OTP Essentials

Use this skill before writing ANY GenServer, Supervisor, Task, or Agent module.

RULES — Follow these with no exceptions

1. Always use @impl true before GenServer/Agent callbacks (init, handle_call, handle_cast, handle_info, terminate)
2. Keep init/1 fast — no blocking calls, no DB queries; use handle_continue for expensive setup
3. Use GenServer.call for request/response, GenServer.cast for fire-and-forget — never cast when you need a result
4. Always define a public API wrapping GenServer calls — callers should never use GenServer.call(pid, ...) directly
5. Use Task.async/Task.await with bounded timeouts — never Task.async without a corresponding Task.await or Task.yield
6. Name processes via Registry, not atoms — atom table is finite and never garbage collected
7. Supervisors own process lifecycle — never start unsupervised long-running processes
8. Handle :DOWN messages from monitored processes — don't let them go unhandled
9. Use Task.Supervisor for fire-and-forget supervised work
10. Prefer ETS over a bottleneck GenServer for shared read-heavy state — one GenServer serializes all access