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Image Builder Composer

Image Builder Composer API Architecture Document

Service Description

The image-builder-composer API routed via serves as a job queue for pending image builds as well as a metadata-store for already built images. When an image build is queued via the API it is turned into a set of jobs that are put on the job queue, and together do the necessary tasks to determine how the image should be built, build the image, upload the image to its destination, and possibly register or import it to its final format.

The image-builder-worker API routed via serves as the other side of the job queue, where jobs can be dequeued to be executed and their results posted.

The actual jobs are executed by workers, which is outside the scope of this document.


osbuild-composer deployments in both and have support for running builds for multiple tenants. Jobs created by a tenant can be picked only by workers belonging to the same tenant.

The tenant of an API request is currently determined from the JWT token that the API caller used. Specifically, the implementation extracts the tenant ID from rh-org-id or account_id fields. This is defined in the deployment template.

Internally in osbuild-composer, the tenant ID is prefixed with org- and saved to the jobqueue as a channel, see composer's source code.

Technology Stack

The service is written in Golang, and the list of dependencies can be found in go.mod.

The ubi8/go-toolset:latest container is used as a builder, and ubi8/ubi-minimal:latest to run the binary. The container images are located here:


The service consists of the composer and the composer-worker apps running in an AppSRE managed cluster, and their backing database.

If either composer or the database are unavailable, the service does not work at all, new images cannot be built, and historical builds cannot be introspected. Already built images that may be in used by customers are unaffected, only their history and metadata can no longer be queried through the service.

If composer-workers is unavailable, new jobs can be queued and old ones can be queried, but workers will not be able to pick up new jobs until the API is back, and they will not be able to report back results correctly for jobs they finish while the API is down.


The public routes are /api/image-builder-composer/v2/ and /api/image-builder/worker/v1/, detailed lists can be found at and


Composer has the following internal and external dependencies.


Composer relies on Red Hat SSO for authentication.


  • AWS RDS for data storage. See the section on state.
  • Quay as a container registry. Without this, the service cannot be redeployed.
  • Github as an upstream repository. Without this, the service cannot be redeployed.
  • Gitlab, AWS EC2, and Openstack for upstream testing. Without this changes to the service cannot land.

Service Diagram

See parent page.

Application Success Criteria

  1. Image builds can be queued successfully
  2. Jobs can be dequeued successfully and correctly
  3. Jobs are tracked correctly
  4. The state of historical or in-flight builds can be queried and introspected successfully


The service depends on a PostgreSQL database, the default postgres12-rds-1 template is used. The database stores metadata about each build, making it possible to enumerate past builds as well as function as a job queue.

If the state is lost historical data would be lost, and pending image builds might never get scheduled, but the user could still use their existing images if they have saved the necessary information. Data loss would not affect the ability to schedule new builds.

Load Testing

The Image Builder API in is currently being load tested on a weekly basis with failure thresholds reflecting the SLIs. The load tests happen against stage CRC, which is backed by composer in An example can be found here.

More information can be found upstream.


The defaults described in App Interface, 1 cpu 512Mi memory per container running in the default three pods are sufficient, and our expectations are that this will remain sufficient for the next twelve months.