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Property-Based Testing with PropEr in Elixir

Modeling the Circuit Breaker: The Test Module

Explore modeling a circuit breaker using state machine properties in PropEr with Elixir. Understand how to define preconditions, update state after commands, and validate postconditions to create robust, reliable tests for real-world stateful systems.

We'll cover the following...

The preconditions

Let’s start off by taking a look at the preconditions.

### Picks whether a command should be valid
def precondition(:unregistered, :ok, _, {:call, _, call, _}) do
  call == :success
end
def precondition(:ok, to, %{errors: n, limit: l}, {:call, _, :err, _}) do
  (to == :tripped and n + 1 == l) or (to == :ok and n + 1 != l)
end
def precondition(
      :ok,
      to,
      %{timeouts: n, limit: l},
      {:call, _, :timeout, _}
    ) do
  (to == :tripped and n + 1 == l) or (to == :ok and n + 1 != l)
end
def precondition(_from, _to, _data, _call) do
  true
end

Notice how both calls to erroneous cases are only valid in mutually exclusive instances:

  • (to ==
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