阅读netty pipeline文档
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netty/netty-doc.md
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# netty document
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## ChannelPipeline
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I/O Request
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via Channel or
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ChannelHandlerContext
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+---------------------------------------------------+---------------+
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| ChannelPipeline | |
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| \|/ |
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| +---------------------+ +-----------+----------+ |
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| | Inbound Handler N | | Outbound Handler 1 | |
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| +----------+----------+ +-----------+----------+ |
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| /|\ | |
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| | \|/ |
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| +----------+----------+ +-----------+----------+ |
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| | Inbound Handler N-1 | | Outbound Handler 2 | |
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| +----------+----------+ +-----------+----------+ |
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| /|\ . |
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| . . |
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| ChannelHandlerContext.fireIN_EVT() ChannelHandlerContext.OUT_EVT()|
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| [ method call] [method call] |
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| . . |
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| . \|/ |
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| +----------+----------+ +-----------+----------+ |
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| | Inbound Handler 2 | | Outbound Handler M-1 | |
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| +----------+----------+ +-----------+----------+ |
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| /|\ | |
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| | \|/ |
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| +----------+----------+ +-----------+----------+ |
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| | Inbound Handler 1 | | Outbound Handler M | |
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| +----------+----------+ +-----------+----------+ |
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| /|\ | |
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+---------------+-----------------------------------+---------------+
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| \|/
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+---------------+-----------------------------------+---------------+
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| | | |
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| [ Socket.read() ] [ Socket.write() ] |
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| |
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| Netty Internal I/O Threads (Transport Implementation) |
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+-------------------------------------------------------------------+
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如上图所示，i/o event将会被`ChannelInboundHandler`或`ChannelOutboundHandler`处理，并且通过调用`ChannelHandlerContext`中的event propagation method（触发事件传播的方法）来进行转发。
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常见的event propagation method如下，例如`ChannelHandlerContext.write(Object)`。
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### inbound event
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inbound event会通过inbound handler自底向上的进行处理，例如上图的左边部分所示。inbound handler通常处理由io thread产生的inbound data，inbound则是通常从remote peer读取。
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> 如果inbound event传播到高于top inbound handler，那么该event将会被丢弃。
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### outbound event
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outbound event会按照自顶向下的顺序被outbound进行处理，outbound handler通常产生outbound traffic或对outbound traffic进行处理。
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> 如果outbound event传播到低于bottom outbound handler, 其会直接被channel关联的io线程处理。
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> 通常，io thread会执行实际的输出操作，例如`SocketChannel.write(ByteBuffer)`。
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### pipeline处理顺序
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例如，按照如下顺序项pipeline中添加handler时，
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```java
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ChannelPipeline p = ...;
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p.addLast("1", new InboundHandlerA());
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p.addLast("2", new InboundHandlerB());
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p.addLast("3", new OutboundHandlerA());
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p.addLast("4", new OutboundHandlerB());
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p.addLast("5", new InboundOutboundHandlerX());
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```
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#### inbound
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对于inbound event，handler的执行顺序为`1,2,3,4,5`
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但由于`3,4`没有实现ChannelInboundHandler，故而Inbound event会跳过`3,4` handler，实际Inbound event的handler顺序为`1,2,5`
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#### outbound
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对于outbound event，handler的执行顺序为`5,4,3,2,1`
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对于outbound evnet，由于`1,2`并没有实现ChannelOutboundHandler，故而outbound event的handler顺序为`5,4,3`。
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> inbound顺序和pipeline handler的添加顺序相同，outbound顺序和pipeline handler添加顺序相反。
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### 将event转发给下一个handler
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如上图所示，再handler中必须调用`ChannelHandlerContext`中的event propagation method来将event转发给下一个handler。`event propagation event`包括的方法如下：
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#### Inbound event propagation method
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- ChannelHandlerContext.fireChannelRegistered()
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- ChannelHandlerContext.fireChannelActive()
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- ChannelHandlerContext.fireChannelRead(Object)
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- ChannelHandlerContext.fireChannelReadComplete()
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- ChannelHandlerContext.fireExceptionCaught(Throwable)
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- ChannelHandlerContext.fireUserEventTriggered(Object)
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- ChannelHandlerContext.fireChannelWritabilityChanged()
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- ChannelHandlerContext.fireChannelInactive()
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- ChannelHandlerContext.fireChannelUnregistered()
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#### Outbound event propagation method
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- ChannelHandlerContext.bind(SocketAddress, ChannelPromise)
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- ChannelHandlerContext.connect(SocketAddress, SocketAddress, ChannelPromise)
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- ChannelHandlerContext.write(Object, ChannelPromise)
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- ChannelHandlerContext.flush()
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- ChannelHandlerContext.read()
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- ChannelHandlerContext.disconnect(ChannelPromise)
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- ChannelHandlerContext.close(ChannelPromise)
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- ChannelHandlerContext.deregister(ChannelPromise)
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### build pipeline
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在一个pipeline中，应该包含一个或多个ChannelHandler用于接收IO事件（read）和请求IO操作（write and close）。例如，一个典型的server其channel的pipeline中应该包含如下handler：
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- protocol decoder：将二进制数据转化为java object
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- protocol encoder：将java object转化为二进制数据
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- business logic handler：执行实际业务操作
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构建pipeline示例如下：
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```java
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static final EventExecutorGroup group = new DefaultEventExecutorGroup(16);
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...
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ChannelPipeline pipeline = ch.pipeline();
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pipeline.addLast("decoder", new MyProtocolDecoder());
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pipeline.addLast("encoder", new MyProtocolEncoder());
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// Tell the pipeline to run MyBusinessLogicHandler's event handler methods
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// in a different thread than an I/O thread so that the I/O thread is not blocked by
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// a time-consuming task.
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// If your business logic is fully asynchronous or finished very quickly, you don't
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// need to specify a group.
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pipeline.addLast(group, "handler", new MyBusinessLogicHandler());
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```
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当为BusinessLogicHandler指定DefaultEventExecutorGroup时，虽然会将操作从EventLoop中卸载，但是其针对每个ChannelHandlerContext仍然是串行进行处理的。故而，串行处理可能仍然会导致性能瓶颈。如果在用例场景下顺序不太重要时，可以考虑使用`UnorderedThreadPoolEventExecutor`来最大化任务的并行执行。
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### ThreadSafety
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channelhandler可以在任何时刻添加到pipeline或从pipeline中移除，ChannelPipeline是线程安全的。例如，可以在交换敏感信息前添加encryption handler并且在交换完信息后将encryption handler移除。
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