Source file aches_lwt.ml

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(*****************************************************************************)
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(** Aches-lwt: a variety of caches for promises

    Aches_lwt is a library that provides caches (limited-size collections with
    automatic discarding of supernumerary elements) for a variety of uses around
    Lwt promises.

    Lwt promises are I/O-dependent values which can take time to resolve. As
    such, using them in a more standard value cache (e.g., as provided by Aches)
    can lead to race conditions. The most common scenario leading to a race
    condition is because of the possible delay between testing whether a value
    is held in a cache and resolving a fresh value:

    {[
      (* with a normal cache holding non-promise values *)
      match find_opt c k with
      | None ->
          (* value is not there *)
          let* v = … in (* make missing values *)
          (* oops the Lwt scheduler has kicked in and there is delay here *)
          replace c k v; (* insert *)
          … (* use [v] *)
      | Some v ->
          … (* use [v] *)
    ]}

    Aches-lwt avoids these race conditions by working directly with promises in
    the backend, providing a carefully selected API and maintaining a strict
    cancelling discipline. *)

(** Lwt-promise caches *)
module Lache = Lache