A Kamal Tip: Dummy Values for Smooth Deploys

Quick detour from our notifications journey to share a handy Kamal tip!

Have you ever encountered an error during deployment because of a missing environment variable, even though it’s set correctly? The issue might stem from how Kamal handles environment variables during its deployment process.

Here’s what happens when Kamal deploys:

1. Creates an image for your application.
2. Uploads the image to your repository (e.g., DockerHub).
3. Downloads the image on the server(s).
4. Boots new containers with the environment variables applied.

The Problem: Environment variables are only included in Step #4. If your application relies on ENV.fetch('FOOBAR') during the image-building process (like asset precompilation), it will fail in Step #1 because those variables aren’t available yet.

The Solution: Provide dummy values for the required environment variables during the build process. Here’s an example:

# Add environment variables needed to boot the application
RUN SECRET_KEY_BASE_DUMMY=1 \
	FOOBAR=123abc \
	./bin/rails assets:precompile

This ensures your application is properly configured without errors during asset precompilation, giving you a smoother deployment process.

Hope this helps! Let me know if you’ve run into other Kamal quirks.