Packaging Runtimes

As we’ve seen, each Flatpak targets a particular runtime, which provides shared binaries, libraries, and data files, and is mounted at /usr when the Flatpak is run.

All Flatpaks in Fedora target the org.fedora.Platform runtime. Using a single runtime simplifies maintenance and avoids users having to download multiple runtimes. In general, you will not need to modify this runtime when creating an application, since any additional packages you need will be bundled with the application. However, if you find a bug in the runtime and want to help fix it, or want to help participate in maintainance of the Fedora runtimes, then it’s useful to know how runtimes are built.

In addition to the org.fedora.Platform, three other runtimes are built in Fedora infrastructure

Runtimes are defined in a similar way to Flatpaks. The content of the runtimes is defined in the flatpak-runtime module. The different profiles of the module define the different runtimes.

Then the runtimes are built into containers - but since there are multiple containers built for the same runtime, the simplification of having both the module and container definition in a single git repository doesn’t work. Instead the container definitions are found in separate git repositories: flatpaks/flatpak-runtime. flatpaks/flatpak-sdk. flatpaks/flatpak-base-runtime. flatpaks/flatpak-base-sdk. These git repositories must be kept tightly in sync with the runtime module, but don’t need to be modified very often.

The package lists in flatpak-runtime.yaml are maintained with scripts that is also found in the flatpak-runtime module. The idea is that the contents of the runtime should be a superset of the org.gnome.Platform runtime, with the addition of libraries that are frequently used by popular non-GNOME applications, such as qt5 and SDL. For more information about the maintenance scripts, see the README.md in the module.

The module definition for flatpak-runtime has an xmd section that includes extra information that is used at application build time to allow the resulting Flatpak to automatically depend on the right runtime and version without requiring it to be separately specified.