# 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.

Thread safety: Full. The functions can be called from any thread.

    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 following 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.

`get` can also be invoked from the controller process.

    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`.

`create` can also be invoked from the controller process.

    val destroy : string -> unit

Destroy this semaphore. Any waiting `decrement` will immediately get (-1L).

Note that there is no protection against unintended re-creation after `destroy`.

`destroy` can also be invoked from the controller process.

    val ctrl_increment : string -> Netplex_types.container_id -> int64

Increment the named semaphore from controller context, substituting an increment from a container (e.g. a container that terminated or is otherwise no longer able to do so). In this case the ID of the container needs to be passed
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"