What is a behavior in Erlang?

A behavior is a formalized process pattern: OTP defines the generic, reusable parts (the message loop, error handling, standard API), and you supply a callback module that fills in the domain-specific parts. It's conceptually close to an interface plus a template method pattern from OOP.

-module(my_server).
-behaviour(gen_server).

init(Args) -> {ok, Args}.
handle_call(_Req, _From, State) -> {reply, ok, State}.

The -behaviour(gen_server) attribute tells the compiler which callbacks your module must export, and it will warn you at compile time if one is missing. The built-in behaviors are gen_server, gen_statem, gen_event, and supervisor, though you can also define custom behaviors for your own reusable patterns.

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