Programming: Concurrency Patterns

Resources

Wikipedia: Concurrency Patterns

https://en.wikipedia.org/wiki/Concurrency_pattern

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 to perform IO from one thread.
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 triggers 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.

ex. update a progressbar in the UI thread from a background thread

• 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

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)