Programming: Concurrency Patterns
Resources
Wikipedia: Concurrency Patterns https://en.wikipedia.org/wiki/Concurrency_pattern
Producer/Consumer
Safely parallelized task enqueueing/execution.
- A producer adds data to a synchronized queue
- A variable number of consumers process data from a synchronized queue in a loop
- When producer is finished, send a poison pill for each worker, informing it to break/exit the loop gracefully
- Join on worker threads (wait to close)
Monitor Object
Safely share an object between multiple threads,
wait in any caller until the original perform is done (ensuring only one task is ever performed with this object ata time).
The caller does not need to know about the perform-lock, it will simply block until the monitor is able to process a new task.Say we only ever want one thread performing IO on a file at any given time
We create a WriterMonitor, which might have methods read/write
- in thread-1, we call read(), which blocks while in the background creates and performs a thread
- in thread-2, we call write(), it will automatically block until thread-1's read() call is finished, then it will take lock and write()
https://www.youtube.com/watch?v=_p-TM1x48zk
# thread-1 # # worker-thread # # thread-2 # proxy.call() >--------+ (start) | +-------< proxy.call() | (wait) (result) <--------+ | (start) | +-------> (result)
Active Object
Safely share an object between threads,
each method call on a proxy-object enqueues an execution in a separate thread.
Unlike Monitor Object, method can be enqueued several times (does not block on perform)
Also unlike Monitor Object, you may choose to perform tasks in a different order than they were enqueued.
Hides the lock/queue logic from the caller.examples:
- update a progressbar in the UI thread from a background thread
- Qt's signals/slots are an expansion of this pattern
Overview:
- Background thread instantiates a proxy object
- On a method call, the proxy returns a Future/Promise (a request is enqueued to UI thread)
- The UI thread eventually performs the task
- You may wait/join on the Future/Promise to wait for it to return it's results
https://www.youtube.com/watch?v=U9Tf7h-etl0
# bg-thread class Proxy # (ActiveObject) methods that will run in sep thread class Future # waitable object, will obtain result when performed # ui-thread class Queue # holds method requests class Scheduler # (EventLoop) chooses next method request to run
Reactor Pattern
The reactor pattern lets you emit and subscribe to events.
examples:
- logging frameworks that send logs to various outputs
- Qt's signals/slots are a variation of this pattern
# The reactor is your eventloop. # It handles emitted events # # This should be a singleton class Reactor: def __init__(self): self._queue = queue.Queue() def send_event(self, event): self._queue.put(event) def register(self, handler): self._handlers.append(handler) def handle_events(self): while True: event = self._queue.get(timeout=0) if event is not None: self._dispatch(event) def _dispatch(self, event): for handler in self._handlers: handler.handle(event) # Events are things that happen class Event: class MouseclickEvent(Event): class KeypressEvent(Event): # EventHandlers decide if they will handle an event class EventHandler: def handle(self, event): pass class KeypressHandler(EventHandler): class MouseclickHandler(EventHandler):Sample Usage
# thread-1 reactor = Reactor() reactor.register(KeypressHandler()) reactor.register(MouseclickHandler()) reactor.handle_events() # thread-2 reactor = Reactor() reactor.send_event(KeypressEvent("a")) # thread-1 handles the eventhttp://www.dre.vanderbilt.edu/~schmidt/PDF/reactor-siemens.pdf