🔗 Tuning TCP ports for your Elixir app

Elixir is great at handling lots of concurrent connections. When you actually try to do this, however, you will bump up against the default OS configuration which limits the number of open filehandles/sockets. You may also run out of TCP ephemeral ports.

The result is poor application performance, e.g. timeouts. If you are running behind Nginx, you may see it as 503 errors, with your application taking five seconds to respond. When you look at the logs, however, the application response time is fine.

What is happening is that the client talks to Nginx, then Nginx talks to your app, but there are not enough filehandles available, so Nginx queues the request. You may start with 1024 by default, which is pitifully small. You will need to raise that at each step in the config, e.g. systemd unit file, Nginx, and Erlang VM.