AWS CDK Init Container dependency with Fargate Pattern

Question

I'm trying to use the CDK to define a ApplicationLoadBalancedFargateService like this:

const cluster = new ecs.Cluster(this, 'Cluster', { vpc });
const loadBalancedService = new ecs_patterns.ApplicationLoadBalancedFargateService(this, "FargateService", {
      cluster,
      taskImageOptions: {
        image: ecs.ContainerImage.fromRegistry("public.ecr.aws/XXXXXXXX/my-node-app:latest"),
        environment: {
          DATABASE_HOST: rdsCluster.attrEndpointAddress,
          DATABASE_NAME: databaseName,
          DATABASE_USERNAME: databaseCredentialsSecret.secretValueFromJson('username').toString(),
          DATABASE_PASSWORD: databaseCredentialsSecret.secretValueFromJson('password').toString(),
        }
      },
    });

I already have an RDS cluster created earlier in the app. I want to run an init container before bringing up this Fargate Service but I'm not quite sure how. I know there is a ContainerDependency in the aws-ecs construct but I don't think I can apply it here.

Answer 1

ECS Containers can have dependencies. In the CDK, you can add a dependency to a ContainerDefinition construct using its addContainerDependencies method.

As you are using the L3 ecs_patterns.ApplicationLoadBalancedFargateService construct, your first step is getting a reference to the underlying service's ContainerDefinition via the task definition's defaultContainer property:

const serviceContainer: : ecs.ContainerDefinition | undefined = loadBalancedService.taskDefinition.defaultContainer;
if (!serviceContainer) throw new Error('LB Service default container must be defined!');

Then add the init Container to the task definition:

const initContainer = loadBalancedService.taskDefinition.addContainer('init', {
  image: ecs.ContainerImage.fromRegistry('init/my-image'),
  essential: false,
});

Finally, create the dependency with a SUCCESS condition. This condition validates that a dependent container runs to completion (and exits with 0) before permitting other containers to start:

serviceContainer.addContainerDependencies({
  container: initContainer,
  condition: ecs.ContainerDependencyCondition.SUCCESS,
});