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qsim is a Go package that lets you build queueing system simulators.

The package provides some building blocks that you can customize and fit together to simulate all kinds of queueing systems, from a grocery store checkout line to a kanban board.

A queueing system in qsim processes arbitrary jobs and is composed of 5 pieces:

  • The arrival process controls how often jobs enter the system.
  • The arrival behavior defines what happens when a new job arrives. When the arrival process generates a new job, the arrival behavior either sends it straight to a processor or appends it to a queue.
  • Queues are simply holding pens for jobs. A system may have many queues associated with different processors.
  • A queueing discipline defines the relationship between queues and processors. It's responsible for choosing the next job to process and assigning that job to a processor.
  • Processors are the entities that remove jobs from the system. A processor may take differing amounts of time to process different jobs. Once a job has been processed, it leaves the queueing system.

To answer questions about a queueing system, we simulate its behavior over a certain number of ticks. We can use callbacks to extract the current system state at any point in the simulation and turn that state into data.

An example: supermarket checkout line.

Suppose you want to model the queueing behavior at a small supermarket with 3 checkout lines. Here's the sort of queueing system you'd create with qsim:

  • Arrival Process: The arrival process is simple. A new job ("shopper") enters the queueing system ("becomes ready for checkout") every n seconds, where n is picked from some probability distribution you define.
  • Arrival Behavior: When a job enters the system, it goes straight to any processor that is idle (i.e. any checkout lane that is empty). If there are no idle processors, the job enters the shortest queue available.
  • Queues: There are 3 queues. At any time they each contain some number of jobs.
  • Queueing Discipline: When a processor finishes a job (a cashier finishes checking a shopper out), the queueing discipline says that the next job in that processor's queue begins processing. Thus the queueing discipline is responsible for keeping track of the 1-to-1 relationship between queues and processors.
  • Processor: There are 3 processors, each of which represents a checkout lane. Each processor takes a certain time to process jobs ("checkout shoppers"), and that processing time is also drawn from a random distribution defined by you.

By putting these building blocks together, you can simulate supermarket checkouts with shocking fidelity. By judiciously placing callbacks, you can answer questions like:

  1. Within how many seconds do 90% of shoppers complete the entire checkout process, from entering the queue to walking out the door?
  2. What happens if a register needs to close for 20 minutes?
  3. How much benefit could be gained from updating the scanners to more modern equipment that can scan items in fewer tries?

If you want to tweak the way the simulation works, all you have to do is modify one of these building blocks.

For an example of this exact simulation, check out:

Another example: porta-potties


Queueing system simulator







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