## A.59 library(thread_pool): Resource bounded thread management

See also
  http_handler/3 and http_spawn/2.

The module `library(thread_pool)` manages threads in pools. A pool defines properties of its member threads and the maximum number of threads that can coexist in the pool. The call thread_create_in_pool/4 allocates a thread in the pool, just like thread_create/3. If the pool is fully allocated it can be asked to wait or raise an error.

The library has been designed to deal with server applications that receive a variety of requests, such as HTTP servers. Simply starting a thread for each request is a bit too simple minded for such servers:

- Creating many CPU intensive threads often leads to a slow-down rather than a speedup.
- Creating many memory intensive threads may exhaust resources
- Tasks that require little CPU and memory but take long waiting for external resources can run many threads.

Using this library, one can define a pool for each set of tasks with comparable characteristics and create threads in this pool. Unlike the worker-pool model, threads are not started immediately. Depending on the design, both approaches can be attractive.

The library is implemented by means of a manager thread with the fixed thread id `__thread_pool_manager`. All state is maintained in this manager thread, which receives and processes requests to create and destroy pools, create threads in a pool and handle messages from terminated threads. Thread pools are not saved in a saved state and must therefore be recreated using the initialization/1 directive or otherwise during startup of the application.

[det]thread_pool_create(+Pool, +Size, +Options)
  Create a pool of threads. A pool of threads is a declaration for creating threads with shared properties (stack sizes) and a limited number of threads. Threads are created using thread_create_in_pool/4. If all threads in the pool are in use, the behaviour depends on the `wait` option of thread_create_in_pool/4 and the `backlog` option described below. Options are passed to thread_create/3, except for
  backlog(+MaxBackLog)
    Maximum number of requests that can be suspended. Default is `infinite`. Otherwise it must be a non-negative integer. Using `backlog(0)` will never delay thread creation for this pool.

  The pooling mechanism does not interact with the `detached` state of a thread. Threads can be created both `detached` and normal and must be joined using thread_join/2 if they are not detached.

[det]thread_pool_destroy(+Name)
  Destroy the thread pool named Name.
  Errors
    `existence_error(thread_pool, Name)`.
[nondet]current_thread_pool(?Name)
  True if Name refers to a defined thread pool.
[nondet]thread_pool_property(?Name, ?Property)
  True if Property is a property of thread pool Name. Defined properties are:
  options(Options)
    Thread creation options for this pool
  free(Size)
    Number of free slots on this pool
  size(Size)
    Total number of slots on this pool
  members(ListOfIDs)
    ListOfIDs is the list or threads running in this pool
  running(Running)
    Number of running threads in this pool
  backlog(Size)
    Number of delayed thread creations on this pool
[det]thread_create_in_pool(+Pool, :Goal, -Id, +Options)
  Create a thread in Pool. Options overrule default thread creation options associated to the pool. In addition, the following option is defined:
  wait(+Boolean)
    If `true` (default) and the pool is full, wait until a member of the pool completes. If `false`, throw a resource_error.
  Errors
    - `resource_error(threads_in_pool(Pool))` is raised if wait is `false` or the backlog limit has been reached.
    - `existence_error(thread_pool, Pool)` if Pool does not exist.
worker_exitted(+PoolName, +WorkerId, :AtExit)
  It is possible that’__thread_pool_manager’no longer exists while closing down the process because the manager was killed before the worker.
  To be done
    Find a way to discover that we are terminating Prolog.
[semidet,multifile]create_pool(+PoolName)
  Hook to create a thread pool lazily. The hook is called if thread_create_in_pool/4 discovers that the thread pool does not exist. If the hook succeeds, thread_create_in_pool/4 retries creating the thread. For example, we can use the following declaration to create threads in the pool `media`, which holds a maximum of 20 threads.

      :- multifile thread_pool:create_pool/1.

      thread_pool:create_pool(media) :-
          thread_pool_create(media, 20, []).