Stefan Scherfke

SimPy: Process Interaction

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As promised, the delay between this and the last topical guide was rather short.

This one is about process interaction. This is what makes event discrete simulation interesting. Without it, you actually wouldn’t even need SimPy.

So this guide is about:

Another possibility for processes to interact are resources. They will be discussed in the next guide.

Sleep until woken up

Imagine you want to model an electric vehicle with an intelligent battery-charging controller. While the vehicle is driving, the controller can be passive but needs to be reactivate once the vehicle is connected to the power grid in order to charge the battery.

In SimPy 2, this pattern was known as passivate / reactivate. In SimPy 3, you can accomplish that with a simple, shared Event:

>>> from random import seed, randint
>>> seed(23)
>>>
>>> import simpy
>>>
>>> class EV:
...     def __init__(self, env):
...         self.env = env
...         self.drive_proc = env.process(self.drive(env))
...         self.bat_ctrl_proc = env.process(self.bat_ctrl(env))
...         self.bat_ctrl_reactivate = env.event()
...
...     def drive(self, env):
...         while True:
...             # Drive for 20-40 min
...             yield env.timeout(randint(20, 40))
...
...             # Park for 1–6 hours
...             print('Start parking at', env.now)
...             self.bat_ctrl_reactivate.succeed()  # "reactivate"
...             self.bat_ctrl_reactivate = env.event()
...             yield env.timeout(randint(60, 360))
...             print('Stop parking at', env.now)
...
...     def bat_ctrl(self, env):
...         while True:
...             print('Bat. ctrl. passivating at', env.now)
...             yield self.bat_ctrl_reactivate  # "passivate"
...             print('Bat. ctrl. reactivated at', env.now)
...
...             # Intelligent charging behavior here …
...             yield env.timeout(randint(30, 90))
...
>>> env = simpy.Environment()
>>> ev = EV(env)
>>> env.run(until=150)
Bat. ctrl. passivating at 0
Start parking at 29
Bat. ctrl. reactivated at 29
Bat. ctrl. passivating at 60
Stop parking at 131

Since bat_ctrl() just waits for a normal event, we no longer call this pattern passivate / reactivate in SimPy 3.

Waiting for another process to terminate

The example above has a problem: it may happen that the vehicles wants to park for a shorter duration than it takes to charge the battery (this is the case if both, charging and parking would take 60 to 90 minutes).

To fix this problem we have to slightly change our model. A new bat_ctrl() will be started every time the EV starts parking. The EV then waits until the parking duration is over and until the charging has stopped:

>>> class EV:
...     def __init__(self, env):
...         self.env = env
...         self.drive_proc = env.process(self.drive(env))
...
...     def drive(self, env):
...         while True:
...             # Drive for 20-40 min
...             yield env.timeout(randint(20, 40))
...
...             # Park for 1–6 hours
...             print('Start parking at', env.now)
...             charging = env.process(self.bat_ctrl(env))
...             parking = env.timeout(randint(60, 360))
...             yield charging & parking
...             print('Stop parking at', env.now)
...
...     def bat_ctrl(self, env):
...         print('Bat. ctrl. started at', env.now)
...         # Intelligent charging behavior here …
...         yield env.timeout(randint(30, 90))
...         print('Bat. ctrl. done at', env.now)
...
>>> env = simpy.Environment()
>>> ev = EV(env)
>>> env.run(until=310)
Start parking at 29
Bat. ctrl. started at 29
Bat. ctrl. done at 83
Stop parking at 305

Again, nothing new (if you’ve read the events guide) and special is happening. SimPy processes are events, too, so you can yield them and will thus wait for them to get triggered. You can also wait for two events at the same time by concatenating them with &.

Interrupting another process

As usual, we now have another problem: Imagine, a trip is very urgent, but with the current implementation, we always need to wait until the battery is fully charged. If we could somehow interrupt that …

Fortunate coincidence, there is indeed a way to do exactly this. You can call interrupt() on a Process. This will throw an Interrupt exception into that process, resuming it immediately:

>>> class EV:
...     def __init__(self, env):
...         self.env = env
...         self.drive_proc = env.process(self.drive(env))
...
...     def drive(self, env):
...         while True:
...             # Drive for 20-40 min
...             yield env.timeout(randint(20, 40))
...
...             # Park for 1 hour
...             print('Start parking at', env.now)
...             charging = env.process(self.bat_ctrl(env))
...             parking = env.timeout(60)
...             yield charging | parking
...             if not charging.triggered:
...                 # Interrupt charging if not already done.
...                 charging.interrupt('Need to go!')
...             print('Stop parking at', env.now)
...
...     def bat_ctrl(self, env):
...         print('Bat. ctrl. started at', env.now)
...         try:
...             yield env.timeout(randint(60, 90))
...             print('Bat. ctrl. done at', env.now)
...         except simpy.Interrupt as i:
...             # Onoes! Got interrupted before the charging was done.
...             print('Bat. ctrl. interrupted at', env.now, 'msg:',
...                   i.cause)
...
>>> env = simpy.Environment()
>>> ev = EV(env)
>>> env.run(until=100)
Start parking at 31
Bat. ctrl. started at 31
Stop parking at 91
Bat. ctrl. interrupted at 91 msg: Need to go!

What process.interrupt() actually does is removing the process’ _resume() method from the callbacks of the event that it is currently waiting for. And it will schedule an event that will throw the Interrupt exception into the interrupted process as soon as possible.

Since we don’t to anything special to the event, the interrupted process can yield the same event again after catching the Interrupt – Imagine someone waiting for a shop to open. The person may get interrupted by a phone call. After finishing the call, he or she checks if the shop already opened and either enters or continues to wait.

This guide is now, of course, also part of the SimPy documentation.