Property-Based Testing with PropEr, Erlang, and Elixir

Modeling the Circuit Breaker

For stateful properties, we’d initially started our test suite by defining generators. We then needed to refine them when it came to writing the command generation, and ended up writing a shim module. This time around, we’re going to start directly with the shim module. It’s a good opportunity to revisit all kinds of possibile calls that can take place.

Since we’re testing the circuit breaker itself, we’ll want to cover these:

• Successful calls and their effect on the internal state of the breaker
• Calls ending in errors
• Calls ending in errors that are whitelisted, and therefore seen as successful
• Calls timing out
• Manual operations to trip and untrip the circuit breaker

Our first step is to ensure the libraries we need ...