Warning: Reason support is experimental. We are looking for beta-tester and contributors.

Module Lwt_pool

module Lwt_pool : sig..end

External resource pools.

For example, instead of creating a new database connection each time you need one, keep a pool of opened connections and reuse ones that are free. The pool also provides a limit on the number of connections that can simultaneously be open.

If you want to have a pool of system threads, consider using Lwt_preemptive.


type 'a t

Pools containing elements of type 'a.

val create : 
  int ->
  ?validate:('a -> bool Lwt.t) ->
  ?check:('a -> (bool -> unit) -> unit) ->
  ?dispose:('a -> unit Lwt.t) -> (unit -> 'a Lwt.t) -> 'a t

create n ?check ?validate ?dispose f creates a new pool with at most n elements. f is used to create a new pool element. Elements are created on demand and re-used until disposed of.

validate : is called each time a pool element is accessed by Lwt_pool.​use, before the element is provided to Lwt_pool.​use's callback. If validate element resolves to true the element is considered valid and is passed to the callback for use as-is. If validate element resolves to false the tested pool element is passed to dispose then dropped, with a new one is created to take element's place in the pool.

check : is called after the resolution of Lwt_pool.​use's callback when the resolution is a failed promise. check element is_ok must call is_ok exactly once with true if element is still valid and false otherwise. If check calls is_ok false then dispose will be run on element and the element will not be returned to the pool.

dispose : is used as described above and by Lwt_pool.​clear to dispose of all elements in a pool. dispose is not guaranteed to be called on the elements in a pool when the pool is garbage collected. Lwt_pool.​clear should be used if the elements of the pool need to be explicitly disposed of.

val use : 'a t -> ('a -> 'b Lwt.t) -> 'b Lwt.t

use p f takes one free element of the pool p and gives it to the function f. The element is put back into the pool after the promise created by f completes.

val clear : 'a t -> unit Lwt.t

clear p will clear all elements in p, calling the dispose function associated with p on each of the cleared elements. Any elements from p which are currently in use will be disposed of once they are released.

The next call to use p after clear p guarantees a freshly created pool element.

Disposals are performed sequentially in an undefined order.

val wait_queue_length : 'a t -> int

wait_queue_length p returns the number of threads currently waiting for an element of the pool p to become available.