Separate processes to handle the map simulations

[zetadin]

Every ~30 seconds need to evaluate how the battles on the map are going and update the amount of equipment each unit has.

Using a Django Background Task for this is not optimal, as repeating tasks there only start their timers after the last task is done. This can be circumvented by removing the processing time from the expected delay interval and scheduling the next task within the current one.

The more complicated method is to manually write an implementation with multiprocessing, but if not implemented carefully this will lead to balooning RAM use under load (eg. with separate process for each active map). With Background Task can use built in queues to schedule everything.

So let's try doing this with Background Tasks with async workers.

Step 1: hook up the Background Tasks app and send periodic updates to the client.

[zetadin]

A more robust solution would be Django Q. It can run without a broker (Redis) relying instead on Django's ORM. When scaling up will be needed, can switch to Redis much easier than rewriting the infrastructure from Background Tasks. Besides, Background Tasks was not maintained for most of 4 years and someone took over its maintenance only a few months ago.

[zetadin]

Django Q only supports task schedules with a minimum of 1 minute intervals because of heart beat to Redis and other brokers. https://github.com/Koed00/django-q/issues/179

A workaround is scheduling 2 simulation steps at once with a sleep between them.

[zetadin]

Huey also has the same issue where scheduler only has a 1 minute resolution.

[zetadin]

With a custom queue-driven implementation I can now schedule tasks that repeat every 10 seconds with a start time <5ms after the scheduled one.

[zetadin]

To know when simulation is needed:

• [x] Subscribe user for map updates when they access that map. List of actively subscribed users should be stored in the map itself.
• [x] Mark map as active while any user is subscribed to it

During each turn of the simulation:

• [x] Unsubscribe user after a period of inactivity
• [x] Mark map inactive when no users are subscribed (after period of inactivity. Need to unsub users too).
• [ ] Determine which units are in contact. This is a directionless graph.
• [ ] Randomize order of edges
• [ ] Determine post-mitigation damage each unit receives
• [ ] Remove that much equipment worth of HP from the unit's active pool
• [ ] Randomize active pool for next turn
• [ ] Run unit AI
• [ ] Update unit positions if they are moving
• [ ] Send an update to all subscribed users