Alexander Wellbrock 5ed0834144 docs/compose: add a introductory section

This section is meant to give a broad overview of this doc.

First introducing the tool in general. Then showing the distinction
between 'compose tree' and the granular approach. Finally transitioning
over to the next section.

2021-03-24 17:05:58 -04:00

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Compose server

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TOC {:toc}

Managing RPM based OSTree commits

With rpm-ostree compose you get a tool to compose your own ostree commits based on a treefile configuration, a couple of RPMs, some post-processing and possibly some custom modifications directly in the resulting tree.

The tool allows to either build a tree commit in one go with a single command: rpm-ostree compose tree. Or to split that process up into smaller chunks with the usage of rpm-ostree compose install, followed by rpm-ostree compose postprocess and finally rpm-ostree compose commit. While the former approach is pretty complete and allows most use-cases the latter is useful if you need some more customization on the resulting filesystem. More customization than the sandboxed post-process functionality of the treefile allows.

In most scenarios you'll want to consider using a more "high level" tool, than rpm-ostree compose.

Using higher level build tooling

Originally rpm-ostree compose tree was intended to be a "high level" tool, but that didn't work out very well in practice. Today, you should consider it as a low level tool. For example, most people that want to generate OSTree commits also want to generate bootable disk images, and rpm-ostree has nothing to do with that.

One example higher level tool that takes care of both OSTree commit generation and bootable disk images is coreos-assembler; it is strongly oriented towards "CoreOS-like" systems which include rpm-ostree and Ignition.

The osbuild project has some support for rpm-ostree based systems. See this blog entry for example.

Background on managing an OSTree repository

Before you get started, it's recommended to read (at least) these two sections of the OSTree manual:

Choosing a base config

Currently, rpm-ostree is fairly coupled to the Fedora project. We are open to supporting other distributions however.

Example base rpm-ostree "manifest repositories" are:

Running `rpm-ostree compose tree`

This program takes as input a manifest file that describes the target system, and commits the result to an OSTree repository.

The input format is a YAML (or JSON) "treefile".

If you're doing this multiple times, it's strongly recommended to create a cache directory:

# rpm-ostree compose tree --unified-core --cachedir=cache --repo=/srv/repo /path/to/manifest.yaml

This will download RPMs from the referenced repos, and commit the result to the OSTree repository, using the ref named by ref.

Once we have that commit, let's export it:

# ostree --repo=repo pull-local build-repo exampleos/8/x86_64/stable

You can tell client systems to rebase to it by combining ostree remote add, and rpm-ostree rebase on the client side.

Generating OSTree commits in a container

rpm-ostree compose tree runs well in an unprivileged (or "run as root") podman container. You can also use other container tools, they are just less frequently tested.

You can also directly install rpm-ostree on a traditional yum/rpm based virtual (or physical) machine - it won't affect your host. However, containers are encouraged.

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