Converging processes

TL;DR

Completing an order is not an easy process. A payment gateway may not accept payments for some time. A coffee can be spilled all over the book you ordered and a new one needs to be taken from a storage. Failures may occur in different moments of the pipeline, but still your ordered is received. Is it a one process or multiple? If one, does anyone takes every single piece into consideration? How to ensure that eventually a client will get their product shipped?

Process managers and sagas

There are two terms used for these processors/processes. They are process managers and sagas. I don’t want to go through the differences and marketing behind using one or another. I want you to focus on a process that handles an order and that reacts to three events:

  • PaymentTimedOut – occurring when the response for Payment was not delivered before the specific timeout
  • PaymentReceived – the payment was received in time
  • OrderCancelled – the order for which we requested this payment was cancelled

What messages will this process receive and in which order? Before answering, take into consideration:

  • the scheduling system that is used to dispatch timeouts,
  • the external gateway system (that has its own SLA),
  • messaging infrastructure

What’s the order then? Is there a predefined one? For sure there isn’t.

Convergence

Your aim is to provide a convergent process. You should review all the permutations of the process inputs. Being given 3 types of input, you should consider 3! = 6 possibilities. That’s why building a few processes instead of one is easier. You can think of them as sub-state machines that later, can be composed into a bigger whole. This isn’t only a code complexity, but a real mental overhead that you need to design against.

Summary

Designing multiple small processes as sub-state machines is easier. When dealing with many events to react to, try to extract smaller state machines and aggregate them on a higher level.