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.
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.
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.
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.
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.
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.
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.
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"