Netty insight

Netty

1. there're 2 EventLoopGroup in netty, bossGroup and workerGroup, (1 implementation NioEventLoopGroup is a kind of thread pool)

2. bossGroup is Acceptor,is responsible for creating Channels for incoming connection requests

3. workerGroup is the Reactor/Selector?, handling I/O requests. 

4. a thread in bossGroup will be listening in the port, Once a connection has been accepted workerGroup assigns an EventLoop to its Channel

5. multiple channels can be registered into 1 EventLoop, multiple EventLoops will exist in workerGroup

6. workerGroup will iterate all the EventLoop, and iterate all the channels in EventLoop, if any of the channel is ready to execute/process

7. it will invoke all the channelHandlers in the channelPipeline
8. ChannelPipelines are containers for chains of ChannelHandlers which executed in order
9. There are, in fact, two ways of sending messages in Netty. You can write directly to the Channel or write to the ChannelHandlerContext object. The main difference is that the former approach causes the message to start from the tail of the ChannelPipeline, while the latter causes the message to start from the next handler in the ChannelPipeline.
10. While the I/O thread must not be blocked at all, thus prohibiting any direct blocking operations within your ChannelHandler, there is a way to implement this requirement.

You can specify an EventExecutorGroup when adding ChannelHandlers to the ChannelPipeline.
This EventExecutorGroup will then be used to obtain an EventExecutor, which will execute all the methods of the ChannelHandler.
This EventExecutor will use a different thread from the I/O thread, thus freeing up the EventLoop.

Channel ch = ...;
ChannelPipeline p = ch.pipeline();
EventExecutor e1 = new DefaultEventExecutor(16);
EventExecutor e2 = new DefaultEventExecutor(8);
 
p.addLast(new MyProtocolCodec());
p.addLast(e1, new MyDatabaseAccessingHandler());
p.addLast(e2, new MyHardDiskAccessingHandler());

http://stackoverflow.com/questions/12928723/netty-4-eventloopgroup-eventloop-eventexecutor-thread-affinity