Extensions

Extensions are reusable configuration fragments that simplify a rockcraft.yaml file for common application frameworks. An extension configures the parts, services, and other keys needed to build and run the application, while still allowing you to customize the generated configuration in the project file.

The top-level extensions key in rockcraft.yaml defines the extension used by the project. When Rockcraft loads the project, it expands the extension and combines its configuration with your project file.

Ubuntu 26.04 LTS framework extensions

The Flask, Django, FastAPI, Go, and Express framework extensions are stable on their supported bases earlier than Ubuntu 26.04 LTS. The Spring Boot extension remains experimental on its supported bases, and the Ubuntu 26.04 LTS implementations of all six extensions are experimental. To use one of these extensions with Ubuntu 26.04 LTS, set ROCKCRAFT_ENABLE_EXPERIMENTAL_EXTENSIONS=1 when running Rockcraft.

Rockcraft selects the extension implementation from build-base when it is set, or from base otherwise. For a rock with base: bare, build-base is mandatory. For example, base: bare with build-base: ubuntu@24.04 uses the stable extension implementation and doesn’t require the experimental extensions environment variable. For migration guidance, see Change from ubuntu@24.04 to ubuntu@26.04.

On Ubuntu 26.04 LTS, these six extensions place application content under /app and create /app-data for application-generated data. /app-data is owned by _daemon_ and is writable by the application service. Use it as the persistent-at-runtime application data directory, rather than for the application content installed under /app.

/app-data is part of the image’s container filesystem. Its contents don’t persist across container replacement unless a volume is mounted at that path.

• Flask framework
• Django framework
• FastAPI framework
• Go framework
• Express framework
• Spring Boot framework