lwt 3.3.0 · OCaml Package

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.

parameter validate
is called each time a pool element is accessed by use, before the element is provided to 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.

parameter check
is called after the resolution of 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.

parameter dispose
is used as described above and by 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. 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 : _ t -> int

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

package lwt