Zapier is an automation platform. I’d give you extra points for wanting to have a complete paper trail in a similar project! Not criticizing your interviewers because I don’t have all the context but with the info I have, I’d say they judged you wrongly here.

And no, don’t use too much external technology – especially one as powerful and complex to set up as Kafka – in interview assignments. Keep it simple. You did well for choosing to roll your own event queue / modification log.

Metrics are good, but if the collection of the metrics cause excessive performance penalties it should be reworked, if everything has to hit the DB all the time, the DB will soon turn into the bottleneck of your application, there is a reason why in every other stack Redis/RabbitMQ/Kafka/Etc are used for queues, and that’s because they are optimized for it in a way that traditional DBs are not.

In the BEAM world you can get pretty far with OTP without resorting to external services, I personally would use a GenServer, and unless strictly necessary to log every single event, (think financial industries) I would aggregate and log them by batch in a reasonable interval.

That’s not very specific, can you give us a clue how did you go about it and what was not liked by the interviewers first?