---
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 `eval`-based migration commands. Only require S3,
   Redis, and similar credentials when that integration is enabled

## Quick Configuration

### runtime.exs (Essential)

```elixir
if config_env() == :prod do
  database_url = System.get_env("DATABASE_URL") || raise "DATABASE_URL is required"
  secret_key_base = System.get_env("SECRET_KEY_BASE") || raise "SECRET_KEY_BASE is required"
  host = System.get_env("PHX_HOST") || raise "PHX_HOST is required"

  config :my_app, MyApp.Repo,
    url: database_url,
    pool_size: String.to_integer(System.get_env("POOL_SIZE") || "10"),
    ssl: true,
    ssl_opts: [verify: :verify_peer]

  config :my_app, MyAppWeb.Endpoint,
    url: [host: host, port: 443, scheme: "https"],
    http: [ip: {0, 0, 0, 0}, port: String.to_integer(System.get_env("PORT") || "4000")],
    secret_key_base: secret_key_base,
    server: true
end
```

### Guard Optional Services

Keep core boot secrets such as `DATABASE_URL` and `SECRET_KEY_BASE` required.
Gate credentials for optional integrations behind the same feature switch that
enables the integration:

```elixir
s3_config =
  if System.get_env("STORAGE_BACKEND") == "s3" do
    [
      access_key_id:
        System.get_env("S3_ACCESS_KEY") ||
          raise("S3_ACCESS_KEY is required when STORAGE_BACKEND=s3"),
      secret_access_key:
        System.get_env("S3_SECRET_KEY") ||
          raise("S3_SECRET_KEY is required when STORAGE_BACKEND=s3")
    ]
  else
    []
  end

config :my_app, :s3_config, s3_config
```

This lets release tasks that do not use S3 start without S3 credentials while
still failing fast when S3 is selected.

### Health Check Plug

```elixir
def call(%{path_info: ["health", "readiness"]} = conn, _opts) do
  case Ecto.Adapters.SQL.query(MyApp.Repo, "SELECT 1", []) do
    {:ok, _} -> send_resp(conn, 200, ~s({"status":"ok"})) |> halt()
    {:error, _} -> send_resp(conn, 503, ~s({"status":"error"})) |> halt()
  end
end
```

## Quick Decisions

### Platform Choice

| Need | Use |
|------|-----|
| Simple, managed | Fly.io |
| Enterprise, existing K8s | Kubernetes |
| Custom infrastructure | Docker + your orchestrator |

### Resource Limits

| Resource | Recommendation |
|----------|----------------|
| CPU | **NO LIMITS** (BEAM scheduler issues) |
| Memory | Set limits (256Mi-512Mi typical) |
| Graceful shutdown | ≥ 60 seconds |

## Deployment Checklist

- [ ] All secrets from environment variables in runtime.exs
- [ ] Optional service credentials required only when their integration is enabled
- [ ] `server: true` in endpoint config
- [ ] SSL verification for database connections
- [ ] Health endpoints: /health/startup, /health/liveness, /health/readiness
- [ ] Graceful shutdown period ≥ 60 seconds
- [ ] No CPU limits (memory limits only)
- [ ] Migrations in deploy process

## Asset Pipeline Notes

Phoenix 1.8 uses esbuild + tailwind (no Node.js required):

- Config in `config/config.exs` under `:esbuild` and `:tailwind`
- `mix assets.deploy` builds for production
- `mix assets.setup` installs binaries on first run
- Custom JS bundlers: configure in `config/config.exs`

## References

For detailed patterns, see:

- `${CLAUDE_SKILL_DIR}/references/docker-config.md` - Multi-stage Dockerfile, best practices
- `${CLAUDE_SKILL_DIR}/references/flyio-config.md` - fly.toml, clustering, commands