Question

I am attempting to write a simple TCP server using Aleph. Everything works fine, except I am unsure of how I should detect when a channel has been closed.

From the documentation:

When the client closes the connection, both sides of the channel will be immediately sealed. The final message from the channel will be nil

However, I never seem to receive this final nil message. If I inspect the channel, I do see that it has been closed. Here is my code:

(use 'lamina.core 'aleph.tcp 'gloss.core)

(defn process-msg [ch msg]
  (if (closed? ch)
    (println "Channel has been closed") ;This never happens
    (do-some-processing msg)))

(start-tcp-server
  (fn [ch client-info]
    (receive-all ch
      (partial process-msg ch))
  {:port 10000, :frame (string :utf-8 :delimiters ["\n"])})

Should I be doing something differently? Is my frame keeping the nil message from being processed? I could have a separate thread monitoring my channels and checking whether or not they have been closed, but this seems like a poor design. I would prefer to use Aleph, but right now its looking like I will need to use a raw Netty handler. Using Netty directly would be fine, but I’d prefer to use Aleph if possible since it feels a little more idiomatic.

Answer 1

To register a callback for when a channel is closed or drained, use (on-closed ch callback) or (on-drained ch callback), which both take a callback with zero arguments.

https://github.com/ztellman/lamina/wiki/Channels