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

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).

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

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