Netty Channel Handlers, Threads and Stacks

Question

Do the instances of Netty's channel handlers such as (SimpleChannelInboundHandler, ChannelInboundHandlerAdapter, etc.) share the same thread and stack or do each have it's own thread and stack? I ask because I instantiate numerous channel handlers and I need them to communicate with each other and I must decide between using thread communication or non- threaded communication.

Thank you for your answer

Answer 1

The answer: it depends

I assume that you must be building a server, some of what I say might not apply.

According to https://netty.io/4.0/api/io/netty/channel/ChannelHandler.html , one of the factors that determines which thread your channel handler runs on is how you add it to the pipeline. Netty allows the capability to use a single instance of your handler for all pipelines (ie Connections to your server), in which case you must accommodate for different threads.

In contrast, if you use a channel initializer to add handlers to the pipeline, then no, you do not need to communicate between threads because each connection uses a different instance of the handler.

This is also given that you are using more than one worker thread to run your channel handlers. You must also account for which channel your handlers are communicating with, if you don't store a state variable using a channel initialized handler, then you must accommodate for inter-thread communication.

Your best bet is to debug the current thread in each handler before making a decision, netty's threading behavior and how it interacts with your program is highly subjective on your implementation and where you are moving your data.

Answer 2

As a general rule, if your handler has state, then it's one handler per channel (pipeline). Otherwise, annotate your handler with @ChannelHandler.Sharable and use the same instance for each channel.