Property-Based Testing with PropEr in Erlang/

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Modeling the Circuit Breaker: The Property

Modeling the Circuit Breaker: The Property

Start off with the test module and take a look at the property and the generators of the FSM tests.

We'll cover the following...

The property

Let’s start by looking at the property.

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prop_test() ->
  ?FORALL(Cmds, proper_fsm:commands(?MODULE),
    begin
      {ok, Pid} = circuit_breaker:start_link(),
      {History,State,Result} = proper_fsm:run_commands(?MODULE, Cmds),
      gen_server:stop(Pid, normal, 5000), %(1)
      ?WHENFAIL(io:format("History: ~p\nState: ~p\nResult: ~p\n",
                          [History,State,Result]),
                aggregate(zip(proper_fsm:state_names(History),
                              command_names(Cmds)), 
                          Result =:= ok))
    end).

There will be four generators:

  1. ok/1
  2. tripped/1
  3. blocked/1,
  4. unregistered/1
  • This generator is added because manual calls aren’t available until the circuit breaker’s service id is registered, and this occurs automatically on first use. This peculiarity will be encoded in the state machine itself.

Here we’ll begin the circuit breaker process by hand, which is paired with a call to gen_server:stop/3 at line 6. The circuit breaker module doesn’t expose a callback to terminate the breaker, usually preferring to let a supervision tree do that work. The gen_server behavior, however, exposes functions that let us ...