
        super(handler, url);
    }

    @Override
    public void received(Channel channel, Object message) throws RemotingException {
        ExecutorService executor = getPreferredExecutorService(message);

        // 这里会过滤消息类型，只有request请求才会交由自定义线程池执行
        if (message instanceof Request) {
            try {
                executor.execute(new ChannelEventRunnable(channel, handler, ChannelState.RECEIVED, message));
            } catch (Throwable t) {
                if (t instanceof RejectedExecutionException) {
                    sendFeedback(channel, (Request) message, t);
                }
                throw new ExecutionException(message, channel, getClass() + " error when process received event.", t);
            }
        } else if (executor instanceof ThreadlessExecutor) {
            executor.execute(new ChannelEventRunnable(channel, handler, ChannelState.RECEIVED, message));
        } else {
            handler.received(channel, message);
        }
    }
}

2.5 ConnectionOrderedDispatcher

连接、断开事件放入队列中，在work线程上逐个执行，而其他类型的消息（请求、响应）则交由自定义业务线程池来执行

public class ConnectionOrderedDispatcher implements Dispatcher {
    public static final String NAME = "connection";
    @Override
    public ChannelHandler dispatch(ChannelHandler handler, URL url) {
        return new ConnectionOrderedChannelHandler(handler, url);
    }
}

public class ConnectionOrderedChannelHandler extends WrappedChannelHandler {

    protected final ThreadPoolExecutor connectionExecutor;
    private final int queuewarninglimit;

    public ConnectionOrderedChannelHandler(ChannelHandler handler, URL url) {
        super(handler, url);
        String threadName = url.getParameter(THREAD_NAME_KEY, DEFAULT_THREAD_NAME);
        // 定义一个单线程的线程池，后续为执行连接、断开事件做准备
        connectionExecutor = new ThreadPoolExecutor(1, 1,
                0L, TimeUnit.MILLISECONDS,
                new LinkedBlockingQueue<Runnable>(url.getPositiveParameter(CONNECT_QUEUE_CAPACITY, Integer.MAX_VALUE)),
                new NamedThreadFactory(threadName, true),
                new AbortPolicyWithReport(threadName, url)
        );  // FIXME There's no place to release connectionExecutor!
        queuewarninglimit = url.getParameter(CONNECT_QUEUE_WARNING_SIZE, DEFAULT_CONNECT_QUEUE_WARNING_SIZE);
    }

    @Override
    public void connected(Channel channel) throws RemotingException {
        try {
            // 连接事件被单线程池执行
            checkQueueLength();
            connectionExecutor.execute(new ChannelEventRunnable(channel, handler, ChannelState.CONNECTED));
        } catch (Throwable t) {
            throw new ExecutionException("connect event", channel, getClass() + " error when process connected event .", t);
        }
    }

    @Override
    public void disconnected(Channel channel) throws RemotingException {
        try {
            // 连接事件被单线程池执行
            checkQueueLength();
            connectionExecutor.execute(new ChannelEventRunnable(channel, handler, ChannelState.DISCONNECTED));
        } catch (Throwable t) {
            throw new ExecutionException("disconnected event", channel, getClass() + " error when process disconnected event .", t);
        }
    }

    @Override
    public void received(Channel channel, Object message) throws RemotingException {
        ExecutorService executor = getPreferredExecutorService(message);
        try {
            // 请求响应信息则交由自定义线程池执行
            executor.execute(new ChannelEventRunnable(channel, handler, ChannelState.RECEIVED, message));
        } catch (Throwable t) {
            if (message instanceof Request && t instanceof RejectedExecutionException) {
                sendFeedback(channel, (Request) message, t);
                return;
            }
            throw new ExecutionException(message, channel, getClass() + " error when process received event .", t);
        }
    }

    @Override
    public void caught(Channel channel, Throwable exception) throws RemotingException {
        ExecutorService executor = getExecutorService();
        try {
            executor.execute(new ChannelEventRunnable(channel, handler, ChannelState.CAUGHT, exception));
        } catch (Throwable t) {
            throw new ExecutionException("caught event", channel, getClass() + " error when process caught event .", t);
        }
    }

    private void checkQueueLength() {
        if (connectionExecutor.getQueue().size() > queuewarninglimit) {
            logger.warn(new IllegalThreadStateException("connectionordered channel handler `queue size: " + connectionExecutor.getQueue().size() + " exceed the warning limit number :" + queuewarninglimit));
        }
    }
}

总结：

Dubbo的分层实在太细了，每层之间也是强解耦的。用户可以随时自定义相关Dispatcher来选择不同的执行请求方案。

下一篇我们再来分析下Dubbo线程池的模型。