[Haskell-cafe] Bounded STM Queues with load shedding

[Alex Hammel]

Forwarding from Haskell-Beginners:

Hi list,

It would be handy for me to have a bounded STM Queue along the lines of TBQueue with the difference that when the queue is full, a call to writeTBQueue is a non-blocking no-op.

This is in the context of an asynchronous logging system. If the load (somehow) gets heavy enough that the log message queue is taking up a lot of memory, It's better to lose a few log messages than to have processes block until the logger catches up.

Are there any off-the-shelf solutions?

Cheers,
Alex

[Yuras Shumovich]

You can try the stm-chans package:
http://hackage.haskell.org/package/stm-chans-3.0.0.2/docs/Control-Concurrent-STM-TBMQueue.html#v:tryWriteTBMQueue

Thanks,
Yuras

>
> Cheers,
> Alex
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[Alex Hammel]

Perfect, thanks!

[Ryan Yates]

Note that this is what orElse is for, turning a blocking API unto a choice.  You could write tryWriteTBQueue as:

    tryWriteTBQueue queue v = (writeTBQueue queue v >> return True) `orElse` (return False)

Ryan