Had a question about buffer management for Netty downstream buffer sending flow.
I have a Netty SimpleChannelInBoundHandler MainHandler that does 2 things:-
Receives buffer and asynchronously creates a channel that proxies that buffer to another destination D
Does its own processing on the received buffer and returns response to the client without waiting on 1.
As part 1, I believe I need to explicitly retain the buffer for sending it downstream to destination channel D. Do I also need to take care of releasing this buffer after writeAndFlush() for this channel? Or would Netty pipeline take care of releasing it?
Based on Netty docs Netty.docs: New and noteworthy in 4.0, it says When an outbound (a.k.a. downstream) message reaches at the beginning of the pipeline, Netty will release it after writing it out. which makes me think that Netty would take care of releasing the explicitly retained buffer after downstream/destination channel pipeline completes. Is this understanding correct? `
Explicitly releasing the buffer throws ReferenceCountUtilException in the downstream pipeline
Your understanding of buffer management in Netty is generally correct, but let’s clarify some important points regarding retaining and releasing buffers, especially when dealing with multiple channels and asynchronous operations.
When you receive a buffer in your SimpleChannelInboundHandler, you should explicitly retain it before sending it downstream. This is crucial because buffers in Netty use reference counting to manage memory, and retaining the buffer increases its reference count. Here’s a quick summary of how this works:
Receiving the Buffer: When your handler receives a buffer, the reference count is typically 1. If you pass this buffer to another channel (e.g., channel D), you need to retain it to ensure it stays valid during its journey.
Sending to Another Channel: After calling retain() on the buffer, you can send it downstream using writeAndFlush(). At this point, the reference count is 2 (1 for your handler and 1 for the destination channel).
Releasing the Buffer: According to the Netty documentation, when an outbound message reaches the beginning of the pipeline, it will be released after being written out. However, this only applies to buffers that were not explicitly retained. Since you retained the buffer before sending it downstream, you are responsible for releasing it.
Avoiding ReferenceCountUtilException: If you try to release the buffer after writeAndFlush(), you might encounter a ReferenceCountUtilException because the reference count is still 1 (your handler’s count). You should not explicitly release the buffer after writeAndFlush(); instead, let the downstream pipeline handle the release. The buffer will eventually be released automatically when it reaches the end of the pipeline, provided no other part of your code retains it again.
In summary, retain the buffer before sending it downstream and do not release it afterward. The downstream pipeline will take care of releasing it after it is written out, preventing memory leaks or ReferenceCountUtilException. If you need to ensure that the buffer is released after processing, you can handle that in the downstream channel’s pipeline but avoid releasing it in your MainHandler.