Are concurrent reads possible in Netty 4?

Question

From these notes:

Netty will never call a ChannelHandler's methods concurrently, unless the ChannelHandler is annotated with @Sharable. This is regardless of the type of handler methods - inbound, outbound, or life cycle event handler methods.

I have no problem with that. My question is, however, if it is possible for two different messages to be read/processed from the same channel at the same time, at different stages of the pipeline.

For instance, consider the following pipeline (ChannelInboundHandlers only):

+-----------+
| Handler 2 |
+-----------+
      ^
      |
+-----------+
| Handler 1 |
+-----------+
      ^
      |
+-----------+
| I/O read  |
+-----------+

I understand that only one thread, at most, may be calling the methods from Handler 1, unless it is Sharable. But, can a thread be processing a message in Handler 2 while another thread is processing a message for the same channel in Handler 1? Or is the channel selected only when the current message reaches the end of the pipeline?

Answer 1

This is only possible if you have added one of the ChannelHandler to the pipeline with a EventExecutorGroup passed in. If you not use one everything will just handled by the EventLoop and so one Thread.