(aws_ecs): Add support for `DefaultCapacityProviderStrategy` to the `Cluster` L2 construct

Step Function state language does not support provider parameter for running ECS task https://docs.aws.amazon.com/step-functions/latest/dg/connect-ecs.html. I think default provider is the only option there. To use it conveniently from CDK, there is an open ticket with a described workaround https://github.com/aws/aws-cdk/issues/7967#issuecomment-651583630.

Proposed Solution

The L2 construct a2s_ecs.Cluster currently doesn’t support a default provider, could you please add it? https://docs.aws.amazon.com/cdk/api/latest/python/aws_cdk.aws_ecs/Cluster.html#aws_cdk.aws_ecs.Cluster.add_asg_capacity_provider

I tried to use L1 constructs and ran into several issues. CfnCluster.default_capacity_provider_strategy (via CapacityProviderStrategyItemProperty) and CfnClusterCapacityProviderAssociations expect provider as a string instead of specific type. This is probably not a problem of CDK as the AWS::ECS::ClusterCapacityProviderAssociations in CloudFormation docs have a similar problem. Examples in the docs are fine. Could you please forward the problem to the correct place? https://docs.aws.amazon.com/cdk/api/latest/docs/@aws-cdk_aws-ecs.CfnCluster.CapacityProviderStrategyItemProperty.html https://docs.aws.amazon.com/cdk/api/latest/docs/@aws-cdk_aws-ecs.CfnClusterCapacityProviderAssociations.html https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-ecs-clustercapacityproviderassociations.html

I tried to create the association using a generic CfnResource, i came up with this:

ecs_cluster = ecs.Cluster(...)
provider = ecs.AsgCapacityProvider(...)

association = CfnResource(self, "capacity_association2",
                type="AWS::ECS::ClusterCapacityProviderAssociations",
                properties= {
                    "CapacityProviders": [
                        provider.node.children[0].ref
                ],
                    "Cluster": ecs_cluster.node.default_child.ref,
                    "DefaultCapacityProviderStrategy": [
                        {
                            "CapacityProvider": provider.node.children[0].ref
                        }
                    ]
                }
            )

In Python CDK, ecs.AsgCapacityProvider did not have a default_child, so to access its reference, I had to use provider.node.children[0].ref

Environment

• CDK CLI Version : 1.108.1
• **Framework Version: ** 1.108.1
• Node.js Version: v10.19.0
• OS : Ubuntu
• Language (Version): 3.8.5

Other

To run an ECS task from step function using a custom provider from CDK, there is currently an open ticket with a described workaround https://github.com/aws/aws-cdk/issues/7967#issuecomment-651583630.

---

This is a 🚀 Feature Request

[piotrromanowski]

Was able to get around this issue using Aspects

class SetDefaultCapacityProviderStrategy implements cdk.IAspect {
  constructor(
    private readonly defaultCapacityProvider: ecs.AsgCapacityProvider
  ) {}

  visit(node: IConstruct): void {
    if (node instanceof ecs.CfnClusterCapacityProviderAssociations) {
      node.defaultCapacityProviderStrategy = [
        {
          capacityProvider: this.defaultCapacityProvider.capacityProviderName,
        },
      ];
    }
  }
}

And then in the construct that’s creating the cluster

    cdk.Aspects.of(this).add(
      new SetDefaultCapacityProviderStrategy(defaultCapacityProvider)
    );
    ```

[piotrromanowski]

I’ve got a branch that I’ll create a PR for once I’ve added tests as suggested by docs https://github.com/aws/aws-cdk/compare/main...piotrromanowski:aws-cdk:Add_cluster_service_capacity_provider_support

Edit: FWIW we’ve been using this in production for a while now and it’s been working out well