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