How to test gen server in erlang?

Question

I am a beginner with erlang, and i write a basic gen server program as follows, I want to know, how to test the server so i can know it works well.

-module(gen_server_test).
-behaviour(gen_server).
-export([start_link/0]).
-export([alloc/0, free/1]).
-export([init/1, handle_call/3, handle_cast/2]).
start_link() ->
    gen_server:start_link({local, gen_server_test}, ch3, [], []).
alloc() ->
    gen_server:call(gen_server_test, alloc).
free(Ch) ->
    gen_server:cast(gen_server_test, {free, Ch}).
init(_Args) ->
    {ok, channels()}.
handle_call(alloc, _From, Chs) ->
    {Ch, Chs2} = alloc(Chs),
    {reply, Ch, Chs2}.
handle_cast({free, Ch}, Chs) ->
    io:format(Ch),
        io:format(Chs),
        Chs2 = free(),
    {noreply, Chs2}.

free() -> 
        io:format("free").
channels() ->
        io:format("channels").
alloc(chs) -> 
        io:format("alloc chs").

BTW: The program can be compiled, and it is not a good program, I just want to print something to make sure it works :)

Answer 1

The beauty of a gen_server implementing module is that it is just a callback module. One need not even have to spawn the underlying gen_server process to test it.

All you need to do is have your test framework (usually eunit) to invoke all the handle_call/cast/info functions by injecting it with different inputs (different gen_server states, different input messages) etc. and ensure it returns the correct response tuple (for eg. {reply, ok, NewState} or {noreply, NewState} etc.)

Ofcourse, this wouldnt work perfectly if your callback functions arn't pure functions. For eg., in your handle_call function, if you are sending a message to another process for eg., or if you are modifying an ets table. In which case, you have to ensure that all the required processes and tables are pre-created before running the test.