How does upstream and downstream handlers work in Netty?

Question

I'm new to Netty and don't really understand the flow of handlers in it. What I want to know is the difference between the Upstream and Downstream handlers.

• Why can't we intercept the message in downstream handlers as we do in upstream?
• Do downstream and upstream mean the same as in normal terms?

Answer 1

The difference is that one handles incoming (upstream) and the other outgoing (downstream). I think the event/message flow gets more clear after looking at the javadoc of ChannelPipeline.

I should also note that upstream events will only get triggered by the IO-Thread so there is no need to synchronize field access in there if you use a new ChannelUpstreamHandler per ChannelPipeline. Downstream events can be triggered by any thread, so access to fields needs to be synchronized.