How can i change the callback module of a gen_server in erlang? (gen_server:swap_handler)

Question

I am building an app which can run in two modes. A sandbox mode and a production one.

In sandbox mode, i want to make many checks in my gen_server against the database : if table doesn't exist then create it ; if column doesn't exist then add it ; if column type doesn't allow the value i want to store then change it, etc.

In production mode, if a tables does not exist or a column does not match the type of the value, it will fail and that is ok.

So, in order to avoid cumbersome code like "case State#state.is_sandbox of true -> ... ", i would like to have two different modules for my gen_server, and i would like to change the current module either in handle_call or handle_info.

Actually, i just want to go from sandbox to production, but i think if it works this way, it could work backwards.

Thanks.

Answer 1

You can add module, which is a name of a module, to the state in gen_server. Then you will need 2 modules - sandbox and production that both implement the same functions (you could create a behaviour for it).

The gen_server callbacks will call module:function which will be a function either from sandbox or production module. The module can be set in init function of the gen_server, to change it, simply add a new function(s) to the gen_server:

use_production() ->
    gen_server:cast(production).

....

handle_cast(production, State) ->
    {noreply, State#state{module = production}).

The same for the sandbox module.

An example of a gen_server's callback with the module:

handle_call(Msg, _From, #state{module = Module} = State) ->
    Module:function(Msq),
    {reply, ok, State}.

The function must be implemented in both sandbox and production modules.

Answer 2

You can get module name using os:getenv/1 (of course you have to set different names in different environments before that)

Answer 3

You could use a gen_event with a single handler instead, which allows you to return a swap_handler tuple (see gen_event/handle_*)

Also, you don't have to use case statements in the gen_server model. If your state contains the sandbox variable, you can define different clauses for your callback functions by binding the sandbox value in the header. For instance:

handle_call(do_stuff, _From, State = #state{sandbox = true}) ->
    do_sandbox_stuff();
handle_call(do_stuff, _From, State) ->
    do_nonsandbox_stuff().

In this setup erlang automatically chooses the correct clause to fire based on the value of the sandbox variable, without you having to define a separate handler or use a case statement. Binding variables in function clauses this way is also good practice for efficiency (since the variables are bound outside of the body of the function, the binding process is done in the scheduler and, as a result, does not count against the function's execution time, whereas all matching is done inside the function body in a case)

Answer 4

Instead of a gen_server you could use gen_fsm, a finite state machine, which handles this case very easily. You just have multiple states which call functions in different modules depending on the state. It basically does all the handling for you, without the need to carry an explicit state parameter. Which is basically implementing an FSM by hand.