Extensions

Extensions initialize your project with YAML and Ops catered to supported web frameworks. They each have a corresponding profiles that you use when initializing your project.

• Django framework extension

• Express framework extension

• FastAPI framework extension

• Flask framework extension

• Go framework extension

• Spring Boot framework extension

12-factor extension behavior by base

The Ubuntu base selects the contract that a 12-factor framework extension generates. There is no version key in charmcraft.yaml.

• Ubuntu 22.04 LTS and Ubuntu 24.04 LTS generate the established contract, where the framework supports the base.

• Ubuntu 26.04 LTS generates an experimental contract. It requires CHARMCRAFT_ENABLE_EXPERIMENTAL_EXTENSIONS=1 when Charmcraft expands or packs the project.

The Spring Boot extension is also experimental on Ubuntu 24.04 LTS in both Charmcraft and Rockcraft. The Spring Boot tutorial shows the required settings. Other Charmcraft framework extensions don’t require the environment variable on lower bases.

On Ubuntu 26.04 LTS, all six framework extensions generate the following contract:

Generated field	Ubuntu 22.04 LTS and 24.04 LTS	Ubuntu 26.04 LTS
Charm part plugin	charm	uv
Workload container	Framework-dependent	app
Workload resource	Framework-dependent	app-image
Peer relation	secret-storage with the secret-storage interface	peers with the peers interface
Secret options	A string secret key and a separate secret-ID option	One app-secret-key option of type secret
Optional build input	None	paas-config.yaml file
OAuth, OpenFGA, and tracing libraries	Libraries fetched from Charmhub	PyPI interface packages provided transitively by paas-charm
Cache relations	Redis and Valkey	Valkey

Charmcraft stages paas-config.yaml only when the file exists in an Ubuntu 26.04 LTS project.