Modeling the Circuit Breaker: The Test Module

The preconditions

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

%% Picks whether a command should be valid under the current state.
precondition(unregistered, ok, _, {call, _, Call, _}) ->
    Call =:= success;
precondition(ok, To, #data{errors=N, limit=L}, {call,_,err,_}) ->
    (To =:= tripped andalso N+1 =:= L) orelse (To =:= ok andalso N+1 =/= L);
precondition(ok, To, #data{timeouts=N, limit=L}, {call,_,timeout,_}) ->
    (To =:= tripped andalso N+1 =:= L) orelse (To =:= ok andalso N+1 =/= L);
precondition(_From, _To, _Data, _Call) ->
    true.

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

• (To =:=

...