config

Elixir Configuration

Guide for proper application configuration in Elixir, with emphasis on understanding and correctly using runtime vs compile-time configuration.

Critical Principle

Runtime configuration is the preferred approach. Only use compile-time configuration when values must affect compilation itself.

Configuration Files

config/config.exs (Compile-Time)

Evaluated during project compilation, before your application starts.

import Config

# Basic configuration
config :my_app, MyApp.Repo,
  database: "my_app_dev",
  username: "postgres",
  password: "postgres",
  hostname: "localhost"

# Environment-specific config
config :my_app,
  environment: config_env()

# Import environment-specific config files
import_config "#{config_env()}.exs"

Key characteristics:

• Runs at compile time
• Uses import Config (not use Mix.Config)
• Can use config_env() and config_target()
• Can import other config files with import_config/1
• Deep-merges keyword lists
• Library config.exs is NOT evaluated when used as a dependency

config/runtime.exs (Runtime)

Evaluated right before applications start in both Mix and releases.

import Config

# Read from environment variables
config :my_app, MyApp.Repo,
  database: System.get_env("DATABASE_NAME") || "my_app_dev",
  username: System.get_env("DATABASE_USER") || "postgres",
  password: System.get_env("DATABASE_PASSWORD") || "postgres",
  hostname: System.get_env("DATABASE_HOST") || "localhost",
  pool_size: String.to_integer(System.get_env("POOL_SIZE") || "10")

# Conditional runtime configuration
if config_env() == :prod do
  config :my_app, MyAppWeb.Endpoint,
    secret_key_base: System.fetch_env!("SECRET_KEY_BASE"),
    http: [port: String.to_integer(System.fetch_env!("PORT"))]
end

Key characteristics:

• Runs at application startup (both dev and prod)
• Executes in both Mix projects and releases
• Used for environment variables and runtime values
• Does NOT support import_config/1
• Can use System.get_env and System.fetch_env!