Event driven simulator in Python, using async/await

eventsim.py

# Copyright (c) 2021 Damon Wischik. See LICENSE for permissions.

import heapq
import asyncio

class EventSimulator(asyncio.AbstractEventLoop):
    '''A simple event-driven simulator, using async/await'''

    def __init__(self):
        self._time = 0
        self._running = False
        self._immediate = []
        self._scheduled = []
        self._exc = None
        
    def get_debug(self):
        # A silly hangover from the way asyncio is written
        return False

    def time(self):
        return self._time
    
    # Methods for running the event loop.
    # For a real asyncio system you need to worry a lot more about running+closed.
    # For a simple simulator, we completely control the passage of time, so most of
    # these functions are trivial.
    #
    # I've split the pending events into _immediate and _scheduled.
    # I could put them all in _scheduled; but if there are lots of
    # _immediate calls there's no need for them to clutter up the heap.
    
    def run_forever(self):
        self._running = True
        while (self._immediate or self._scheduled) and self._running:
            if self._immediate:
                h = self._immediate[0]
                self._immediate = self._immediate[1:]
            else:
                h = heapq.heappop(self._scheduled)
                self._time = h._when
                h._scheduled = False   # just for asyncio.TimerHandle debugging?
            if not h._cancelled:
                h._run()
            if self._exc is not None:
                raise self._exc

    def run_until_complete(self, future):
        raise NotImplementedError

    def _timer_handle_cancelled(self, handle):
        # We could remove the handle from _scheduled, but instead we'll just skip
        # over it in the "run_forever" method.
        pass

    def is_running(self):
        return self._running
    def is_closed(self):
        return not self._running
    def stop(self):
        self._running = False
    def close(self):
        self._running = False
    def shutdown_asyncgens(self):
        pass

    def call_exception_handler(self, context):
        # If there is any exception in a callback, this method gets called.
        # I'll store the exception in self._exc, so that the main simulation loop picks it up.
        self._exc = context.get('exception', None)


    # Methods for scheduling jobs.
    #
    # If the job is a coroutine, the end-user should call asyncio.ensure_future(coro()).
    # The asyncio machinery will invoke loop.create_task(). Asyncio will then
    # run the coroutine in pieces, breaking it apart at async/await points, and every time it
    # will construct an appropriate callback and call loop.call_soon(cb).
    #
    # If the job is a plain function, the end-user should call one of the loop.call_*()
    # methods directly.
    
    def call_soon(self, callback, *args):
        h = asyncio.Handle(callback, args, self)
        self._immediate.append(h)
        return h

    def call_later(self, delay, callback, *args):
        if delay < 0:
            raise Exception("Can't schedule in the past")
        return self.call_at(self._time + delay, callback, *args)

    def call_at(self, when, callback, *args):
        if when < self._time:
            raise Exception("Can't schedule in the past")
        h = asyncio.TimerHandle(when, callback, args, self)
        heapq.heappush(self._scheduled, h)
        h._scheduled = True  # perhaps just for debugging in asyncio.TimerHandle?
        return h

    def create_task(self, coro):
        # Since this is a simulator, I'll run a plain simulated-time event loop, and
        # if there are any exceptions inside any coroutine I'll pass them on to the
        # event loop, so it can halt and print them out. To achieve this, I need the
        # exception to be caught in "async mode" i.e. by a coroutine, and then
        # use self._exc to pass it on to "regular execution mode".
        async def wrapper():
            try:
                await coro
            except Exception as e:
                print("Wrapped exception")
                self._exc = e
        return asyncio.Task(wrapper(), loop=self)

    def create_future(self):
        # Not sure why this is here rather than in AbstractEventLoop.
        return asyncio.Future(loop=self)

LICENSE

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.

sim.py

# Copyright (c) 2021 Damon Wischik. See LICENSE for permissions.

import asyncio, heapq
from eventsim import EventSimulator


class ProcessorSharingQueue:
    def __init__(self, service_rate=1, loop=None):
        self._service_rate = service_rate
        self._queue = []
        self._loop = loop if loop else asyncio.get_event_loop()
        self._done = None
        self._work_done = 0
        self._last_time = self._loop.time()
    def process(self, work):
        t = self._advance_clock()
        fut = self._loop.create_future()
        w = work / self._service_rate
        heapq.heappush(self._queue, (self._work_done+w, t, fut))
        if self._done:
            self._done.cancel()
        self._schedule()
        return fut
    def complete(self):
        t = self._advance_clock()
        (_, tstart, fut) = heapq.heappop(self._queue)
        fut.set_result(t - tstart)
        self._schedule()
    def _advance_clock(self):
        t = self._loop.time()
        if self._queue:
            self._work_done += (t - self._last_time) / len(self._queue)
        self._last_time = t
        return t
    def _schedule(self):
        if not self._queue:
            self._done = None
        else:
            w,_,_ = self._queue[0]
            dt = (w - self._work_done) * len(self._queue)
            self._done = self._loop.call_later(dt, self.complete)


class FIFOQueue:
    def __init__(self, service_rate=1, loop=None):
        self._service_rate = service_rate
        self._queue = []
        self._loop = loop if loop else asyncio.get_event_loop()
        self._done = None
    def process(self, work):
        fut = self._loop.create_future()
        w = work / self._service_rate
        self._queue.append((w, fut))
        if not self._done:
            self._done = self._loop.call_later(w, self.complete)
        return fut
    def complete(self):
        w,fut = self._queue[0]
        fut.set_result(w)
        self._queue = self._queue[1:]
        if self._queue:
            w,_ = self._queue[0]
            self._done = self._loop.call_later(w, self.complete)
        else:
            self._done = None



#------------------------------------------------

loop = EventSimulator()
asyncio.set_event_loop(loop)

q = ProcessorSharingQueue()
            
async def queueing_job(i=1):
    print(loop.time(), "Start job {}".format(i))
    await asyncio.sleep(i)
    print(loop.time(), "Sending job {}".format(i))
    xmit = q.process(work=4)
    await xmit
    print(loop.time(), "Done job {} in time {}".format(i, xmit.result()))
    
asyncio.ensure_future(queueing_job(1))
asyncio.ensure_future(queueing_job(3))
loop.run_forever()

Writing an event-driven simulator in Python, using async/await

The first file eventsim.py is a generic event loop, implementing asyncio.AbstractEventLoop. It doesn't implement any of the methods to do with actual real network access etc., it just maintains a virtual clock and a heap of events waiting to fire. Because it implements asyncio.AbstractEventLoop, all the Python 3.5 cleverness of async/await can use it automatically.

The second file sim.py is an example, using the generic event loop to simulate either a processor sharing queue or a FIFO queue.

Looks like this codes dose not run on Python3.7, now Python will pass a new keyword parameter context to call_soon.

@damonjw Thanks for the great work, I tried to use it with Pyodide and it works! A minor fix I did is to change add **kwargs to def call_soon(self, callback, *args, **kwargs):.

Would you mind to attach a license to this file so I can use it in a open source project (MIT license)?

I stumbled upon this code whilst researching if/how I could use asyncio (with manual control of time passage) to do Discrete Event Simulations. It seems so obvious to me, but this is the first application of the idea I found. Thanks, very useful!