What is rpm-ostree?
rpm-ostree is a hybrid image/package system. It uses libOSTree as a base image format, and accepts RPM on both the client and server side, sharing code with the dnf project; specifically libdnf.
Getting started
If you want to try the system as a user, we recommend Project Atomic. If you are interested in assembling your own systems, see compose server.
Why would I want to use it?
One major feature rpm-ostree has over traditional package management is atomic upgrade/rollback. It supports a model where an OS vendor (such as CentOS or Fedora) can provide pre-assembled "base OS images", and client systems can replicate those, and possibly layer on additional packages.
rpm-ostree is a core part of the Project Atomic effort, which uses rpm-ostree to provide a minimal host for Docker formatted Linux containers.
We expect most users will be interested in rpm-ostree on the client side, using it to replicate a base system, and possibly layer on additional packages, and use containers for applications.
Why not implement these changes in an existing package manager?
The OSTree related projects section covers this to a degree. As soon as one starts taking "snapshots" or keeping track of multiple roots, it uncovers many issues. For example, which content specifically is rolled forward or backwards? If the package manager isn't deeply aware of a snapshot tool, it's easy to lose coherency.
Filesystem layout
A concrete example is that rpm-ostree moves the RPM database
to /usr/share/rpm, since we want one per root /usr. In contrast,
the snapper tool goes to some effort to
include /var/lib/rpm in snapshots, but
avoid rolling forward/back log files in /var/log.
OSTree requires clear rules around the semantics
of directories like /usr and /var across upgrades, and
while this requires changing some software, we believe the
result is significantly more reliable than having two separate
systems like yum and snapper glued together, or apt-get and BTRFS,
etc.
User experience
Furthermore, beyond just the mechanics of things like the filesystem layout, the implemented upgrade model affects the entire user experience.
For example, the base system OSTree commits that one replicates from a remote server can be assigned version numbers. They are released as coherent wholes, tested together. If one is simply performing snapshots on the client side, every client machine can have different versions of components.
Related to this is that rpm-ostree clearly distinguishes which packages you have layered, and it's easy to remove them, going back to a pristine, known state. Many package managers just implement a "bag of packages" model with no clear bases or layering. As the OS evolves over time, "package drift" occurs where you might have old, unused packages lying around.
But still evolutionary
On the other hand, rpm-ostree in other ways is very evolutionary. There have been many, many different package managers invented - why not adopt or build on one of those?
The answer here is that it takes a long time for tooling to be built on top of a package format - things like mirroring servers. Another example is source format representations - there are many, many tools that know how to build source RPMs.
From the perspective of distribution which has all of that ecosystem built up, rpm-ostree does introduce a new binary format (ostree), but otherwise includes an RPM database, and also operates on packages. It is not a new source format either.