I have a supervisor with N worker processes. As usual the supervisor can send a message to a worker process and there is a handle_cast that sends a reply from a worker to the supervisor.
How can I check that exactly all N workers have replied to the supervisor? Is it possible to implement this with any kind of event handling - i.e. to tell the supervisor "Ok, everyone has replied" and not to make the supervisor to check for the "All N processes have replied" status every second in some kind of ETS child registry table?
If you are talking about an OTP supervisor, no you can't send a message to a worker from it. A supervisor is a very limited behavior with the purpose of starting, monitoring, restarting and stopping processes. Nothing else.
So to solve your particular problem, you would have to have a process that is responsible for sending a message to all workers. This process could also keep a list of all workers in its state, "tick off" (or remove from the list) the workers that have responded. You can achieve this with a list of PIDs, and receiving responses from the processes (or by monitoring the processes with erlang:monitor/2 if they're exiting when they're done) and see who's left.