What is a gen_event and when would you choose it over gen_server?

gen_event is the OTP behavior for a pub/sub-style event manager: one process holds a list of independent handler callback modules, and any event sent to the manager is dispatched in turn to every registered handler, each with its own private state.

{ok, Pid} = gen_event:start_link(),
gen_event:add_handler(Pid, logger_handler, []),
gen_event:add_handler(Pid, metrics_handler, []),
gen_event:notify(Pid, {order_placed, OrderId}).

Choose gen_event when you have one stream of events that multiple, independently pluggable consumers need to react to — logging, metrics, and alerting handlers all reacting to the same application events, added and removed at runtime without touching the code that raises the events. A plain gen_server is the better fit when there's a single consumer with one coherent piece of state, rather than a fan-out to many independent listeners.

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