Java Dubbo协议下的服务端线程使用详解

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2023-04-06
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标签   Dubbo
目录
  • Provider端线程模型
  • AllDispatcher
  • DirectDispatcher
  • ExecutionDispatcher
  • MessageOnlyDispatcher
  • ConnectionOrderedDispatcher

Provider端线程模型

在了解服务端线程模型之前，先了解一下Dubbo对Channel上的操作抽象，Dubbo将Channel上的操作成了5中行为，分别是：建立连接、断开连接、发送消息、接收消息、异常捕获，Channel上的操作的接口为org.apache.dubbo.remoting.ChannelHandler，该接口是SPI的，用户可以自己扩展，接口代码如下：

该接口抽象的五种Channel上的行为解释如下：

  • 建立连接：connected，主要是的职责是在channel记录read、write的时间，以及处理建立连接后的回调逻辑，比如dubbo支持在断开后自定义回调的hook（onconnect），即在该操作中执行。
  • 断开连接：disconnected，主要是的职责是在channel移除read、write的时间，以及处理端开连接后的回调逻辑，比如dubbo支持在断开后自定义回调的hook（ondisconnect），即在该操作中执行。
  • 发送消息：sent，包括发送请求和发送响应。记录write的时间。
  • 接收消息：received，包括接收请求和接收响应。记录read的时间。
  • 异常捕获：caught，用于处理在channel上发生的各类异常。

Dubbo框架的线程模型与以上这五种行为息息相关，Dubbo协议Provider端线程模型提供了五种实现，虽说都是五种但是别把二者混淆，线程模型的顶级接口是org.apache.dubbo.remoting.Dispatcher，该接口也是SPI的，提供的五种实现分别是AllDispatcher、DirectDispatcher、MessageOnlyDispatcher、ExecutionDispatcher、ConnectionOrderedDispatcher，默认的使用的是AllDispatcher。

org.apache.dubbo.remoting.ChannelHandler作为Channel上的行为的顶级接口对应Dubbo协议Provider端的5种线程模型同样也提供了对应的5种实现，分别是AllChannelHandler、DirectChannelHandler、MessageOnlyChannelHandler、ExecutionChannelHandler、ConnectionOrderedChannelHandler，这里Channel上行为的具体实现不展开讨论。

Channel上行为和线程模型之间使用策略可以参考org.apache.dubbo.remoting.transport.dispatcher.ChannelHandlers的源代码，这里不做详细的介绍，下面的各个章节只针对5种线程模型做简单的介绍。

AllDispatcher

IO线程上的操作：

  • 接口响应序列化
  • sent操作

Dubbo线程池上的操作：

  • received、connected、disconnected、caught都是在Dubbo线程池上执行
  • 服务端反序列化操作的Dubbo线程池上执行

AllDispatcher代码如下，AllDispatcher的dispatch方法实例化了AllChannelHandler，AllChannelHandler实现了received、connected、disconnected、caught操作在dubbo线程池中，代码如下：

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 = getSharedExecutorService();
        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 = getSharedExecutorService();
        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 {
            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 = getSharedExecutorService();
        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);
        }
    }
}

DirectDispatcher

该线程模型Channel上的所有行为均在IO线程中执行，并没有在Dubbo线程池中执行

DirectDispatcher与AllDispatcher相似，实例化了DirectChannelHandler，DirectChannelHandler只实现了received行为，但是received中获取的线程池如果是ThreadlessExecutor才会提交task，否则也是在ChannelHandler中执行received行为，ThreadlessExecutor和普通线程池最大的区别是不会管理任何线程，这里不展开讨论。

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);
            }
        } else {
            handler.received(channel, message);
        }
    }
}

ExecutionDispatcher

在IO线程中执行的操作有：

  • sent、connected、disconnected、caught操作在IO线程上执行。
  • 序列化响应在IO线程上执行。

在Dubbo线程中执行的操作有：

  • received都是在Dubbo线程上执行的。
  • 反序列化请求的行为在Dubbo中做的。

同样的，我们可以直接看ExecutionChannelHandler源码，逻辑是当message的类型是Request时received行为在Dubbo线程池执行。感兴趣的可以自己看源码，这里不做介绍。

MessageOnlyDispatcher

Message Only Dispatcher所有的received行为和反序列化都是在dubbo线程池中执行的

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

ConnectionOrderedDispatcher

该线程模型与AllDispatcher类似，sent操作和相应的序列化是在IO线程上执行；connected、disconnected、received、caught操作在dubbo线程池上执行，他们的区别是在connected、disconnected行为上ConnectionOrderedDispatcher做了线程池隔离，并且在Dubbo connected thread pool中提供了链接限制、告警灯能力，我们直接看ConnectionOrderedChannelHandler源码，代码如下：

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);
        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 {
            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 {
            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 {
            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 = getSharedExecutorService();
        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));
        }
    }
}
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