module Netplex_semaphore:Netplex-wide semaphores
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 :
add_pluginmethod with this object as argument. This can e.g. be done in the
post_add_hookof the processor.
val increment :
string -> int64
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
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 can also be invoked from the controller process.
val create :
?protected:bool -> string -> int64 -> bool
trueif the creation is successful, and
falseif the semaphore already existed.
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
It is automatically created at the first use time with a value of 0
create can also be invoked from the controller process.
val destroy :
string -> unit
decrementwill immediately get (-1L).
Note that there is no protection against unintended re-creation
destroy can also be invoked from the controller process.
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"