How to test gen_server internal state with eunit

Question

Is it possible to inspect the internal state of a gen_server after a callback function has been called? I would rather not change the API of my server here.

Answer 1

You could use sys:get_state/1 which works nicely with all gen's.

Answer 2

Maybe, you would found useful another approach to unit-testing gen_servers. Instead of running gen_server process and testing its behaviour, you can directly test gen_server callbacks and then inspect its state transitions.

For example:

-module(foo_server).

%% Some code skipped

handle_call({do_stuf, Arg}, _From, State) ->
    NewState = modify_state(
    {reply, {stuf_done, Arg}, NewState}.

%% Some code skipped 

-ifdef(TEST)

do_stuf_test_() ->
    {setup,
        fun() ->
            {ok, InitState} = foo_server:init(SomeInitParams),
            InitState
        end,
        fun(State) ->
            ok = foo_server:terminate(shutdown, State)
        end,
        fun(State) ->
            Result = foo_server:handle_call({do_stuf, hello}, undefined, State),
            [
                ?_assertMatch({reply, {stuf_done, hello}, _}, Result)
            ]
        end
    }
}.

-endif.

See discussion of this approach here Also, if you dealing with realy complex states and state transitions, maybe you would be found proper helpful.