What is Modularity?
Modularity introduces a new optional repository to Fedora called Modular (often referred to as the "Application Stream" or AppStream for short) that ships additional versions of software on independent life cycles. This enables users to keep their operating system up-to-date while having the right version of an application for their use case, even when the default version in the distribution changes.
Different users have different needs. Developers often want the latest version, and system administrators often want stability for long periods of time. There are many Linux distributions out there, each targeting a different audience. A clear example of this is Fedora and CentOS.
Fedora generally ships the latest stable versions of its component packages when it is released twice per year. That is convenient for desktop users and developers. This can be an issue for Fedora servers because it is sometimes necessary to have a stable version of certain packages for a longer period, mostly because of third-party applications.
At the same time, some people consider Fedora to move too slowly and want even newer software on their system. Some upstreams release their software faster than twice a year, and some users want these versions as they get released.
On the other hand, CentOS targets long-term stability and releases a new version once every few years. This is convenient for server administrators as there are fewer changes over longer periods of time. The issue is that some of the software gets too old for modern applications, and newer versions might be needed.
In other words, it would be convenient to be able to choose some parts of the system to update infrequently, while other parts update at a faster pace.
There are many container images out there. A large portion of them are built manually, not actively maintained, not patched with security fixes, and still used by many people. This is especially true for the ones using software of a different version than the distribution the container image is based on provides.
If Fedora had multiple versions of software that is actively maintained and built, could we use it to produce containers? Could these containers get automatically rebuilt every time the packages get updated?
Fedora contributors maintain their packages in multiple branches, one for each release. This is still required even when the packages are the same across releases. This leads to a series of manual steps associated with the build process.
Could we enable packagers to maintain packages in branches that would follow the package version instead of an arbitrary distribution release version? Having a single branch that builds across multiple releases would save a lot of work for packagers.
This is all possible with modularity!