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CoreOS Assembler Design

This document describes some of the current high level design of the project. It assumes some familiarity with the primary

  1. Builds
  2. Change detection
  3. Managing data


coreos-assembler operates on a “build directory”, which can contain multiple builds. A build is a pairing of an OSTree commit (stored as *-ostree.tar) as well as an optional set of disk images.

This is in contrast to rpm-ostree which just generates OSTree commits, and doesn’t have anything to do with disk images. Another way to say this is that coreos-assembler ties together OSTree commits with disk images under a single build schema, and gives them the same version numbering for example.

The default for cosa build is to generate a new OSTree commit and a qemu image. This supports e.g. cosa run.

The OSTree commit data is generated via rpm-ostree, using src/config/manifest.yaml. Image configuration uses src/config/image.yaml.

Physically, a coreos-assembler build is represented primarily by a new subdirectory in builds/$version, and inside that directory there’s a meta.json that contains a lot of relevant metadata, including the OSTree commit.

There is also a builds/builds.json which maintains the list of builds. The reason for this is that HTTP doesn’t offer a way to enumerate a directory.

After a build is generated there are a variety of buildextend-$x commands, for example buildextend-ec2 which can upload to AWS, and buildextend-metal which generates a bare metal disk image.

By default, builds are pruned (as is the OSTree repository), although one can use build --no-prune to prevent this.

For more information on OSTree and build systems, see the libostree docs.

Change detection

All of the filesystem content of a build goes into the ostree commit. Images are just wrappers for that, containing the partition layout, etc. rpm-ostree has a lot of built-in intelligence around change detection; if you run coreos-assembler build and the rpm-md repositories haven’t changed, and you haven’t edited the manifest, it will simply not generate a new build.

You can detect this situation in a pipeline by comparing readlink builds/latest.

However, coreos-assembler builds on top of rpm-ostree and also generates disk images. It uses supermin to run a virtual machine that runs code to write the ostree content along with the filesystem layout into a disk image.

If you want to force a build, use coreos-assembler build --force. A common reason to do this is when something changes in the tooling itself and you want that change.

Managing data

cosa offers buildprep which downloads builds from https:// or s3://, as well as a buildupload which is oriented around S3 today. However, there are a wide variety of S3-compatible storage systems, so you are not tied to AWS.