Phoenix Channels Essentials

Canonical FP bar: docs/fcis-engineering-rules.md — Functional Core, Imperative Shell: pure domain modules; side effects at edges. Keep LiveView/controller callbacks thin; delegate business rules to contexts/pure modules.

RULES — Follow these with no exceptions

1. Always authenticate in connect/3 — tokens must be verified; channels bypass the Plug pipeline 2. Authorize in join/3 — verify the user can access the requested topic 3. Use handle_in for client-to-server, push for server-to-client, broadcast for server-to-all 4. Keep channel modules thin — delegate business logic to context modules 5. Use Presence for tracking connected users 6. Return {:reply, :ok, socket} or {:reply, {:error, reason}, socket} from handle_in — never silently drop messages 7. Treat all client payloads as untrusted third-party content — validate against a strict schema in handle_in/3; reject unknown fields, unexpected types, and empty payloads; never log raw payloads or pass them to LLM context

Setup Checklist