Appstream: The next step

With the release of GNOME-Software 3.12, it’s also time for another update about what is going on behind the scenes of Appstream, the project providing all the data needed to make software-centers work. So here is the long overdue update 🙂

If you have attended my FOSDEM talk or seen the slides, you know about the concept of “component metadata” to describe (almost) all software components which make up a Linux system, as well as their public interfaces they provide for users and other software to access. This metadata specification was originally designed as part of the Listaller project for use with the 3rd-party software installer.
But we already have the Appstream specification, which describes applications available on a distribution. And technically, applications are nothing but specialized components.
When I was asked about extending the Appstream spec with fonts and codecs, I decided to give up the Listaller components concept and merge it with the Appstream project, effectively creating a single source of metadata for all software in distributions.

Appstream and components were separate, because Appstream was designed just for application metadata shown in software-centers, while component-metadata was more technical and not user-visible. This separation, however, did not reflect reality. Software centers want to show things components were designed for (codecs, fonts, …), and application meta-information also can contain technical items not shown in a software-center, but useful for other functionality (e.g. for mimehandler finding). So, if you like clear naming of things (like I do), you can now think of the upcoming Appstream release 0.6 as Appstream not being a stream of application metadata, but rather metadata provided for use in applications. 😉

Extending Appstream with components has a number of benefits:
First of all, users will soon get systems which are able to automatically install missing firmware, codecs, fonts and even libraries or Python-modules. This happens in a distro-agnostic way, so any application can request installations of missing components through Appstream and PackageKit without having to care about distribution details.

For distributors, you will get more information about the software upstream provides: Which public interfaces does it make available? What is the upstream’s homepage? Summary? Short release-descriptions? Exposed library symbols? etc. This data was available before, of course, but we made it machine-readable and standardized now. This will also allow us to match software across distributions quickly, allowing efficient exchange of patches between distributions, and answering questions upstream has like “in which distribuions is my software available, and in which version?”. Tracking upstream software is also easier now. Last but not least, the metadata format gives upstreams a small say in how their software gets packaged (= how it gets split / if there is a metapackage depending on the split parts).

For upstream authors, it is much easier to maintain just a few Appstream XML metafiles, instead of adding a Listaller component file as well (which was not XML). Most data for creating the metainfo files is already available, some can even be filled in automatically by the build-system.

With all the advantages this new system has comes a small disadvantage: These changes broke the backwards-compatibility of the Appstream spec, so to use Appstream data in version 0.6 or higher, you need to adjust your parsers. But of course you are using a library to access the data anyway, which already does that for you 😉 GNOME-Software 3.12 has almost complete support for the new (but still not released) Appstream 0.6 spec through it’s libappstream-glib library. The libappstream library in Appstream’s Git branch will get 0.6 support as well very soon. The only thing you need to do is adjust your code for the new library version, which broke it’s API as well due to a rewrite in C (I hope that we can provide language bindings again very soon).
Soon, there will also be a Qt library for accessing Appstream, since some people expressed a demand for it (and working with GObject in Qt C++ code always feels a litle bit odd).
The 0.6 specification is still being fine-tuned to iron out some issues which might come up and mainly to improve the documentation and adjust all tools. It will be released in 2-3 weeks, and bring not only components support, but also lots of other small improvements and additions for better description of applications/components. In some cases, tag names have changed to have less ambiguous names.

Thanks to Richard Hughes for helping with the spec – our discussions about the Appstream spec are highly productive and in the end lead to a better specification. Also, the new 0.6 specification includes many extensions used in GNOME-Software by default now, which makes it easier for other distributions to provide them for GS.

The current draft for the Appstream 0.6 specifications can be found here. Some details might still change before the final release. As soon as the spec is final, there will be a ML announcement for distributors as well (watch the XDG distributions list), so there is enough time for distributors to future-proof their Appstream distro-data generators (I will also describe the changes more detailed then).

If you are confused now about the whole components stuff: Fear not! 😉 I will do some more detailed posts in a few weeks explaining upstream projects how to provide metadata properly (after the 0.6 release). I will also finish the wiki page for Appstream AppData in KDE projects, where we can then use 0.6-style data in KDE right away.