[Haskell-cafe] pool: Why doesn't it block?

Edward Z. Yang ezyang at MIT.EDU
Sun Jun 12 11:06:59 CEST 2011

The documentation seems to indicate that the behaviour should be blocking,
so if it's not, might be a bug.

> withResource :: MonadCatchIO m => Pool a -> (a -> m b) -> m b
> Temporarily take a resource from a Pool, perform an action with it, and
> return it to the pool afterwards. 
> ....
> * If the maximum number of resources has been reached, this function blocks
>   until a resource becomes available. 



Excerpts from Ertugrul Soeylemez's message of Sun Jun 12 03:47:37 -0400 2011:
> Hello Michael, hello fellow haskellers,
> there is something, which has bothered me for quite a while, but now it
> has become a serious problem for me, because I see it as a bug, and
> there is no elegant way to work around it.
> I wonder if it's the right semantics for Data.Pool to simply fail with
> an exception, if the pool is exhausted.  It would be much more
> appropriate, if it would just block, until a resource becomes available.
> Otherwise it's just /safe/ for multi-threading, but not really /useful/
> for it.
> I noticed this when I launched 512 worker threads, but my pool had only
> 16 database connections.  I need the pool to block, until a resource is
> available.
> It's also common that my Yesod site just returns an internal server
> error, when the pool is exhausted, so you can only handle as many
> connections successfully as there are database connections.  I would
> expect Yesod to wait for a connection to become available instead of
> simply blowing the request.
> Blocking should at least be an option and be somehow reachable from
> Yesod/persistent.
> Greets,
> Ertugrul

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