Actions Runner Controller (ARC) is an open-source project, now maintained by GitHub, for managing and scaling self-hosted runners on demand.
See how a queued GitHub Actions workflow automatically triggers the creation of new runner pods in a Kubernetes cluster.
Kubernetes operators use control loops, also known as reconciliation loops, to continuously check and enforce a desired state for resources.
The controller runs in your Kubernetes cluster and communicates with GitHub to manage runner scale sets based on workflow demands.
Isolate ARC from production workloads to prevent noisy neighbor problems and security risks from running arbitrary code.
ARC installs a controller manager with multiple control loops, each responsible for reconciling a specific custom resource like runner scale sets.
Runner scale sets are pools of homogeneous runners, and a dedicated listener pod makes scaling decisions based on messages from GitHub.
A workflow trigger sends a message to the listener, which patches a resource to increase the runner count, prompting a controller to create a new runner pod.
A tour of the Actions Runner Controller codebase highlights its structure and the distinction between legacy community modes and the new GitHub-supported mode.
The controller uses custom resource definitions (CRDs) to extend the Kubernetes API, creating new resource types like AutoscalingRunnerSet.
The core logic of the operator resides in the reconciliation function, which compares the current state to the desired state and applies necessary changes.
The listener runs as a separate application that establishes a long-poll HTTPS connection to the GitHub Actions backend to receive job messages.
ARC is distributed as OCI-compliant Helm charts and a container image containing the controller manager and listener applications.
Key challenges include scaling quickly to meet demand, scaling down to zero, and managing resource limitations within the Kubernetes cluster.
Beyond learning a tech stack, focus on software engineering fundamentals like data structures, algorithms, and writing maintainable code.
Raiffeisen Bank International AG
Vienna, Austria
ROSEN Technology and Research Center GmbH
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50:02Liviu Costea
1:04:54Ixchel Ruiz
47:42Roberth Strand
1:05:06Chris Ayers
46:06Dominik Kress
34:21Aarno Aukia
40:00Rob Richardson
39:20Dan Erez
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