Deploying runner scale sets with Actions Runner Controller

Legal notice

About runner scale sets

Runner scale sets is a group of homogeneous runners that can be assigned jobs from GitHub Actions. The number of active runners owned by a runner scale set can be controlled by auto-scaling runner solutions such as Actions Runner Controller (ARC).

You can use runner groups to manage runner scale sets. Similar to self-hosted runners, you can add runner scale sets to existing runner groups. However, runner scale sets can belong to only one runner group at a time and can only have one label assigned to them. For more information on runner groups, see Managing access to self-hosted runners using groups.

To assign jobs to a runner scale set, you must configure your workflow to reference the runner scale set's name. For more information, see Using Actions Runner Controller runners in a workflow.

Deploying a runner scale set

To deploy a runner scale set, you must have ARC up and running. For more information, see Quickstart for Actions Runner Controller.

You can deploy runner scale sets with ARC's Helm charts or by deploying the necessary manifests. Using ARC's Helm charts is the preferred method, especially if you do not have prior experience using ARC.

1. To configure your runner scale set, run the following command in your terminal, using values from your ARC configuration.

   When you run the command, keep the following in mind.

   • Update the INSTALLATION_NAME value carefully. You will use the installation name as the value of runs-on in your workflows.

   • Update the NAMESPACE value to the location you want the runner pods to be created.

   • Set the GITHUB_CONFIG_URL value to the URL of your repository, organization, or enterprise. This is the entity that the runners will belong to.

   • This example command installs the latest version of the Helm chart. To install a specific version, you can pass the --version argument with the version of the chart you want to install. You can find the list of releases in the actions-runner-controller repository.

     Bash

     INSTALLATION_NAME="arc-runner-set"
     NAMESPACE="arc-runners"
     GITHUB_CONFIG_URL="http(s)://<HOSTNAME>/<'enterprises/your_enterprise'/'org'/'org/repo'>"
     GITHUB_PAT="<PAT>"
     helm install "${INSTALLATION_NAME}" \
         --namespace "${NAMESPACE}" \
         --create-namespace \
         --set githubConfigUrl="${GITHUB_CONFIG_URL}" \
         --set githubConfigSecret.github_token="${GITHUB_PAT}" \
         oci://ghcr.io/actions/actions-runner-controller-charts/gha-runner-scale-set
     

     For additional Helm configuration options, see values.yaml in the ARC repository.

2. To check your installation, run the following command in your terminal.

   Bash

   helm list -A
   

   You should see an output similar to the following.

   NAME            NAMESPACE       REVISION        UPDATED                                 STATUS          CHART                                       APP VERSION
   arc             arc-systems     1               2023-04-12 11:45:59.152090536 +0000 UTC deployed        gha-runner-scale-set-controller-0.4.0       0.4.0
   arc-runner-set  arc-systems     1               2023-04-12 11:46:13.451041354 +0000 UTC deployed        gha-runner-scale-set-0.4.0                  0.4.0
   

3. To check the manager pod, run the following command in your terminal.

   Bash

   kubectl get pods -n arc-systems
   

   If the installation was successful, the pods will show the Running status.

   NAME                                                   READY   STATUS    RESTARTS   AGE
   arc-gha-runner-scale-set-controller-594cdc976f-m7cjs   1/1     Running   0          64s
   arc-runner-set-754b578d-listener                       1/1     Running   0          12s
   

If your installation was not successful, see Troubleshooting Actions Runner Controller errors for troubleshooting information.

Using advanced configuration options

ARC offers several advanced configuration options.

Configuring the runner scale set name

To configure the runner scale set name, you can define an INSTALLATION_NAME or set the value of runnerScaleSetName in your copy of the values.yaml file.

## The name of the runner scale set to create, which defaults to the Helm release name
runnerScaleSetName: "my-runners"

Make sure to pass the values.yaml file in your helm install command. See the Helm Install documentation for more details.

Choosing runner destinations

Runner scale sets can be deployed at the repository, organization, or enterprise levels.

To deploy runner scale sets to a specific level, set the value of githubConfigUrl in your copy of the values.yaml to the URL of your repository, organization, or enterprise.

The following example shows how to configure ARC to add runners to octo-org/octo-repo.

githubConfigUrl: "http(s)://<HOSTNAME>/<'enterprises/your_enterprise'/'org'/'org/repo'>"

For additional Helm configuration options, see values.yaml in the ARC repository.

Using a GitHub App for authentication

If you are not using enterprise-level runners, you can use GitHub Apps to authenticate with the GitHub API. For more information, see Authenticating to the GitHub API.

You can either create a Kubernetes secret, or specify values in your values.yaml file.

Option 1: Create a Kubernetes secret (recommended)

Once you have created your GitHub App, create a Kubernetes secret and pass the reference to that secret in your copy of the values.yaml file.

kubectl create secret generic pre-defined-secret \
  --namespace=arc-runners \
  --from-literal=github_app_id=123456 \
  --from-literal=github_app_installation_id=654321 \
  --from-literal=github_app_private_key='-----BEGIN RSA PRIVATE KEY-----********'

In your copy of the values.yaml pass the secret name as a reference.

githubConfigSecret: pre-defined-secret

Option 2: Specify values in your values.yaml file

Alternatively, you can specify the values of app_id, installation_id and private_key in your copy of the values.yaml file.

## githubConfigSecret is the Kubernetes secret to use when authenticating with GitHub API.
## You can choose to use a GitHub App or a personal access token (classic)
githubConfigSecret:
  ## GitHub Apps Configuration
  ## IDs must be strings, use quotes
  github_app_id: "123456"
  github_app_installation_id: "654321"
  github_app_private_key: |
    -----BEGIN RSA PRIVATE KEY-----
    ...
    HkVN9...
    ...
    -----END RSA PRIVATE KEY-----

For additional Helm configuration options, see values.yaml in the ARC repository.

Managing access with runner groups

You can use runner groups to control which organizations or repositories have access to your runner scale sets. For more information on runner groups, see Managing access to self-hosted runners using groups.

To add a runner scale set to a runner group, you must already have a runner group created. Then set the runnerGroup property in your copy of the values.yaml file. The following example adds a runner scale set to the Octo-Group runner group.

runnerGroup: "Octo-Group"

For additional Helm configuration options, see values.yaml in the ARC repository.

Configuring an outbound proxy

To force HTTP traffic for the controller and runners to go through your outbound proxy, set the following properties in your Helm chart.

proxy:
  http:
    url: http://proxy.com:1234
    credentialSecretRef: proxy-auth # a Kubernetes secret with `username` and `password` keys
  https:
    url: http://proxy.com:1234
    credentialSecretRef: proxy-auth # a Kubernetes secret with `username` and `password` keys
  noProxy:
    - example.com
    - example.org

ARC supports using anonymous or authenticated proxies. If you use authenticated proxies, you will need to set the credentialSecretRef value to reference a Kubernetes secret. You can create a secret with your proxy credentials with the following command.

  kubectl create secret generic proxy-auth \
    --namespace=arc-runners \
    --from-literal=username=proxyUsername \
    --from-literal=password=proxyPassword \

  kubectl create secret generic proxy-auth \
    --namespace=arc-runners \
    --from-literal=username=proxyUsername \
    --from-literal=password=proxyPassword \

For additional Helm configuration options, see values.yaml in the ARC repository.

Setting the maximum and minimum number of runners

The maxRunners and minRunners properties provide you with a range of options to customize your ARC setup.

Example: Unbounded number of runners

If you comment out both the maxRunners and minRunners properties, ARC will scale up to the number of jobs assigned to the runner scale set and will scale down to 0 if there aren't any active jobs.

## maxRunners is the max number of runners the auto scaling runner set will scale up to.
# maxRunners: 0

## minRunners is the min number of idle runners. The target number of runners created will be
## calculated as a sum of minRunners and the number of jobs assigned to the scale set.
# minRunners: 0

Example: Minimum number of runners

You can set the minRunners property to any number and ARC will make sure there is always the specified number of runners active and available to take jobs assigned to the runner scale set at all times.

## maxRunners is the max number of runners the auto scaling runner set will scale up to.
# maxRunners: 0

## minRunners is the min number of idle runners. The target number of runners created will be
## calculated as a sum of minRunners and the number of jobs assigned to the scale set.
minRunners: 20

Example: Set maximum and minimum number of runners

In this configuration, Actions Runner Controller will scale up to a maximum of 30 runners and will scale down to 20 runners when the jobs are complete.

## maxRunners is the max number of runners the auto scaling runner set will scale up to.
maxRunners: 30

## minRunners is the min number of idle runners. The target number of runners created will be
## calculated as a sum of minRunners and the number of jobs assigned to the scale set.
minRunners: 20

Example: Jobs queue draining

In certain scenarios you might want to drain the jobs queue to troubleshoot a problem or to perform maintenance on your cluster. If you set both properties to 0, Actions Runner Controller will not create new runner pods when new jobs are available and assigned.

## maxRunners is the max number of runners the auto scaling runner set will scale up to.
maxRunners: 0

## minRunners is the min number of idle runners. The target number of runners created will be
## calculated as a sum of minRunners and the number of jobs assigned to the scale set.
minRunners: 0

Custom TLS certificates

Some environments require TLS certificates that are signed by a custom certificate authority (CA). Since the custom certificate authority certificates are not bundled with the controller or runner containers, you must inject them into their respective trust stores.

githubServerTLS:
  certificateFrom:
    configMapKeyRef:
      name: config-map-name
      key: ca.crt
  runnerMountPath: /usr/local/share/ca-certificates/

When you do this, ensure you are using the Privacy Enhanced Mail (PEM) format and that the extension of your certificate is .crt. Anything else will be ignored.

The controller executes the following actions.

• Creates a github-server-tls-cert volume containing the certificate specified in certificateFrom.
• Mounts that volume on path runnerMountPath/<certificate name>.
• Sets the NODE_EXTRA_CA_CERTS environment variable to that same path.
• Sets the RUNNER_UPDATE_CA_CERTS environment variable to 1 (as of version 2.303.0, this will instruct the runner to reload certificates on the host).

ARC observes values set in the runner pod template and does not overwrite them.

For additional Helm configuration options, see values.yaml in the ARC repository.

Using a private container registry

To use a private container registry, you can copy the controller image and runner image to your private container registry. Then configure the links to those images and set the imagePullPolicy and imagePullSecrets values.

Configuring the controller image

You can update your copy of the values.yaml file and set the image properties as follows.

image:
  repository: "custom-registry.io/gha-runner-scale-set-controller"
  pullPolicy: IfNotPresent
  # Overrides the image tag whose default is the chart appVersion.
  tag: "0.4.0"

imagePullSecrets:
  - name: <registry-secret-name>

The listener container inherits the imagePullPolicy defined for the controller.

Configuring the runner image

You can update your copy of the values.yaml file and set the template.spec properties as follows.

template:
  spec:
    containers:
      - name: runner
        image: "custom-registry.io/actions-runner:latest"
        imagePullPolicy: Always
        command: ["/home/runner/run.sh"]
    imagePullSecrets:
      - name: <registry-secret-name>

For additional Helm configuration options, see values.yaml in the ARC repository.

Updating the pod specification for the runner pod

You can fully customize the PodSpec of the runner pod and the controller will apply the configuration you specify. The following is an example pod specification.

template:
  spec:
    containers:
      - name: runner
        image: ghcr.io/actions/actions-runner:latest
        command: ["/home/runner/run.sh"]
        resources:
          limits:
            cpu: 500m
            memory: 512Mi
        securityContext:
          readOnlyRootFilesystem: true
          allowPrivilegeEscalation: false
          capabilities:
            add:
              - NET_ADMIN

For additional Helm configuration options, see values.yaml in the ARC repository.

Updating the pod specification for the listener pod

You can customize the PodSpec of the listener pod and the controller will apply the configuration you specify. The following is an example pod specification.

listenerTemplate:
  spec:
    containers:
    # If you change the name of the container, the configuration will not be applied to the listener,
    # and it will be treated as a side-car container.
    - name: listener
      securityContext:
        runAsUser: 1000
      resources:
        limits:
          cpu: "1"
          memory: 1Gi
        requests:
          cpu: "1"
          memory: 1Gi

For additional Helm configuration options, see values.yaml in the ARC repository.

Using Docker-in-Docker or Kubernetes mode for containers

If you are using container jobs and services or container actions, the containerMode value must be set to dind or kubernetes.

• For more information on container jobs and services, see Running jobs in a container.
• For more information on container actions, see Creating a Docker container action.

Using Docker-in-Docker mode

Docker-in-Docker mode is a configuration that allows you to run Docker inside a Docker container. In this configuration, for each runner pod created, ARC creates the following containers.

• An init container
• A runner container
• A dind container

To enable Docker-in-Docker mode, set the containerMode.type to dind as follows.

containerMode:
  type: "dind"

The template.spec will be updated to the following default configuration.

template:
  spec:
    initContainers:
      - name: init-dind-externals
        image: ghcr.io/actions/actions-runner:latest
        command:
          ["cp", "-r", "/home/runner/externals/.", "/home/runner/tmpDir/"]
        volumeMounts:
          - name: dind-externals
            mountPath: /home/runner/tmpDir
    containers:
      - name: runner
        image: ghcr.io/actions/actions-runner:latest
        command: ["/home/runner/run.sh"]
        env:
          - name: DOCKER_HOST
            value: unix:///var/run/docker.sock
        volumeMounts:
          - name: work
            mountPath: /home/runner/_work
          - name: dind-sock
            mountPath: /var/run
      - name: dind
        image: docker:dind
        args:
          - dockerd
          - --host=unix:///var/run/docker.sock
          - --group=$(DOCKER_GROUP_GID)
        env:
          - name: DOCKER_GROUP_GID
            value: "123"
        securityContext:
          privileged: true
        volumeMounts:
          - name: work
            mountPath: /home/runner/_work
          - name: dind-sock
            mountPath: /var/run
          - name: dind-externals
            mountPath: /home/runner/externals
    volumes:
      - name: work
        emptyDir: {}
      - name: dind-sock
        emptyDir: {}
      - name: dind-externals
        emptyDir: {}

The values in template.spec are automatically injected and cannot be overridden. If you want to customize this setup, you must unset containerMode.type, then copy this configuration and apply it directly in your copy of the values.yaml file.

For additional Helm configuration options, see values.yaml in the ARC repository.

Using Kubernetes mode

In Kubernetes mode, ARC uses runner container hooks to create a new pod in the same namespace to run the service, container job, or action.

Prerequisites

Kubernetes mode relies on persistent volumes to share job details between the runner pod and the container job pod. For more information, see the Persistent Volumes section in the Kubernetes documentation.

To use Kubernetes mode, you must do the following.

• Create persistent volumes available for the runner pods to claim.
• Use a solution to automatically provision persistent volumes on demand.

For testing, you can use a solution like OpenEBS.

Configuring Kubernetes mode

To enable Kubernetes mode, set the containerMode.type to kubernetes in your values.yaml file.

containerMode:
  type: "kubernetes"
  kubernetesModeWorkVolumeClaim:
    accessModes: ["ReadWriteOnce"]
    storageClassName: "dynamic-blob-storage"
    resources:
      requests:
        storage: 1Gi

For additional Helm configuration options, see values.yaml in the ARC repository.

Jobs without a job container are forbidden on this runner, please add a 'container:' to your job or contact your self-hosted runner administrator.

template:
  spec:
    containers:
      - name: runner
        image: ghcr.io/actions/actions-runner:latest
        command: ["/home/runner/run.sh"]
        env:
          - name: ACTIONS_RUNNER_REQUIRE_JOB_CONTAINER
            value: "false"

Customizing container modes

When you set the containerMode in the values.yaml file for the gha-runner-scale-set helm chart, you can use either of the following values:

• dind or
• kubernetes

Depending on which value you set for the containerMode, a configuration will automatically be injected into the template section of the values.yaml file for the gha-runner-scale-set helm chart.

• See the dind configuration.
• See the kubernetes configuration.

To customize the spec, comment out or remove containerMode, and append the configuration you want in the template section.

Example: running dind-rootless

Before deciding to run dind-rootless, make sure you are aware of known limitations.

## githubConfigUrl is the GitHub url for where you want to configure runners
## ex: https://<HOSTNAME>/enterprises/my_enterprise or https://<HOSTNAME>/myorg
githubConfigUrl: "https://<HOSTNAME>/actions/actions-runner-controller"

## githubConfigSecret is the k8s secrets to use when auth with GitHub API.
## You can choose to use GitHub App or a PAT token
githubConfigSecret: my-super-safe-secret

## maxRunners is the max number of runners the autoscaling runner set will scale up to.
maxRunners: 5

## minRunners is the min number of idle runners. The target number of runners created will be
## calculated as a sum of minRunners and the number of jobs assigned to the scale set.
minRunners: 0

runnerGroup: "my-custom-runner-group"

## name of the runner scale set to create. Defaults to the helm release name
runnerScaleSetName: "my-awesome-scale-set"

## template is the PodSpec for each runner Pod
## For reference: https://kubernetes.io/docs/reference/kubernetes-api/workload-resources/pod-v1/#PodSpec
template:
  spec:
    initContainers:
    - name: init-dind-externals
      image: ghcr.io/actions/actions-runner:latest
      command: ["cp", "-r", "/home/runner/externals/.", "/home/runner/tmpDir/"]
      volumeMounts:
        - name: dind-externals
          mountPath: /home/runner/tmpDir
    - name: init-dind-rootless
      image: docker:dind-rootless
      command:
        - sh
        - -c
        - |
          set -x
          cp -a /etc/. /dind-etc/
          echo 'runner:x:1001:1001:runner:/home/runner:/bin/ash' >> /dind-etc/passwd
          echo 'runner:x:1001:' >> /dind-etc/group
          echo 'runner:100000:65536' >> /dind-etc/subgid
          echo 'runner:100000:65536' >> /dind-etc/subuid
          chmod 755 /dind-etc;
          chmod u=rwx,g=rx+s,o=rx /dind-home
          chown 1001:1001 /dind-home
      securityContext:
        runAsUser: 0
      volumeMounts:
        - mountPath: /dind-etc
          name: dind-etc
        - mountPath: /dind-home
          name: dind-home
    containers:
    - name: runner
      image: ghcr.io/actions/actions-runner:latest
      command: ["/home/runner/run.sh"]
      env:
        - name: DOCKER_HOST
          value: unix:///var/run/docker.sock
      volumeMounts:
        - name: work
          mountPath: /home/runner/_work
        - name: dind-sock
          mountPath: /var/run
    - name: dind
      image: docker:dind-rootless
      args:
        - dockerd
        - --host=unix:///var/run/docker.sock
      securityContext:
        privileged: true
        runAsUser: 1001
        runAsGroup: 1001
      volumeMounts:
        - name: work
          mountPath: /home/runner/_work
        - name: dind-sock
          mountPath: /var/run
        - name: dind-externals
          mountPath: /home/runner/externals
        - name: dind-etc
          mountPath: /etc
        - name: dind-home
          mountPath: /home/runner
    volumes:
    - name: work
      emptyDir: {}
    - name: dind-externals
      emptyDir: {}
    - name: dind-sock
      emptyDir: {}
    - name: dind-etc
      emptyDir: {}
    - name: dind-home
      emptyDir: {}

Understanding runner-container-hooks

When the runner detects a workflow run that uses a container job, service container, or Docker action, it will call runner-container-hooks to create a new pod. The runner relies on runner-container-hooks to call the Kubernetes APIs and create a new pod in the same namespace as the runner pod. This newly created pod will be used to run the container job, service container, or Docker action. For more information, see the runner-container-hooks repository.

Configuring hook extensions

As of ARC version 0.4.0, runner-container-hooks support hook extensions. You can use these to configure the pod created by runner-container-hooks. For example, you could use a hook extension to set a security context on the pod. Hook extensions allow you to specify a YAML file that is used to update the PodSpec of the pod created by runner-container-hooks.

There are two options to configure hook extensions.

• Store in your custom runner image. You can store the PodSpec in a YAML file anywhere in your custom runner image. For more information, see About Actions Runner Controller.
• Store in a ConfigMap. You can create a config map with the PodSpec and mount that config map in the runner container. For more information, see ConfigMaps in the Kubernetes documentation.

Example: Using config map to set securityContext

Create a config map in the same namespace as the runner pods. For example:

apiVersion: v1
kind: ConfigMap
metadata:
  name: hook-extension
  namespace: arc-runners
data:
  content: |
    metadata:
      annotations:
        example: "extension"
    spec:
      containers:
        - name: "$job" # Target the job container
          securityContext:
            runAsUser: 1000

• The .metadata.labels and metadata.annotations fields will be appended as is, unless their keys are reserved. You cannot override the .metadata.name and metadata.namespace fields.
• The majority of the PodSpec fields are applied from the specified template, and will override the values passed from your Helm chart values.yaml file.
• If you specify additional volumes they will be appended to the default volumes specified by the runner.
• The spec.containers are merged based on the names assigned to them.
  • If the name of the container is $job:
    • The spec.containers.name and spec.containers.image fields are ignored.
    • The spec.containers.env, spec.containers.volumeMounts, and spec.containers.ports fields are appended to the default container spec created by the hook.
    • The rest of the fields are applied as provided.
  • If the name of the container is not $job, the fields will be added to the pod definition as they are.

Enabling metrics

ARC can emit metrics about your runners, your jobs, and time spent on executing your workflows. Metrics can be used to identify congestion, monitor the health of your ARC deployment, visualize usage trends, optimize resource consumption, among many other use cases. Metrics are emitted by the controller-manager and listener pods in Prometheus format. For more information, see Exposition formats in the Prometheus documentation.

To enable metrics for ARC, configure the metrics property in the values.yaml file of the gha-runner-scale-set-controller chart.

The following is an example configuration.

metrics:
  controllerManagerAddr: ":8080"
  listenerAddr: ":8080"
  listenerEndpoint: "/metrics"

Once these properties are configured, your controller-manager and listener pods emit metrics via the listenerEndpoint bound to the ports that you specify in your values.yaml file. In the above example, the endpoint is /metrics and the port is :8080. You can use this endpoint to scrape metrics from your controller-manager and listener pods.

To turn off metrics, update your values.yaml file by removing or commenting out the metrics: object and its properties.

Available metrics for ARC

The following table shows the metrics emitted by the controller-manager and listener pods.

[1]: Listener metrics that have the counter type are reset when the listener pod restarts.

Using ARC with Dependabot and code scanning

You can use Actions Runner Controller to create dedicated runners for your GitHub Enterprise Server instance that Dependabot can use to help secure and maintain the dependencies used in repositories on your enterprise. For more information, see Managing self-hosted runners for Dependabot updates on your enterprise.

You can also use ARC with CodeQL to identify vulnerabilities and errors in your code. For more information, see About code scanning with CodeQL. If you're already using code scanning and want to configure a runner scale set to use default setup, set INSTALLATION_NAME=code-scanning. For more information about code scanning default setup, see Configuring default setup for code scanning.

Actions Runner Controller does not use multiple labels to route jobs to specific runner scale sets. Instead, to designate a runner scale set for Dependabot updates or code scanning with CodeQL, use a descriptive installation name in your Helm chart, such as dependabot or code-scanning. You can then set the runs-on value in your workflows to the installation name as the single label, and use the designated runner scale set for Dependabot updates or code scanning jobs.

If you're using default setup for code scanning, the analysis will automatically look for a runner scale set with the installation name code-scanning .

Upgrading ARC

Because there is no support for upgrading or deleting CRDs with Helm, it is not possible to use Helm to upgrade ARC. For more information, see Custom Resource Definitions in the Helm documentation. To upgrade ARC to a newer version, you must complete the following steps.

1. Uninstall all installations of gha-runner-scale-set.
2. Wait for resources cleanup.
3. Uninstall ARC.
4. If there is a change in CRDs from the version you currently have installed, to the upgraded version, remove all CRDs associated with actions.github.com API group.
5. Reinstall ARC again.

For more information, see Deploying a runner scale set.

If you would like to upgrade ARC but are concerned about downtime, you can deploy ARC in a high availability configuration to ensure runners are always available. For more information, see High availability and automatic failover.

Deploying a canary image

You can test features before they are released by using canary releases of the controller-manager container image. Canary images are published with tag format canary-SHORT_SHA. For more information, see gha-runner-scale-set-controller on the Container registry.

1. Update the tag in the gha-runner-scale-set-controller values.yaml file to: canary-SHORT_SHA
2. Update the field appVersion in the Chart.yaml file for gha-runner-scale-set to: canary-SHORT_SHA
3. Re-install ARC using the updated Helm chart and values.yaml files.

High availability and automatic failover

ARC can be deployed in a high availability (active-active) configuration. If you have two distinct Kubernetes clusters deployed in separate regions, you can deploy ARC in both clusters and configure runner scale sets to use the same runnerScaleSetName. In order to do this, each runner scale set must be assigned to a distinct runner group. For example, you can have two runner scale sets each named arc-runner-set, as long as one runner scale set belongs to runner-group-A and the other runner scale set belongs to runner-group-B. For information on assigning runner scale sets to runner groups, see Managing access to self-hosted runners using groups.

If both runner scale sets are online, jobs assigned to them will be distributed arbitrarily (assignment race). You cannot configure the job assignment algorithm. If one of the clusters goes down, the runner scale set in the other cluster will continue to acquire jobs normally without any intervention or configuration change.

Using ARC across organizations

A single installation of Actions Runner Controller allows you to configure one or more runner scale sets. These runner scale sets can be registered to a repository, organization, or enterprise. You can also use runner groups to control the permissions boundaries of these runner scale sets.

As a best practice, create a unique namespace for each organization. You could also create a namespace for each runner group or each runner scale set. You can install as many runner scale sets as needed in each namespace. This will provide you the highest levels of isolation and improve your security. You can use GitHub Apps for authentication and define granular permissions for each runner scale set.

Legal notice

Portions have been adapted from https://github.com/actions/actions-runner-controller/ under the Apache-2.0 license:

Copyright 2019 Moto Ishizawa

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.