ecs-arc

ecs-arc

A Go-based controller that autoscales GitHub Actions self-hosted runners as ECS tasks. It uses the actions/scaleset library to replace the Kubernetes dependency of actions-runner-controller with Amazon ECS, supporting EC2 Managed Instances, ECS Anywhere (EXTERNAL), and Fargate Spot launch types.

Prerequisites

• A GitHub App installed on your organization with the following permissions:
  • Organization permissions: Self-hosted runners (Read & Write)
• An AWS account with an ECS cluster
• ECS task definitions for your runner sizes (the controller reads these at startup)
• The official ARC runner image: ghcr.io/actions/actions-runner

Quick Start

1. Store your GitHub App private key in AWS Secrets Manager:

aws secretsmanager create-secret \
  --name "ecs-arc/github-app-private-key" \
  --secret-string file://private-key.pem

1. Grab deploy/template.yaml from this repo (it's a static CloudFormation template -- no generator, no release asset). Pin to a tag if you want a specific version.

2. Deploy the CloudFormation template. The stack creates an empty SSM Parameter (/prod/ecs-arc/runners by default) that you will populate in step 4.

ECS Anywhere (EXTERNAL) controller -- no VPC/subnets/security groups needed for the controller itself:

aws cloudformation deploy \
  --template-file template.yaml \
  --stack-name ecs-arc \
  --parameter-overrides \
    ControllerLaunchType=EXTERNAL \
    GitHubAppClientId=Iv1.abc123 \
    GitHubAppInstallationId=12345 \
    GitHubAppPrivateKeyArn=arn:aws:secretsmanager:us-east-1:123456789:secret:gh-app-key-AbCdEf \
    GitHubOrg=my-org \
    ExistingClusterName=my-cluster \
  --capabilities CAPABILITY_NAMED_IAM

Fargate Spot controller (default):

aws cloudformation deploy \
  --template-file template.yaml \
  --stack-name ecs-arc \
  --parameter-overrides \
    GitHubAppClientId=Iv1.abc123 \
    GitHubAppInstallationId=12345 \
    GitHubAppPrivateKeyArn=arn:aws:secretsmanager:us-east-1:123456789:secret:gh-app-key-AbCdEf \
    GitHubOrg=my-org \
    PrivateSubnetIds=subnet-aaa,subnet-bbb \
    ServiceSecurityGroupId=sg-xxx \
  --capabilities CAPABILITY_NAMED_IAM

ControllerLaunchType only affects how the controller itself runs. Runner tasks are configured per-scale-set in the TOML (see step 4) and can independently use FARGATE, FARGATE_SPOT, EXTERNAL, or EC2 Managed Instances.

1. Populate the SSM parameter with your runner TOML config. Start from deploy/sample-runners.toml:

aws ssm put-parameter \
  --name "/prod/ecs-arc/runners" \
  --type String \
  --overwrite \
  --value "$(cat runners.toml)"

The controller polls this parameter every SSM_POLL_INTERVAL (default 5m) and reconciles ECS task definitions to match. New [[runner]] or [[template]] entries spawn scale sets; removed entries deregister them.

1. Target runners in your workflows using the scale set name (task definition family name, optionally prefixed by SCALESET_NAME_PREFIX):

jobs:
  build:
    runs-on: runner-small

Configuration

Environment Variables

The CloudFormation template sets all of these on the controller task. Only set them manually if you are running the controller outside the provided template.

TOML Runner Configuration

All per-scale-set configuration (CPU, memory, min/max runners, subnets, security groups, capacity provider, launch type, DinD, max runtime, ...) lives in a TOML document. The controller reconciles ECS task definitions and GitHub scale sets to match. The TOML can be stored in SSM Parameter Store (production) or a local file (development); set SSM_PARAMETER_NAME or TOML_CONFIG_FILE respectively (exactly one).

See deploy/sample-runners.toml for a working example. In short:

• [defaults] -- baseline applied to every runner (e.g. compatibility, network_mode, max_runtime).
• [[runner]] -- declares a single concrete runner variant.
• [[template]] -- expands sizes × features into many runner variants automatically, with optional [template.exclude] combinations.

Updating the config source is the only way to add, remove, or resize scale sets at runtime.

IAM Permissions

The CloudFormation template creates all required IAM roles automatically. If you are setting up IAM manually, the controller task role needs the following permissions:

Controller Task Role

The controller manages runner ECS tasks and reconciles their task definitions from the TOML in SSM. The cluster-scoped ecs:TagResource lets RunTask apply per-task tags (ecs-arc:scale-set, ecs-arc:runner-name). ecs:RegisterTaskDefinition/DeregisterTaskDefinition/ListTaskDefinitionFamilies are account-global (not cluster-scoped) and are required for the reconciler; ecs:TagResource must also be allowed in that statement so the reconciler can tag task definitions at registration time with ecs-arc:managed=true (task definitions have no cluster context, so the cluster-condition form cannot satisfy it).

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Sid": "EcsClusterScoped",
      "Effect": "Allow",
      "Action": [
        "ecs:RunTask",
        "ecs:StopTask",
        "ecs:DescribeTasks",
        "ecs:ListTasks",
        "ecs:TagResource"
      ],
      "Resource": "*",
      "Condition": {
        "ArnEquals": {
          "ecs:cluster": "arn:aws:ecs:REGION:ACCOUNT_ID:cluster/CLUSTER_NAME"
        }
      }
    },
    {
      "Sid": "EcsTaskDefinitionManagement",
      "Effect": "Allow",
      "Action": [
        "ecs:RegisterTaskDefinition",
        "ecs:DeregisterTaskDefinition",
        "ecs:DescribeTaskDefinition",
        "ecs:ListTaskDefinitionFamilies",
        "ecs:ListTaskDefinitions",
        "ecs:TagResource"
      ],
      "Resource": "*"
    },
    {
      "Sid": "SsmReadRunnerConfig",
      "Effect": "Allow",
      "Action": "ssm:GetParameter",
      "Resource": "arn:aws:ssm:REGION:ACCOUNT_ID:parameter/PATH/TO/RUNNERS"
    },
    {
      "Sid": "PassRunnerRoles",
      "Effect": "Allow",
      "Action": "iam:PassRole",
      "Resource": [
        "arn:aws:iam::ACCOUNT_ID:role/RUNNER_EXECUTION_ROLE",
        "arn:aws:iam::ACCOUNT_ID:role/RUNNER_TASK_ROLE"
      ],
      "Condition": {
        "StringEquals": {
          "iam:PassedToService": "ecs-tasks.amazonaws.com"
        }
      }
    }
  ]
}

Controller Execution Role

The execution role needs the standard ECS task execution policy plus access to the Secrets Manager secret containing the GitHub App private key:

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Sid": "SecretsAccess",
      "Effect": "Allow",
      "Action": "secretsmanager:GetSecretValue",
      "Resource": "arn:aws:secretsmanager:REGION:ACCOUNT_ID:secret:SECRET_NAME"
    }
  ]
}

This is in addition to the AWS managed policy AmazonECSTaskExecutionRolePolicy which handles ECR image pulls and CloudWatch Logs.

CMK-encrypted secrets. If the GitHub App private-key secret is encrypted with a customer-managed KMS key (anything other than the AWS-managed aws/secretsmanager key), the execution role additionally needs kms:Decrypt on that CMK. The bundled CloudFormation template does not add this grant; either use the AWS-managed key, or extend the execution role yourself with something like:

{ "Effect": "Allow", "Action": "kms:Decrypt", "Resource": "arn:aws:kms:REGION:ACCOUNT_ID:key/KEY_ID" }

SSM SecureString. The CloudFormation template creates the runner-config SSM parameter as Type: String, so no KMS grant is needed to read it. If you switch the parameter to SecureString backed by a customer-managed KMS key, the controller task role will also need kms:Decrypt on that key.

Runner Execution Role

Attach the AWS managed policy AmazonECSTaskExecutionRolePolicy. No additional permissions are needed unless your runner image is in a private ECR registry that requires cross-account access.

Runner Task Role

Add permissions based on what your GitHub Actions workflows need (e.g. S3 access, ECR push, etc.). The template creates this role with no extra policies -- customize it for your workloads.

Architecture

The controller runs as a single ECS service (desired count 1). A reconciler polls a pluggable ConfigSource (SSM parameter or local file) for the TOML config and emits events; the controller spawns one goroutine per active scale set in response:

main process
  |- reconciler        -> source.Fetch -> register/deregister task defs -> events
  |- goroutine: scale set "runner-small"  -> listener.Run(ctx, scaler)
  |- goroutine: scale set "runner-medium" -> listener.Run(ctx, scaler)
  |- goroutine: scale set "runner-large"  -> listener.Run(ctx, scaler)

Reconciliation Flow

1. The reconciler reads the TOML from its configured source on startup and every SSM_POLL_INTERVAL. The source is either SSMSource (backed by SSM_PARAMETER_NAME, version token = parameter version) or FileSource (backed by TOML_CONFIG_FILE, version token = SHA-256 of contents).
2. Templates are expanded into concrete runner configs; the diff against observed state emits Create / Update / Remove events.
3. Create events trigger RegisterTaskDefinition and start a scale set goroutine. Remove deregisters the task definition, cancels the goroutine, and deletes the scale set on GitHub (only if it carries the ecs-arc.managed label). Update currently logs a warning -- configuration changes to a live scale set require a controller restart.
4. On startup, task definitions that are tagged as managed but no longer present in the TOML are deregistered, and managed scale sets on GitHub whose names no longer correspond to a desired family are deleted (orphan cleanup). Only resources carrying the ecs-arc.managed system label are ever deleted; foreign scale sets in the same runner group are left alone.

Scaling Flow

1. The listener polls GitHub for desired runner count via long-polling message sessions.
2. HandleDesiredRunnerCount computes the target (clamped by min/max), then calls GenerateJitRunnerConfig + ecs:RunTask for each new runner needed.
3. The JIT config is injected via the ACTIONS_RUNNER_INPUT_JITCONFIG environment variable as a container override.
4. When a job starts, HandleJobStarted marks the runner as busy.
5. When a job completes, HandleJobCompleted removes the runner from tracking; the ephemeral ECS task exits on its own.
6. A reaper goroutine periodically stops tasks stuck in PENDING (>5min) or exceeding max_runtime.

Workflow Targeting

The scale set name is the runner label used in workflows. It is derived from the TOML runner family, optionally prefixed by SCALESET_NAME_PREFIX. For example, with SCALESET_NAME_PREFIX=prod and a TOML family runner-small:

runs-on: prod-runner-small

Without a prefix, the family name is used directly as the label.

Development

Building

go build ./cmd/ecs-arc

Running Locally

export GITHUB_APP_CLIENT_ID=Iv1.abc123
export GITHUB_APP_INSTALLATION_ID=12345
export GITHUB_APP_PRIVATE_KEY="$(cat path/to/private-key.pem)"
export GITHUB_ORG=my-org

export ECS_CLUSTER=my-cluster

# Choose ONE of these (not both):
export SSM_PARAMETER_NAME=/dev/ecs-arc/runners
# export TOML_CONFIG_FILE=./deploy/sample-runners.toml

export RUNNER_EXECUTION_ROLE_ARN=arn:aws:iam::123456789012:role/runner-execution
export RUNNER_TASK_ROLE_ARN=arn:aws:iam::123456789012:role/runner-task
export RUNNER_LOG_GROUP=/ecs-arc/runners

go run ./cmd/ecs-arc controller

Subnets, security groups, and capacity providers are configured per runner in the TOML, not via environment variables.

Testing

go test ./... -v -race

Docker

Pre-built multi-arch images (linux/amd64, linux/arm64) are published to GitHub Container Registry:

# Latest tip from main
docker pull ghcr.io/niranjan94/ecs-arc:tip

# Specific release
docker pull ghcr.io/niranjan94/ecs-arc:1.0.0

# Latest stable release
docker pull ghcr.io/niranjan94/ecs-arc:latest

To build locally:

docker build -t ecs-arc:dev .

License

See LICENSE for details.