deploy

name	deploy
description	Elixir/Phoenix deployment patterns — Dockerfile, fly.toml, runtime.exs, mix release, rel/ overlays. Use when configuring Fly.io, Docker, CI/CD, health checks, or production migrations.
effort	medium
paths	["config/runtime.exs","Dockerfile","fly.toml","rel/**/*"]

Elixir/Phoenix Deployment Reference

Quick reference for deploying Elixir/Phoenix applications.

Iron Laws — Never Violate These

1. Config at runtime, not compile time — Secrets in config.exs get baked into the release binary. Use runtime.exs with env vars so secrets are resolved at boot
2. Graceful shutdown ≥ 60 seconds — Shorter timeouts kill in-flight requests and WebSocket connections mid-operation, causing data loss for users
3. Health checks required — Without startup/liveness/readiness endpoints, orchestrators can't distinguish a booting node from a dead one, leading to cascading restarts
4. SSL verification for database — Skipping verify: :verify_peer allows MITM attacks between your app and database; production data traverses the connection
5. No CPU limits — The BEAM scheduler assumes it owns all cores; cgroups CPU limits cause scheduler collapse where the VM thinks it has more cores than it can use, leading to latency spikes
6. Guard optional service credentials — runtime.exs runs whenever a release boots, including -based migration commands. Only require S3, Redis, and similar credentials when that integration is enabled