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Module Netplex_semaphore

module Netplex_semaphore: sig .. end
Netplex-wide semaphores

Semaphores are counters with atomic increment and decrement operations. They are very useful for counting the number of uses of a shared resource, and allow the identification of the first use (so the resource must be made available at all), and the last use (the resource can be released).

This implementation works in both multi-processing and multi-threading netplex environments. It is, however, not very fast, because the counters live in the controller, and the increment/decrement operations are realized by RPC's. It is good enough when these operations are only infrequently called, e.g. in the post-start and pre-finish processor callbacks.

This interface is designed so that a later re-implementation with POSIX semaphores is relatively straight-forward.

val plugin : Netplex_types.plugin
To enable semaphores, call the controller's add_plugin method with this object as argument. This can e.g. be done in the post_add_hook of the processor.

The folloing functions can only be invoked in container contexts. Outside of such a context the exception Netplex_cenv.Not_in_container_thread is raised.
val increment : string -> int64
Increment the named semaphore by 1, and return the new value. If the semaphore does not exist yet, it is created with an initial value of 0, which is then incremented.

Semaphore names are global to the whole netplex system. By convention, these names are formed like "service_name.local_name", i.e. they are prefixed by the socket service to which they refer.

val decrement : ?wait:bool -> string -> int64
Decrement the named semaphore by 1, and return the new value. Semaphore values cannot become negative. If the value is already 0, it is not decremented anymore if wait = false. However, (-1) is then returned nevertheless.

If the value is already 0 and wait=true, the operation waits until the value exceeds 0, and when this happens, the semaphore is then decremented again. If several waiters exist, only one waiter gets the chance to decrement.

val get : string -> int64
Get the value of the named semaphore. Useful e.g. for monitoring the semaphore. If the semaphore does not exist, a value of 0 is returned.
val create : ?protected:bool -> string -> int64 -> bool
Create the semaphore with this initial value. Returns true if the creation is successful, and false if the semaphore already existed.

If protected, the semaphore is automatically decremented by some value when the container calling this function terminates. This value is pi - d where pi is the number of increments and d is the number of (successful) decrements requested by the container.

A semaphore needs not to be explicitly created by calling create. It is automatically created at the first use time with a value of 0 and protected=true.

Example (code fragment):

Override the processor callbacks as follows to count the number of containers for the service:

    method post_add_hook sockserv ctrl =
      ctrl # add_plugin Netplex_semaphore.plugin

    method post_start_hook container =
      let sem_name = container#socket_service#name ^ ".counter" in
      let n =
        Netplex_semaphore.increment sem_name in
      if n=1 then
        prerr_endline "First container"

    method pre_finish_hook container =
      let sem_name = container#socket_service#name ^ ".counter" in
      let n =
        Netplex_semaphore.decrement sem_name in
      if n=0 then
        prerr_endline "Last container"

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