#snappy #snapcraft #development

Broader use of bases for snaps

8 March 2019

We have regular Engineering Sprints at Canonical. These are a great opportunity for engineering teams to come together and discuss progress and roadmaps, as well as code and work together. Many of us have just returned from one such event in Malta, getting home just before the widely report storm hit the island (see Destruction across Malta as gale-force winds batter islands).

And one of the things we discussed over the course of that week was the implications of using bases when snaps use type other than app (a base is a special kind of snap that provides a minimal set of libraries common to most applications).

Bases for more than apps

So far, the only place developers have been using bases is when they declare type: app in their snap.yaml, and it’s also the implicit snap type when nothing is declared.

But from snapcraft’s point of view, the base keyword can also be used with snap types of gadget and kernel, and these would then work internally inside snapd.

Discussions at the sprint the following initial questions about using base with gadget and kernel snaps:

  • Is it useful?
  • How would everything connect, internally, inside snapd.
  • How would it affect the runtime? (as a kernel or gadget snap could be userspace independent, if all the potential hooks they used were architecture independent and would require a base for the cases, it would be relevant.

As a result of these questions, for cases where a base is not required, we are introducing a new base called none. From a runtime perspective, this will is a real base but is has nothing in it.

But what will happen at build time? Where are builds supposed to take place? Where do we build some other snap types we haven’t mentioned before, such as base or snapd?

To solve the above, we are also introducing a new keyword, build-base. This takes a value, such as core18, which becomes the build environment used to create snaps targeting, for example, core18.

Status of the core16 base

With the release of Snapcraft 3.0, all new features are now tied to the use of the base keyword. If base isn’t specified, your snap will build just as it used to, only without the new features. This way, we don’t force anyone to use or migrate to our new features and can maintain a only single version of snapcraft, regardless of whether you’re using bases or do not.

We originally planned to have a core16 base snap to support a 16.04 base with the release of Snapcraft 3.0. However, the release of core16 is currently behind schedule.

To help mitigate the effects of this delay, we’re going to activate a slight trick in snapcraft. The trick is that snapcraft will soon allow specifying core as a base but it will write out a snap.yaml with no base. This means that core essentially is a base internally - it’s just treated differently because it’s coupled with snapd (which, with the core16 work, has also been stripped out into its own snap).


Here’s what you’ll be able to do with the upcoming releases of snapcraft:

  • Use none as a base (requires work in snapd as well).
  • Specify a build-base when using base: none.
  • Use core as a base in snapcraft.yaml.

And you can already use the base keyword for snaps of type app, kernel and gadget.