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

Michael Snoyman michael at snoyman.com
Sun Jun 12 11:15:41 CEST 2011

Wrong package, that's resource-pool (which I wasn't aware of until
this moment). Ertugrul is referring to pool:

My original intention of splitting pool off from persistent was so
others could use it. If Bryan's maintaining resource-pool instead, I'd
have no problem deprecating pool and using resource-pool in its place.


On Sun, Jun 12, 2011 at 12:06 PM, Edward Z. Yang <ezyang at mit.edu> wrote:
> 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.
> http://hackage.haskell.org/packages/archive/resource-pool/
> Cheers,
> Edward
> 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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