Elixir Configuration

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

When to Activate

Use this skill when:

• Setting up or modifying application configuration
• Choosing between config.exs and runtime.exs
• Deciding between Application.compile_env and Application.get_env
• Debugging configuration-related issues
• Working with releases or deployment configuration
• Migrating from use Mix.Config to import Config
• Writing libraries that need 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
• Perfect for environment variables and runtime values
• Does NOT support import_config/1
• Can use System.get_env and System.fetch_env!

config/dev.exs, config/test.exs, config/prod.exs

Environment-specific compile-time configuration, typically imported from config.exs:

# config/config.exs
import_config "#{config_env()}.exs"

# config/dev.exs
import Config

config :my_app, MyApp.Repo,
  show_sensitive_data_on_connection_error: true,
  pool_size: 10

# config/test.exs
import Config

config :my_app, MyApp.Repo,
  pool: Ecto.Adapters.SQL.Sandbox,
  pool_size: 10

# config/prod.exs
import Config

# Production-specific compile-time config only
config :my_app, MyAppWeb.Endpoint,
  cache_static_manifest: "priv/static/cache_manifest.json"

Accessing Configuration

Runtime Access (Preferred)

Use in function bodies to read configuration at runtime:

Application.get_env/3

defmodule MyApp.Service do
  def start_link do
    # Get with default value
    timeout = Application.get_env(:my_app, :timeout, 5000)
    GenServer.start_link(__MODULE__, timeout, name: __MODULE__)
  end
end

When to use:

• Reading config in function bodies (most common)
• When a sensible default exists
• When config might change between environments

Application.fetch_env!/2

defmodule MyApp.Mailer do
  def deliver(email) do
    # Raise if not configured (for required config)
    api_key = Application.fetch_env!(:my_app, :mailgun_api_key)
    send_email(email, api_key)
  end
end

When to use:

• Required configuration that must exist
• When you want explicit errors for missing config
• When no sensible default exists

Application.fetch_env/2

defmodule MyApp.Cache do
  def get(key) do
    case Application.fetch_env(:my_app, :cache_adapter) do
      {:ok, adapter} -> adapter.get(key)
      :error -> nil  # No caching configured
    end
  end
end

When to use:

• Optional configuration
• When you need pattern matching on result
• When absence of config is a valid state

Compile-Time Access (Use Sparingly)

Use only when configuration must affect compilation:

Application.compile_env/3

defmodule MyApp.JSONEncoder do
  # Only use compile_env when the value affects compilation
  @json_library Application.compile_env(:my_app, :json_library, Jason)

  def encode(data) do
    # The specific library is compiled into the module
    @json_library.encode(data)
  end
end

When to use:

• Configuration affects which code gets compiled
• Performance-critical paths where indirection is costly
• Compile-time optimizations or code generation

Warning: Mix tracks compile-time config and raises errors if values diverge between compile and runtime.

Application.compile_env!/2

defmodule MyApp.Adapter do
  # Raises at compile time if not configured
  @adapter Application.compile_env!(:my_app, :storage_adapter)

  def store(data) do
    @adapter.put(data)
  end
end

When to use:

• Required compile-time configuration
• Adapters or behaviors selected at compile time

Common Patterns

Pattern 1: Environment Variables in Runtime

Correct approach:

# config/runtime.exs
import Config

config :my_app,
  api_url: System.get_env("API_URL") || "http://localhost:4000",
  api_key: System.fetch_env!("API_KEY")  # Required in production

Access in code:

defmodule MyApp.Client do
  def call(endpoint) do
    api_url = Application.fetch_env!(:my_app, :api_url)
    api_key = Application.fetch_env!(:my_app, :api_key)
    HTTPoison.get("#{api_url}/#{endpoint}", [{"Authorization", api_key}])
  end
end

Pattern 2: Development vs Production Config

config/config.exs:

import Config

# Shared configuration for all environments
config :my_app, :shared_setting, "value"

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

config/dev.exs:

import Config

config :my_app, MyApp.Repo,
  database: "my_app_dev",
  show_sensitive_data_on_connection_error: true

config/runtime.exs:

import Config

# Runtime config for all environments
if config_env() == :prod do
  # Production-specific runtime config
  database_url = System.fetch_env!("DATABASE_URL")

  config :my_app, MyApp.Repo,
    url: database_url,
    pool_size: String.to_integer(System.get_env("POOL_SIZE") || "10")
end

Pattern 3: Storing config_env() for Runtime Access

Problem: Can't call config_env() at runtime.

Solution: Store it in config:

# config/config.exs
import Config

config :my_app, :environment, config_env()

# Then in your code:
defmodule MyApp do
  def environment do
    Application.fetch_env!(:my_app, :environment)
  end

  def development? do
    environment() == :dev
  end
end

Pattern 4: Optional Features Based on Config

defmodule MyApp.Telemetry do
  def setup do
    case Application.fetch_env(:my_app, :telemetry_backend) do
      {:ok, :datadog} -> setup_datadog()
      {:ok, :prometheus} -> setup_prometheus()
      :error -> :ok  # Telemetry disabled
    end
  end
end

Pattern 5: Child Spec with Runtime Config

defmodule MyApp.Application do
  use Application

  def start(_type, _args) do
    children = [
      MyApp.Repo,
      {MyApp.Worker, Application.fetch_env!(:my_app, :worker_opts)},
      MyAppWeb.Endpoint
    ]

    opts = [strategy: :one_for_one, name: MyApp.Supervisor]
    Supervisor.start_link(children, opts)
  end
end

Anti-Patterns to Avoid

❌ Using compile_env for Runtime Values

# DON'T: Using compile_env for environment variables
defmodule MyApp.Service do
  @api_key Application.compile_env(:my_app, :api_key)

  def call do
    # This won't work correctly in releases!
    HTTPoison.get(url, [{"Authorization", @api_key}])
  end
end

Why it's wrong: Environment variables aren't available at compile time in releases.

Correct approach:

defmodule MyApp.Service do
  def call do
    # Read at runtime
    api_key = Application.fetch_env!(:my_app, :api_key)
    HTTPoison.get(url, [{"Authorization", api_key}])
  end
end

❌ Reading Other Application's Config

# DON'T: Directly access other app's configuration
defmodule MyApp do
  def logger_level do
    Application.get_env(:logger, :level)  # Fragile coupling
  end
end

Why it's wrong: Creates tight coupling and breaks encapsulation.

Correct approach:

# Configure it in your own app
# config/config.exs
config :my_app, :log_level, :info

# Then read your own config
defmodule MyApp do
  def log_level do
    Application.get_env(:my_app, :log_level, :info)
  end
end

❌ Using Application Config in Libraries

# DON'T: In a library
defmodule MyLibrary do
  def process(data) do
    # Library reading its own application environment
    timeout = Application.get_env(:my_library, :timeout, 5000)
    do_work(data, timeout)
  end
end

Why it's wrong: Library config.exs is not evaluated when used as a dependency.

Correct approach:

# DO: Accept options as arguments
defmodule MyLibrary do
  def process(data, opts \\ []) do
    timeout = Keyword.get(opts, :timeout, 5000)
    do_work(data, timeout)
  end
end

# Users configure in their application
defmodule MyApp.Worker do
  def run do
    opts = Application.get_env(:my_app, :my_library_opts, [])
    MyLibrary.process(data, opts)
  end
end

❌ Using Mix Module in Application Code

# DON'T: Use Mix.env() in application code
defmodule MyApp do
  def environment do
    Mix.env()  # Won't work in releases!
  end
end

Why it's wrong: Mix is not available in production releases.

Correct approach:

# Store it in config
# config/config.exs
config :my_app, :environment, config_env()

# Access from application environment
defmodule MyApp do
  def environment do
    Application.fetch_env!(:my_app, :environment)
  end
end

Config Functions Reference

In Configuration Files

Function	Description	Where to Use
config/2	Configure app with keyword list	All config files
config/3	Configure app key with value	All config files
config_env/0	Get current environment (:dev, :test, :prod)	All config files
config_target/0	Get build target	All config files
import_config/1	Import other config files	Not in runtime.exs

In Application Code

Function	Return Type	Use Case
Application.get_env/3	value | default	Runtime with default
Application.fetch_env/2	{:ok, value} | :error	Runtime with pattern matching
Application.fetch_env!/2	value (raises if missing)	Required runtime config
Application.compile_env/3	value	Compile-time with default
Application.compile_env!/2	value (raises if missing)	Required compile-time config

Migration Guide

From use Mix.Config to import Config

Old (deprecated):

use Mix.Config

config :my_app, :key, "value"

if Mix.env() == :prod do
  config :my_app, :production, true
end

import_config "#{Mix.env()}.exs"

New:

import Config

config :my_app, :key, "value"

if config_env() == :prod do
  config :my_app, :production, true
end

import_config "#{config_env()}.exs"

Changes:

1. Replace use Mix.Config with import Config
2. Replace Mix.env() with config_env()
3. Remove wildcard imports (not supported)

Moving Runtime Config to runtime.exs

Before (all in config.exs):

# config/config.exs
import Config

config :my_app,
  api_key: System.get_env("API_KEY"),  # Wrong place!
  static_value: "something"

After (split correctly):

# config/config.exs
import Config

config :my_app,
  static_value: "something"

# config/runtime.exs
import Config

config :my_app,
  api_key: System.get_env("API_KEY") || raise("API_KEY not set")

Best Practices Summary

1. Default to Runtime Configuration: Use Application.get_env/3 in function bodies
2. Use runtime.exs for Environment Variables: Never read env vars in config.exs
3. Use compile_env Only When Necessary: Only when config affects compilation
4. Libraries Should Not Use Application Config: Accept options as function arguments
5. Never Use Mix in Application Code: Use config_env() in config files, store result
6. Validate Required Config Early: Use fetch_env!/2 in application start for required values
7. Provide Sensible Defaults: Use get_env/3 with defaults for optional config
8. Document Configuration: Add comments explaining what each config key does
9. Use runtime.exs for Releases: Essential for Elixir releases and deployments
10. Store config_env() for Runtime Use: Can't call config_env() outside config files

Debugging Configuration

Check Current Configuration

# In IEx
Application.get_all_env(:my_app)

# Check specific key
Application.fetch_env(:my_app, :some_key)

# See all applications
Application.loaded_applications()

Common Issues

Problem: Config not available in tests

# config/test.exs
import Config

config :my_app, :test_value, "configured"

Problem: Different values in dev vs release

Check that runtime.exs is being used and environment variables are set correctly.

Problem: Compile-time config not updating

# Clean and recompile
mix clean
mix compile

Resources

• Config Module Docs: https://hexdocs.pm/elixir/Config.html
• Application Module Docs: https://hexdocs.pm/elixir/Application.html
• Runtime Configuration Guide: https://hexdocs.pm/mix/Mix.Tasks.Release.html#module-runtime-configuration

Key Insights

"Reading the application environment at runtime is the preferred approach."

"If you are writing a library to be used by other developers, it is generally recommended to avoid the application environment, as the application environment is effectively a global storage."

"config/config of a library is not evaluated when the library is used as a dependency, as configuration is always meant to configure the current project."

Configuration is a cross-cutting concern. Default to runtime configuration with Application.get_env/3, and only reach for compile-time configuration when you have a specific need for it that justifies the trade-offs.