CoreOS Assembler Design
This document describes some of the current high level design of the project. It assumes some familiarity with the primary README.md.
Builds
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 buildfetch 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.