NOTE: This question has been ported over from Programmers since it appears to be more appropriate here given the limitation of the language I'm using (VBA), the availability of appropriate tags here and the specificity of the problem (on the inference that Programmers addresses more theoretical Computer Science questions).
I'm attempting to build a Discrete Event Simulation library by following this tutorial and fleshing it out. I am limited to using VBA, so "just switch to [insert language here] and it's easy!" is unfortunately not possible. I have specifically chosen to implement this in Access VBA to have a convenient location to store configuration information and metrics.
How should I handle logging metrics in my Discrete Event Simulation engine?
If you don't want/need background, skip to The Design or The Question section below...
Simulation
The goal of a simulation of the type in question is to model a process to perform analysis of it that wouldn't be feasible or cost-effective in reality.
The canonical example of a simulation of this kind is a Bank:
- Customers enter the bank and get in line with a statistically distributed frequency
- Tellers are available to handle customers from the front of the line one by one taking an amount of time with a modelable distribution
- As the line grows longer, the number of tellers available may have to be increased or decreased based on business rules
You can break this down into generic objects:
- Entity: These would be the customers
- Generator: This object generates Entities according to a distribution
- Queue: This object represents the line at the bank. They find much real world use in acting as a buffer between a source of customers and a limited service.
- Activity: This is a representation of the work done by a teller. It generally processes Entities from a Queue
Discrete Event Simulation
Instead of a continuous tick by tick simulation such as one might do with physical systems, a "Discrete Event" Simulation is a recognition that in many systems only critical events require process and the rest of the time nothing important to the state of the system is happening.
In the case of the Bank, critical events might be a customer entering the line, a teller becoming available, the manager deciding whether or not to open a new teller window, etc.
In a Discrete Event Simulation, the flow of time is kept by maintaining a Priority Queue of Events instead of an explicit clock. Time is incremented by popping the next event in chronological order (the minimum event time) off the queue and processing as necessary.
The Design
I've got a Priority Queue implemented as a Min Heap for now.
In order for the objects of the simulation to be processed as events, they implement an ISimulationEvent interface that provides an EventTime property and an Execute method. Those together mean the Priority Queue can schedule the events, then Execute them one at a time in the correct order and increment the simulation clock appropriately.
The simulation engine is a basic event loop that pops the next event and Executes it until there are none left. An event can reschedule itself to occur again or allow itself to go idle. For example, when a Generator is Executed it creates an Entity and then reschedules itself for the generation of the next Entity at some point in the future.
The Question
How should I handle logging metrics in my Discrete Event Simulation engine?
In the midst of this simulation, it is necessary to take metrics. How long are Entities waiting in the Queue? How many Acitivity resources are being utilized at any one point? How many Entities were generated since the last metrics were logged?
It follows logically that the metric logging should be scheduled as an event to take place every few units of time in the simulation.
The difficulty is that this ends up being a cross-cutting concern: metrics may need to be taken of Generators or Queues or Activities or even Entities. Consider also that it might be necessary to take derivative calculated metrics: e.g. measure a, b, c, and ((a-c)/100) + Log(b).
I'm thinking there are a few main ways to go:
Have a single, global Stats object that is aware of all of the simulation objects. Have the Generator/Queue/Activity/Entity objects store their properties in an associative array so that they can be referred to at runtime (VBA doesn't support much in the way of reflection). This way the statistics can be attached as needed
Stats.AddStats(Object, Properties). This wouldn't support calculated metrics easily unless they are built into each object class as properties somehow.Have a single, global Stats object that is aware of all of the simulation objects. Create some sort of ISimStats interface for the Generator/Queue/Activity/Entity classes to implement that returns an associative array of the important stats for that particular object. This would also allow runtime attachment,
Stats.AddStats(ISimStats). The calculated metrics would have to be hardcoded in the straightforward implementation of this option.Have multiple Stats objects, one per Generator/Queue/Activity/Entity as a child object. This might make it easier to implement simulation object-specific calculated metrics, but clogs up the Priority Queue a little bit with extra things to schedule. It might also cause tighter coupling, which is bad :(.
Some combination of the above or completely different solution I haven't thought of?
Let me know if I can provide more (or less) detail to clarify my question!
