(ecs): defer the Fargate/Ec2Service essential container exception to the validation phase

[misterjoshua]

I'd like FargateService and Ec2Service to defer throwing their A TaskDefinition must have at least one essential container exceptions until the validate phase.

Use Case

When I'm creating an ECS service with a container that needs to discover its siblings in other tasks via service discovery, I may need to get the service's cloudmap service and add that to an environment variable on the default container.

There's a problem here because the container must exist before the service can exist or we get the exception, but the container needs to know the service discovery service after the service has been created.

Right now, I can use a Lazy.string to set the environment variable later, but the way to do this is cumbersome.

Proposed Solution

If the services defer this validation error to the validate phase, we can still catch the user's mistake, but while also adding the ability to create the default container after the service exists. I can appreciate that one project philosophy is to validate as soon as possible, but I think this exception is thrown too soon.

Here's what I'd like to be able to do:

const taskDefinition = new ecs.FargateTaskDefinition(stack, 'FargateTaskDef');
// Currently, this part throws immediately:
const service = new ecs.FargateService(stack, 'Service', {
  cluster,
  taskDefinition,
  cloudMapOptions: {},
});

const container = taskDefinition.addContainer('MainContainer', {
  image: ecs.ContainerImage.fromRegistry('hello'),
  environment: {
    SERVICE_DISCOVERY: cdk.Fn.join('.', [
      service.cloudMapService!.serviceName,
      service.cloudMapService!.namespace.namespaceName,
    ]),
  },
});
container.addPortMappings({ containerPort: 80 });

I'd be fine if I needed to set a feature flag in the parent scope to defer the error, but I'm not sure that is necessary as it seems the intent of the exception is to catch a user that has misconfigured their task definition, which still happens in the case of this deferral.

Other

Here's how I currently work around this limitation:

// [1] Set this later when we have a service available
let serviceDiscoveryLazy: string | undefined;

const taskDefinition = new ecs.FargateTaskDefinition(stack, 'TD');
const mainContainer = taskDefinition.addContainer('main', {
  image: ecs.ContainerImage.fromRegistry('something'),
  essential: true,
  environment: {
    // [2] We set serviceDiscoveryLazy after we create the service
    SERVICE_DISCOVERY: cdk.Lazy.string({ produce: () => serviceDiscoveryLazy }),
  },
});
mainContainer.addPortMappings({ containerPort: 80 });

const service = new ecs.FargateService(stack, 'Service', {
  cluster,
  taskDefinition,
  // Enables cloudmap service with all-defaults
  cloudMapOptions: {},
});

// [3] Set the lazy value used above so the lazy string can evaluate.
serviceDiscoveryLazy = cdk.Fn.join('.', [
  service.cloudMapService!.serviceName,
  service.cloudMapService!.namespace.namespaceName,
]);

Ideally, I'd like to add the default container after the service has been created without needing to use this kind of lazy evaluation.

• 👋 I may be able to implement this feature request
• ⚠️ This feature might incur a breaking change

---

This is a 🚀 Feature Request

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