Adding Queues

In the minimal framework, processes could only wait for time. In real simulations, processes often need to wait for each other. This step introduces our first coordination primitive, a first-in/first-out (FIFO) queue that processes can await using get() and put().

Design

The queue maintains two internal lists:

self._items    # items currently in the queue
self._getters  # Event objects for waiting consumers

If an item is available, Queue.get() removes it and returns it, resuming the caller at the current simulation time. If an item is not available, on the other hand, Queue.get() create a new Event, appends it to the queue._getters list, and suspends the calling process by awaiting the event.

When a process calls queue.put() to add something to the queue, the method checks to see if a consumer is already waiting. If so, the consumer is immediately resumed with the item. Otherwise, the item is appended to queue._items.

Notice that queue.get() and queue.put() do not advance simulated time. If putting something in the queue or getting something from it takes time, the simulation needs to use Timeout to model that explicitly.