Data.Pool

A high-performance striped pooling abstraction for managing flexibly-sized collections of resources such as database connections.

"Striped" means that a single Pool consists of several sub-pools, each managed independently. A single stripe is fine for many applications, and probably what you should choose by default. More stripes will lead to reduced contention in high-performance multicore applications, at a trade-off of causing the maximum number of simultaneous resources in use to grow.

Documentation

data Pool a

Instances

Show (Pool a)

Typeable (* -> *) Pool

data LocalPool a

A single striped pool.

Instances

Typeable (* -> *) LocalPool

createPool

Arguments

:: IO a

Action that creates a new resource.

-> (a -> IO ())

Action that destroys an existing resource.

-> Int

The number of stripes (distinct sub-pools) to maintain. The smallest acceptable value is 1.

-> NominalDiffTime

Amount of time for which an unused resource is kept open. The smallest acceptable value is 0.5 seconds.

The elapsed time before destroying a resource may be a little longer than requested, as the reaper thread wakes at 1-second intervals.

-> Int

Maximum number of resources to keep open per stripe. The smallest acceptable value is 1.

Requests for resources will block if this limit is reached on a single stripe, even if other stripes have idle resources available.

-> IO (Pool a)

Create a striped resource pool.

Although the garbage collector will destroy all idle resources when the pool is garbage collected it's recommended to manually destroyAllResources when you're done with the pool so that the resources are freed up as soon as possible.

withResource :: MonadBaseControl IO 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 pool has an idle resource available, it is used immediately.
• Otherwise, if the maximum number of resources has not yet been reached, a new resource is created and used.
• If the maximum number of resources has been reached, this function blocks until a resource becomes available.

If the action throws an exception of any type, the resource is destroyed, and not returned to the pool.

It probably goes without saying that you should never manually destroy a pooled resource, as doing so will almost certainly cause a subsequent user (who expects the resource to be valid) to throw an exception.

takeResource :: Pool a -> IO (a, LocalPool a)

Take a resource from the pool, following the same results as withResource. Note that this function should be used with caution, as improper exception handling can lead to leaked resources.

This function returns both a resource and the LocalPool it came from so that it may either be destroyed (via destroyResource) or returned to the pool (via putResource).

tryWithResource :: forall m a b. MonadBaseControl IO m => Pool a -> (a -> m b) -> m (Maybe b)

Similar to withResource, but only performs the action if a resource could be taken from the pool without blocking. Otherwise, tryWithResource returns immediately with Nothing (ie. the action function is not called). Conversely, if a resource can be borrowed from the pool without blocking, the action is performed and it's result is returned, wrapped in a Just.

tryTakeResource :: Pool a -> IO (Maybe (a, LocalPool a))

A non-blocking version of takeResource. The tryTakeResource function returns immediately, with Nothing if the pool is exhausted, or Just (a, LocalPool a) if a resource could be borrowed from the pool successfully.

destroyResource :: Pool a -> LocalPool a -> a -> IO ()

Destroy a resource. Note that this will ignore any exceptions in the destroy function.

putResource :: LocalPool a -> a -> IO ()

Return a resource to the given LocalPool.

destroyAllResources :: Pool a -> IO ()

Destroy all resources in all stripes in the pool. Note that this will ignore any exceptions in the destroy function.

This function is useful when you detect that all resources in the pool are broken. For example after a database has been restarted all connections opened before the restart will be broken. In that case it's better to close those connections so that takeResource won't take a broken connection from the pool but will open a new connection instead.

Another use-case for this function is that when you know you are done with the pool you can destroy all idle resources immediately instead of waiting on the garbage collector to destroy them, thus freeing up those resources sooner.