[Haskell-cafe] Database connection pool

[Bas van Dijk]

On Thu, May 6, 2010 at 3:24 PM, Michael Snoyman <michael at snoyman.com> wrote:
>
>
> On Thu, May 6, 2010 at 9:13 AM, Bryan O'Sullivan <bos at serpentine.com> wrote:
>>
>> On Wed, May 5, 2010 at 10:51 PM, Michael Snoyman <michael at snoyman.com>
>> wrote:
>>>
>>> * When a connection is released, is goes to the end of the pool, so
>>> connections get used evenly (not sure if this actually matters in practice).
>>
>> In practice, you're better off letting idle connections stay that way,
>> because then your DB server can close connections and free up resources. In
>> other words, when you're done with a connection, put it at the front of the
>> reuse queue, not the back.
>> You'll also want to handle the possibility that a connection that you grab
>> from the pool has been closed by the server. Many connection pooling
>> implementations I've seen get this wrong in subtle or expensive ways.
>
> Thanks for the feedback. I've gone ahead and implemented a simple resource
> pool module. Since I need it to work with monad transformer stacks, I've
> built it on top of MonadCatchIO-transformers. I've put the code up in a gist
> on github[1]. I would appreciate if anyone could review this, especially to
> make sure the exception handling code is correct. block and unblock in
> particular concern me.
> Thanks,
> Michael
> [1] http://gist.github.com/392078

I also have a suggestion for your design. (Note however that I don't
have much experience with resource pools.)

In your current design a Pool has a fixed maximum number of opened
resources. I can imagine situations where the maximum number of opened
resources can change dynamically. For example due to plugging in (or
out) a new blade server at run-time which will increase (or decrease)
the maximum number of resources that can be handled.

So what about changing:

createPool :: IO a -> Int -> IO (Pool a)
to:
createPool :: IO (Maybe a) -> IO (Pool a)

so, instead of statically storing the maximum number of  opened
resources (Int), the resource creation function will decide itself
when it has created enough (Maybe a).

Regards,

Bas