design(aws-ecs) Design proposal for Queue Worker Service L3 construct

design/aws-ecs-autoscaling-queue-worker.md

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+# AWS ECS - L3 Construct for Autoscaling ECS/Fargate Service that Processes Items in a SQS Queue
+
+To address issue [#2396](https://github.com/awslabs/aws-cdk/issues/2396), the AWS ECS CDK construct library should provide a way for customers to create a queue worker service (an AWS ECS/Fargate service that processes items from an sqs queue). This would mean adding new ECS CDK constructs `Ec2QueueWorkerService` and `FargateQueryWorkerService`, that would take in the necessary properties required to create a task definition, an SQS queue as well as an ECS/Fargate service and enable autoscaling for the service based on cpu usage and the SQS queue's approximateNumberOfMessagesVisible metric. 
+
+## General approach
+
+The new `ecs.QueueWorkerServiceBase`, `ecs.Ec2QueueWorkerService` and `ecs.FargateQueueWorkerService` classes will create L3 constructs for:
+
+* Ec2QueueWorkerService
+* FargateQueueWorkerService
+
+A `QueueWorkerService` will create a task definition with the specified container (on both EC2 and Fargate). An AWS SQS `Queue` will be created and autoscaling of the ECS Service will be dependent on both CPU as well as the SQS queue's `ApproximateNumberOfMessagesVisible` metric. 
+
+The `QueueWorkerService` constructs (for EC2 and Fargate) will use the following existing constructs:
+
+* Ec2TaskDefinition/FargateTaskDefinition - To create a Task Definition for the container to start
+* SQSQueue - The queue that the worker is processing from
+* Ec2Service/FargateService - The Service running the container
+
+## Code changes
+
+Given the above, we should make the following changes to support queue workers on ECS (for both EC2 and Fargate):
+1. Create `QueueWorkerServiceBaseProps` interface and `QueueWorkerServiceBase` construct
+2. Create `Ec2QueueWorkerServiceProps` interface  and `Ec2QueueWorkerService` construct
+3. Create `FargateQueueWorkerServiceProps` interface  and `FargateQueueWorkerService` construct
+
+### Part 1: Create `QueueWorkerServiceBaseProps` interface  and `QueueWorkerServiceBase` construct  
+
+The `QueueWorkerServiceBaseProps` interface will contain common properties used to construct both the Ec2QueueWorkerService and the FargateQueueWorkerService:
+
+```ts
+/**
+ * Properties to define an Query Worker service
+ */
+export interface QueueWorkerServiceBaseProps {
+  /**
+   * Cluster where service will be deployed
+   */
+  readonly cluster: ICluster;
+
+  /**
+   * The image to start.
+   */
+  readonly image: ContainerImage;
+
+  /**
+   * The CMD value to pass to the container. A string with commands delimited by commas.
+   *
+   * @default none
+   */
+  readonly command?: string;
+
+  /**
+   * Number of desired copies of running tasks
+   *
+   * @default 1
+   */
+  readonly desiredTaskCount?: number;
+
+  /**
+   * Flag to indicate whether to enable logging
+   *
+   * @default true
+   */
+  readonly enableLogging?: boolean;
+
+  /**
+   * The environment variables to pass to the container.
+   *
+   * @default none
+   */
+  readonly environment?: { [key: string]: string };
+
+  /**
+   * A name for the queue.
+   *
+   * If specified and this is a FIFO queue, must end in the string '.fifo'.
+   *
+   * @default CloudFormation-generated name
+   */
+  readonly queueName?: string;
+
+  /**
+   * Maximum capacity to scale to.
+   *
+   * @default (desiredTaskCount * 2)
+   */
+  readonly maxScalingCapacity?: number
+
+  /**
+   * The intervals for scaling based on the SQS queue's ApproximateNumberOfMessagesVisible metric.
+   *
+   * Maps a range of metric values to a particular scaling behavior.
+   * https://docs.aws.amazon.com/autoscaling/ec2/userguide/as-scaling-simple-step.html
+   * 
+   * @default [{ upper: 0, change: -1 },{ lower: 100, change: +1 },{ lower: 500, change: +5 }]
+   */
+  readonly scalingSteps: autoScaling.ScalingInterval[];
+}
+```
+
+### Part 2: Create `Ec2QueueWorkerServiceProps` interface  and `Ec2QueueWorkerService` construct  
+
+The `Ec2QueueWorkerServiceProps` interface will contain properties to construct the Ec2TaskDefinition, SQSQueue and Ec2Service:
+
+```ts
+/**
+ * Properties to define an ECS service
+ */
+export interface Ec2QueueWorkerServiceProps {
+  /**
+   * The minimum number of CPU units to reserve for the container.
+   *
+   * @default none
+   */
+  readonly cpu?: number;
+
+  /**
+   * The hard limit (in MiB) of memory to present to the container.
+   *
+   * If your container attempts to exceed the allocated memory, the container
+   * is terminated.
+   *
+   * At least one of memoryLimitMiB and memoryReservationMiB is required for non-Fargate services.
+   */
+  readonly memoryLimitMiB?: number;
+
+  /**
+   * The soft limit (in MiB) of memory to reserve for the container.
+   *
+   * When system memory is under contention, Docker attempts to keep the
+   * container memory within the limit. If the container requires more memory,
+   * it can consume up to the value specified by the Memory property or all of
+   * the available memory on the container instance—whichever comes first.
+   *
+   * At least one of memoryLimitMiB and memoryReservationMiB is required for non-Fargate services.
+   */
+  readonly memoryReservationMiB?: number;
+}
+```
+
+An example use case:
+```ts
+// Create the vpc and cluster used by the Queue Worker task
+const vpc = new ec2.VpcNetwork(stack, 'Vpc', { maxAZs: 1 });
+const cluster = new ecs.Cluster(stack, 'EcsCluster', { vpc });
+cluster.addCapacity('DefaultAutoScalingGroup', {
+  instanceType: new ec2.InstanceType('t2.micro')
+});
+
+// Create the Queue Worker  task
+new Ec2QueueWorkerService(stack, 'EcsQueueWorkerService', {
+  cluster,
+  image: ecs.ContainerImage.fromRegistry('amazon/amazon-ecs-sample'),
+  desiredTaskCount: 2,
+  maxScalingCapacity: 5,
+  memoryReservationMiB: 512,
+  cpu: 256,
+  queueName: 'EcsWorkerQueue'
+});
+```
+
+### Part 3: Create `FargateQueueWorkerServiceProps` interface  and `FargateQueueWorkerService` construct  
+
+The `FargateQueueWorkerServiceProps` interface will contain properties to construct the FargateTaskDefinition, SQSQueue and FargateService:
+
+```ts
+/**
+ * Properties to define an Fargate service
+ */
+export interface FargateQueueWorkerServiceProps {
+    /**
+   * The number of cpu units used by the task.
+   * Valid values, which determines your range of valid values for the memory parameter:
+   * 256 (.25 vCPU) - Available memory values: 0.5GB, 1GB, 2GB
+   * 512 (.5 vCPU) - Available memory values: 1GB, 2GB, 3GB, 4GB
+   * 1024 (1 vCPU) - Available memory values: 2GB, 3GB, 4GB, 5GB, 6GB, 7GB, 8GB
+   * 2048 (2 vCPU) - Available memory values: Between 4GB and 16GB in 1GB increments
+   * 4096 (4 vCPU) - Available memory values: Between 8GB and 30GB in 1GB increments
+   *
+   * This default is set in the underlying FargateTaskDefinition construct.
+   *
+   * @default 256
+   */
+  readonly cpu?: string;
+
+  /**
+   * The amount (in MiB) of memory used by the task.
+   *
+   * This field is required and you must use one of the following values, which determines your range of valid values
+   * for the cpu parameter:
+   *
+   * 0.5GB, 1GB, 2GB - Available cpu values: 256 (.25 vCPU)
+   *
+   * 1GB, 2GB, 3GB, 4GB - Available cpu values: 512 (.5 vCPU)
+   *
+   * 2GB, 3GB, 4GB, 5GB, 6GB, 7GB, 8GB - Available cpu values: 1024 (1 vCPU)
+   *
+   * Between 4GB and 16GB in 1GB increments - Available cpu values: 2048 (2 vCPU)
+   *
+   * Between 8GB and 30GB in 1GB increments - Available cpu values: 4096 (4 vCPU)
+   *
+   * This default is set in the underlying FargateTaskDefinition construct.
+   *
+   * @default 512
+   */
+  readonly memoryMiB?: string;
+}
+```
+
+An example use case:
+```ts
+// Create the vpc and cluster used by the Queue Worker task
+const vpc = new ec2.VpcNetwork(stack, 'Vpc', { maxAZs: 2 });
+const cluster = new ecs.Cluster(stack, 'FargateCluster', { vpc });
+
+// Create the Queue Worker task
+new FargateQueueWorkerService(stack, 'FargateQueueWorkerService', {
+  cluster,
+  image: ecs.ContainerImage.fromRegistry('amazon/amazon-ecs-sample'),
+  desiredTaskCount: 2,
+  maxScalingCapacity: 5,
+  queueName: 'FargateWorkerQueue'
+});
+```