CI runners

Run each CI job in a fresh microVM that is deleted when the job ends. Nothing from one job can leak into the next: no files, no processes and no credentials.

Containers on a shared runner host all share one kernel. A microVM gives each job its own kernel, so a job can run Docker, load kernel modules or do other privileged work without putting the host or other jobs at risk.

This is a good fit when you want to:

• Self-host runners on your own hardware, for cost, performance or data-locality reasons.
• Run jobs from untrusted contributors, such as pull requests from forks.
• Give jobs root access without sharing a host between them.

How it works

muster runs each GitHub Actions job in its own microVM:

1. A Battery warm pool keeps pre-booted microVMs ready on your Flintlock hosts.
2. When a job is queued, muster claims a VM from the pool and starts a GitHub runner inside it.
3. When the job ends, muster deletes the VM and Battery boots a replacement.

The warm pool means a job does not wait for a VM to boot.