Monitors can be used to coordinate access to a resource from several threads so that only one thread comes into contact with the resource. Of course, the resources we talk about must have an engine interface. For example, the resource could be an RPC client. In contrast to a critical section (protected with mutexes), the accesses are not serialized, but simply pushed onto the same event system, and run there concurrently. Effectively, the monitor translates heavy-weight threading (kernel threads) into light-weight threading (engines).
A thread monitor governs which threads have access to a set of engines running together on an event system
let result = monitor_run mon f outer_esys arg:
Runs another engine within the
monitor. The engine
inner_e is created as
let inner_e = f inner_esys arg
inner_esys is the event system of the monitor. It is
inner_e uses this event system.
inner_e reaches a final state, the thread leaves the
monitor, and the result of
inner_e is returned (or the
recorded exception is thrown).
The "special effect" of the monitor is that several threads can
share the same monitor, even at the same time. All parallel running
inner_e will use the same event system
Of course, this also means that they run in the same thread.
Note that it is possible that the monitor switches the thread
it uses for executing the event system (and the engines). In
particular, this can happen when one of the inner engines finishes,
and the thread that put this engine into the monitor was chosen
as the thread for executing the event system. When this happens,
another thread waiting for the monitor will be selected for
running the event system.
If an inner engine establishes configurations that are not bound to the lifetime of the inner engine, there is no guarantee that these additional events and handlers will be considered when the inner engine ends. For example, if the engine starts a timer to run an action later, it is not said that the action is carried out at all if the engine terminates before.
Exceptions falling through to the caller of the event system are logged and otherwise ignored.
It is possible to use monitors in single-threaded programs - this is treated as if only one thread was using the monitor in a multi-threaded program.
let result = monitor_do mon f arg: Just runs
f arg in the
scope of the monitor, and returns the result.
let result = monitor_async f arg: For calling RPC-style
asynchronous clients. If
f is called like
f arg emit,
the result is passed back by calling
emit (fun () -> result).
In this example we want to achieve that several threads can use the same RPC client. This type of client is not thread-safe, but it is able to run asynchronously. Because of this, the requests coming from several threads can be concurrently handled.
Prepare the RPC client and the monitor: We assume that
the generated RPC module with the client, and that we access program
P at version
let esys = Unixqueue.create_unix_event_system let rpc = M_clnt.P.V.create_client ~esys connector protocol let mon = Uq_mt.create_monitor esys
It is essential that the RPC client and the monitor use the same
Next, let's assume we are in a spawned thread, and we want to
call an RPC procedure
M_clnt.P.V.proc. The trick here is to
use the asynchronous variant
M_clnt.P.V.proc'async, and to
call it via the monitor:
let result = Uq_mt.monitor_async mon (M_clnt.P.V.proc'async rpc) arg
The procedure is called with argument
arg and returns the
The monitor waits until the asynchronous call is done, so this appears
as a synchronous call to the user.
If you need to call functions to configure and control the RPC
let sockaddr = Uq_mt.monitor_do mon Rpc_client.get_peer_name rpc