Question

I want to extend gen_server (create a gen_server_extra) with some additional functionality. The requirements are:

1. The gen_server_extra processes should behave like a regular gen_server‘s. E.g, they should accept calls via gen_server:call, integrate with SASL, fit OTC supervision tree, etc.
2. gen_server_extra processes should have an additional functionality, provided by gen_server_extra. That basically means some of the messages will be handled by gen_server_extra code, without passing them to the callback module. The rest of the messages are passed to callback module as is.
3. gen_server_extra functionality requires its own state which should be hidden from the callback module.

What is the simplest approach to do that?

Answer 1

The best, most modular approach would be to implement a new behavior in a module (e.g. gen_ext_server) and wrap the gen_server behavior from there.

First, make sure your behavior is identical to gen_server:

-module(gen_ext_server).
-behavior(gen_server).

% Exports...

behaviour_info(Type) -> gen_server:behaviour_info(Type).

Implement all callbacks needed for gen_server, keep the name of the callback module that implements your behavior in your state:

init([Mod|ExtraArgs]) ->
    % ...
    ModState = Mod:init(ExtraArgs),
    #state{mod = Mod, mod_state = ModState, internal = [...]}

Then, in each gen_server callback, implement your behavior and then call the callback module if needed:

handle_call(internal, _From, State) ->
    % Do internal stuff...
    {reply, ok, State};
handle_call(Normal, From, State = #state{mod = Mod, mod_state = ModState}) ->
    case Mod:handle_call(Normal, From, ModState) of
        {reply, Reply, NewState} ->
            {reply, Reply, #state{mod_state = NewState};
        ... ->
            ...
    end.

Implement similar functionality for handle_cast/2, handle_info/2, terminate/1 etc.