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…aviour (under feature flag) (#13130)

AWS CloudFormation previously required a desiredCount to be specified to create an Amazon ECS Service. They have recently removed that required field, and now provide the following default behaviour, if the desired count is not specified.
* when creating a service, the service is created and starts up with a desired count of 1.
* when updating a service, the service is updated and starts up with the same desired count as it had prior to the deployment.

The AWS-CDK currently adds a default desiredCount of 1 when creating or updating an Amazon ECS service, and this could lead to issues when a service has scaled up due to autoscaling. When updating a service using the AWS-CDK, the service would then be scaled down to the original desiredCount, leading to potential outages.

This change introduces a new feature flag `REMOVE_DEFAULT_DESIRED_COUNT` to flip the default behaviour of setting the desiredCount for any service created using any of the following ECS Patterns constructs. Without the feature flag set, the desiredCount of any service created is automatically defaulted to 1 (which is the current behaviour). With the feature flag set (to true), the default will be removed and the desiredCount will only be passed to the service if it is passed in as input.

* ApplicationLoadBalancedEc2Service
* ApplicationLoadBalancedFargateService
* NetworkLoadBalancedEc2Service
* NetworkLoadBalancedFargateService
* QueueProcessingEc2Service
* QueueProcessingFargateService

**BREAKING CHANGE:** the desiredCount property stored on the above constructs will be optional, allowing them to be undefined. This is enabled through the `@aws-cdk/aws-ecs-patterns:removeDefaultDesiredCount` feature flag. We would recommend all aws-cdk users to set the `REMOVE_DEFAULT_DESIRED_COUNT` flag to true for all of their existing applications. 

Fixes: #12990


*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

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AWS Cloud Development Kit (AWS CDK)

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The AWS Cloud Development Kit (AWS CDK) is an open-source software development framework to define cloud infrastructure in code and provision it through AWS CloudFormation.

It offers a high-level object-oriented abstraction to define AWS resources imperatively using the power of modern programming languages. Using the CDK’s library of infrastructure constructs, you can easily encapsulate AWS best practices in your infrastructure definition and share it without worrying about boilerplate logic.

The CDK is available in the following languages:

Jump To: Developer Guide | API Reference | Getting Started | Getting Help | Contributing | RFCs | Roadmap | More Resources

Developers use the CDK framework in one of the supported programming languages to define reusable cloud components called constructs, which are composed together into stacks, forming a "CDK app".

They then use the AWS CDK CLI to interact with their CDK app. The CLI allows developers to synthesize artifacts such as AWS CloudFormation Templates, deploy stacks to development AWS accounts and "diff" against a deployed stack to understand the impact of a code change.

The AWS Construct Library includes a module for each AWS service with constructs that offer rich APIs that encapsulate the details of how to use AWS. The AWS Construct Library aims to reduce the complexity and glue-logic required when integrating various AWS services to achieve your goals on AWS.

Modules in the AWS Construct Library are designated Experimental while we build them; experimental modules may have breaking API changes in any release. After a module is designated Stable, it adheres to semantic versioning, and only major releases can have breaking changes. Each module's stability designation is available on its Overview page in the AWS CDK API Reference. For more information, see Versioning in the CDK Developer Guide.

Getting Started

For a detailed walkthrough, see the tutorial in the AWS CDK Developer Guide.

At a glance

Install or update the AWS CDK CLI from npm (requires Node.js ≥ 10.13.0). We recommend using a version in Active LTS ⚠️ versions 13.0.0 to 13.6.0 are not supported due to compatibility issues with our dependencies.

$ npm i -g aws-cdk

(See Manual Installation for installing the CDK from a signed .zip file).

Initialize a project:

$ mkdir hello-cdk
$ cd hello-cdk
$ cdk init sample-app --language=typescript

This creates a sample project looking like this:

export class HelloCdkStack extends cdk.Stack {
  constructor(scope: cdk.App, id: string, props?: cdk.StackProps) {
    super(scope, id, props);

    const queue = new sqs.Queue(this, 'HelloCdkQueue', {
      visibilityTimeout: cdk.Duration.seconds(300)

    const topic = new sns.Topic(this, 'HelloCdkTopic');

    topic.addSubscription(new subs.SqsSubscription(queue));

Deploy this to your account:

$ cdk deploy

Use the cdk command-line toolkit to interact with your project:

  • cdk deploy: deploys your app into an AWS account
  • cdk synth: synthesizes an AWS CloudFormation template for your app
  • cdk diff: compares your app with the deployed stack

Getting Help

The best way to interact with our team is through GitHub. You can open an issue and choose from one of our templates for bug reports, feature requests, documentation issues, or guidance.

If you have a support plan with AWS Support, you can also create a new support case.

You may also find help on these community resources:


The AWS CDK Roadmap project board lets developers know about our upcoming features and priorities to help them plan how to best leverage the CDK and identify opportunities to contribute to the project. See for more information and FAQs.


We welcome community contributions and pull requests. See for information on how to set up a development environment and submit code.

More Resources