12-Factor App Principle #3: Config

Goal

Store all configuration settings outside of the application’s code so you can change behavior without changing the code itself.

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What It Means

Configuration refers to anything that can vary between deployments — for example, database URLs, API keys, or feature flags.

• Store in environment variables – Keep configuration in environment variables instead of hardcoding them in code.
• Environment-specific – Each environment (development, staging, production) can have its own configuration without altering the code.
• No secrets in code – Avoid placing sensitive information like passwords in the source code.

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Why It Matters

• Flexibility – Easily adapt the app to different environments without code changes.
• Security – Protect sensitive data by keeping it out of version control.
• Speed – Faster deployments and updates since config changes don’t require code changes.
• Reduced risk – Minimizes accidental leaks of credentials.

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Example

A Node.js web app:

1. Stores its database connection string in an environment variable DATABASE_URL.
2. In development, DATABASE_URL points to a local database.
3. In production, DATABASE_URL points to a managed cloud database.
4. The code remains the same in both cases — only the environment variable changes.

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Best Practices

• Use environment variables for all configs.
• Keep separate config for each environment.
• Avoid committing config files with secrets to version control.
• Use a centralized config management system for large projects.
• Document all environment variables for easier onboarding.

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Takeaway

Keep configuration separate from code.

This improves security, simplifies deployments, and allows quick environment-specific changes without touching the source code.