CODEWITHSUNDEEP
javasocketsnetty
user1129382
user1129382

Reputation: 33

Understanding Netty's use of Threads

In netty, events that flow through a channel pipeline occur in order as each channel is effectively only assigned to one thread and each handler calls each other in turn. This makes sense and alleviates many syncronisation issues.

However if you use an IdleStateHandler, from my reading of the source, it appears that the channelIdle event will be 'processed' in the context of the Timers thread (the thread the HashedWheelTime is using for example).

Is this case or have I missed something? If it is the case, does this mean that it is possible for an idlestate event and an IO event (example a messageRecieved event) to be executing on the same channel at the same time?

Also, as I could save the ChannelHandler ctx and use it in a different thread to 'write' to a channel for example, thus also have an event going downstream in one thread and upstream in another thread at the same time on the same channel?

Finally which thread do ChannelFutures execute in?

All of these use cases are perfectly acceptable and not a criticism of Netty at all, I am actually very fond of the library and make use of it everywhere. Its just that as I try to do more and more complex things with it, I want to understand more about how it works under the hood so that I can ensure I am using the proper and just the right amount (no more, no less) of syncronisation (locking).

Upvotes: 3

Views: 4038

Answers (2)

Jestan Nirojan
Jestan Nirojan

Reputation: 2486

However if you use an IdleStateHandler, from my reading of the source, it appears that the channelIdle event will be 'processed' in the context of the Timers thread (the thread the HashedWheelTime is using for example).

Is this case or have I missed something? If it is the case, does this mean that it is possible for an idlestate event and an IO event (example a messageRecieved event) to be executing on the same channel at the same time?

Yes, you have missed the concept of ordered event execution available in the Netty, If you are not haveing ExecutionHandler with OrderedMemoryAwareThreadPoolExecutor in the pipeline, you can not have the ordered event execution, specially channel state events from IdleStateHandler.

Also, as I could save the ChannelHandler ctx and use it in a different thread to 'write' to a channel for example, thus also have an event going downstream in one thread and upstream in another thread at the same time on the same channel?

Yes its correct.

More over, If you want to have ordered event execution in the downstream, you have have an downstream execution handler impl with OrderedMemoryAwareThreadPoolExecutor.

Finally which thread do ChannelFutures execute in?

The are executed by Oio/Nio Worker threads,

But if your down stream handler is consuming some type of event and firing another type of event to below the downstream, then your downstream handler can optionally handle the future execution. this can be done by get the future form downstream event and calling

future.setSuccess()

Upvotes: 1

Norman Maurer
Norman Maurer

Reputation: 23557

Yes the channelIdle event and downstream / upstream event could be fired at the same time. So if your handler does implement at least two of the three you need to add proper synchronization. I guess we should make it more clear in the javadocs..

Now the other questions..

You can call Channel.write(...) from every thread you want too.. So its possible to just store it somewhere and call write whenever you want. This also gives the "limitation" that you need have proper synchronization for "downstream" handlers.

ChannelFutures are executed from within the worker thread.

Upvotes: 4

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