Capacity Providers

Capacity providers let ECS manage and scale the compute behind your tasks.

A capacity provider tells a cluster where to get compute. For EC2 it links to an Auto Scaling group; for serverless it can be FARGATE or FARGATE_SPOT.

Cluster auto scaling

With an EC2 capacity provider and managed scaling, ECS automatically adds or removes instances to fit pending tasks — so you scale the fleet by scaling tasks.

Strategies

A strategy mixes providers by weight, e.g. mostly Spot with a small on-demand base.

Example

{
  "capacityProviders": [
    "ec2-asg",
    "FARGATE_SPOT"
  ],
  "defaultCapacityProviderStrategy": [
    {
      "capacityProvider": "ec2-asg",
      "weight": 1,
      "base": 2
    },
    {
      "capacityProvider": "FARGATE_SPOT",
      "weight": 4
    }
  ]
}

When to use it

A team attaches an Auto Scaling group as an ECS capacity provider so ECS automatically adds EC2 instances when tasks cannot be scheduled.
A platform engineer sets a mixed capacity provider strategy to run 80% of tasks on Spot EC2 and 20% on On-Demand for cost savings with resilience.
An SRE team sets managed termination protection on the capacity provider so ECS drains tasks before Auto Scaling terminates an EC2 instance.

More examples

Create an ASG-Backed Capacity Provider

Creates a capacity provider backed by an Auto Scaling group with managed scaling targeting 80% utilization and termination protection enabled.

aws ecs create-capacity-provider \
  --name my-asg-cp \
  --auto-scaling-group-provider \
    autoScalingGroupArn=arn:aws:autoscaling:us-east-1:123456789012:autoScalingGroup:abc:autoScalingGroupName/my-asg,\
managedScaling='{status=ENABLED,targetCapacity=80}',\
managedTerminationProtection=ENABLED

Attach Capacity Provider to Cluster

Attaches the ASG capacity provider and Fargate Spot to the cluster with an 80/20 default strategy favouring EC2.

aws ecs put-cluster-capacity-providers \
  --cluster my-ec2-cluster \
  --capacity-providers my-asg-cp FARGATE_SPOT \
  --default-capacity-provider-strategy \
    '[{"capacityProvider":"my-asg-cp","weight":4},{"capacityProvider":"FARGATE_SPOT","weight":1}]'

Use Capacity Provider in Service

Creates a service with a base of 2 tasks guaranteed on the ASG capacity provider, with all additional tasks also placed there.

aws ecs create-service \
  --cluster my-ec2-cluster \
  --service-name worker-svc \
  --task-definition worker:3 \
  --desired-count 10 \
  --capacity-provider-strategy \
    '[{"capacityProvider":"my-asg-cp","weight":1,"base":2}]'

Discussion

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