A self-hosted runner is a feature provided by CircleCI that allows you to use your own infrastructure for running jobs. This means you can build and test on a wider variety of architectures and have additional control over the environment. Self-hosted runners can be installed in a Kubernetes cluster using a container runner, or within a machine execution environment using a machine runner.

 

Self-hosted runners are particularly useful for unique compute requirements where you need to run jobs on an environment or architecture that CircleCI does not offer as a resource class. They also provide privileged access and controls, enabling you to run jobs on on-premises or limited-access infrastructure due to stricter isolation requirements.

 

When using self-hosted runners, you are responsible for building your own infrastructure with any software you need for your workflows to run successfully. For example, if your application needs access to internal databases or sensitive resources for proper testing, you can deploy it to self-hosted runners behind your firewall.

 

Self-hosted runners are grouped into uniquely named resource classes that are used for identification and to assign jobs. It is a best practice to keep the underlying compute for self-hosted runners consistent within resource classes — each machine should be identically configured with the same architecture and environment.