Elixir/Phoenix Deployment Reference

Quick reference for deploying Elixir/Phoenix applications.

Iron Laws — Never Violate These

• 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

• Graceful shutdown ≥ 60 seconds — Shorter timeouts kill in-flight requests and WebSocket connections mid-operation, causing data loss for users

• Health checks required — Without startup/liveness/readiness endpoints, orchestrators can't distinguish a booting node from a dead one, leading to cascading restarts

• SSL verification for database — Skipping verify: :verify_peer allows MITM attacks between your app and database; production data traverses the connection

• 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

Quick Configuration

runtime.exs (Essential)

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

Health Check Plug

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

• 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:

• references/docker-config.md

• Multi-stage Dockerfile, best practices

• references/flyio-config.md

• fly.toml, clustering, commands