When I stopped writing CloudFormation and picked up CDK, I expected a slightly better experience. The tooling and type checking from TypeScript seemed useful, but I wondered if it was worth the extra layer of abstraction. After four months of using CDK, the productivity leap is surprising. Technical solutions that used to be expensive and daunting are now fun to build.

I often hear developers ask, “Why would I want to move from a declarative language like YAML to an imperative one like JavaScript?” Developers that gave it five minutes though, never looked back.

For me, this is a reminder that supporting abstraction is critical for humane developer tools. The value from a DSL that embraces The Rule of Least Power is inconsequential if it stifles abstraction.

I’ve seen inhumane tools for working with cloud infrastructure. They veer into some common ditches.

Overreliance on templates

When programming becomes repetitive and developers complain that tasks take too long, templates are the first tools that emerge. But because templates don’t create abstraction, they leave a wake of technical debt and provide little value over copy-and-pasting.

Building Workflow GUIs

At first glance, a GUI that helps developers with a common workflow seems useful. Unfortunately, filling out a form is hard to repeat, test, review, and deploy to multiple environments. GUIs are great for providing information but are an abstraction bottleneck when receiving information.

Abstraction spikes

Rather than craft the fundamental elements needed to solve a problem, it can be faster to create a one-off solution. Time passes, the problem shifts, and developers are stuck waiting for another one-off solution.

For instance, SAM templates lower the complexity of using CloudFormation but only address a narrow problem space. Developers must eject from the paradigm when their needs expand or the landscape changes. Golang famously provides two data structures that implement generics as a special case (map and slice), but doesn’t let developers do the same.

These three pitfalls share something important: they demo well. However they all thwart abstraction.

Humane Developer Tools

Humane developer tools are composed, composable, and usable.


The alternative to “abstraction spikes” are solutions built on smaller primitives. And, perhaps counterintuitively, it’s important to expose those primitives to developers.

A good example is the evolution of async/await in JavaScript. Arguably, being able to write await fetch() is the most important feature, but developers can write their own awaitable functions with the Promise primitive. This allows developers to build with a consistent approach for all asynchronous code they use.


Using a developer tool should not prohibit the use of others. You can only use a single application template to setup your project, but you can combine many libraries. A good developer tool should know where its value lies and where to stop.


A product is not usable in a vacuum. User abilities and context determine a tool’s usability.

And developers tend to have specific needs that the end users they serve don’t. Developers need to reliably automate and repeat tasks. They need artifacts for peer review and to facilitate rollbacks. Likely, they have skills that a tool could exploit instead of introducing new concepts.

CDK Affords Abstraction

Let’s look at a simplified example of a cluster that runs two related services.

Type: AWS::AutoScaling::AutoScalingGroup
  DesiredCapacity: 3

Type: AWS::ECS::Service
  DesiredCount: 1

Type: AWS::ECS::Service
  DesiredCount: 2

When we increase the DesiredCount of the first service, we have to remember to change the DesiredCapacity on the autoscaling group. This is a hidden concept that would be difficult to detect in a large project.

Terraform provides more expressive power than CloudFormation, so we could solve that problem with HCL.

resource "aws_ecs_service" "service_1" {
  desired_count = 1

resource "aws_ecs_service" "service_2" {
  desired_count = 3

resource "aws_autoscaling_group" "asg" {
  desired_capacity = "${aws_ecs_service.service_1.desired_count + aws_ecs_service.service_2.desired_count}"

That reifies our hidden concept. So what does CDK bring to the table?

We could take the same approach:

const service1 = Service(this, 'Service1', {
  desiredCount: 1,

const service2 = Service(this, 'Service2', {
  desiredCount: 2,

const asg = AutoScalingGroup(this, 'Asg', {
  desiredCapacity: service1.desiredCount + service2.desiredCount,

But with CDK we can go further1. We can use object-oriented programming to express intent and hide information.

const service1 = Service(this, 'Service1', {
  desiredCount: 1,

const service2 = Service(this, 'Service2', {
  desiredCount: 2,

const asg = MyCluster(this, 'Cluster', {
  services: [service1, service2]

The caller no longer knows what the cluster does with the services. It may just add their desiredCounts together or it may use some other information to decide how many machines it needs.

Going Big

Code reuse in the small is easy to pull off. Could we realize the promise of integrating large, reusable modules with a tool like CDK? I’m still finding out but I’m optimistic.

Consider this heroic project which creates hundreds of AWS resources to deploy Airflow with CloudFormation templates. Using this this project involves a mix of manual steps and ad hoc scripts. If you need to maintain this system over time, you’re likely to modify the CloudFormation templates, breaking the abstraction. Every new extension point for this project requires a new top-level parameter for users to consider.

Contrast that with a hypothetical solution using CDK. All of the messy deployment and installation concerns are dealt with by CDK, npm, and other common developer tools, freeing a library author to focus on the implementation and user interface of the solution. A CDK user wouldn’t even need to look at README to get started. They would just type and learn how to setup Airflow:

Maybe all you want is to get started with some sane defaults.

export class AirflowStack extends core.Stack {
  constructor(scope: core.App, id: string, props?: AirflowStackProps) {
    super(scope, id, props);

    new airflow.System(this, 'Airflow');

Or maybe you need robust replication on the Airflow database and want to use MySQL instead of Postgres. If you can adhere to an the interface for Storage, you could meet requirements that the library author never intended.

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

    const storage = new mystoragelib.AuroraMySQLStorage(this, 'DB', {
      replication: new airflow.CrossRegionReplication({
        regions: ['us-west-2', 'us-east-1'],
        replicasPerRegion: 2,

    new airflow.System(this, 'Airflow', {

Of course object-oriented programming is a demanding discipline and abstraction can hinder understanding. With more power, we risk creating a confusing system that is hard to adapt. But when we’re building something complex, it is the risk we have to take. Our only choice is to build abstractions well and embrace Software’s Primary Technical Imperative.


  1. For version 0.12 Terraform discusses module composition and I’m confident an experienced user could create an example roughly equivalent to the object-oriented approach shown for CDK. However I’ve never seen a Terraform example like this in the wild and the larger concern is the patterns a DSL like HCL affords. The key-value slant of HCL and its readme make its original intentions clear:

    Full programming languages such as Ruby enable complex behavior a configuration language shouldn’t usually allow…

    HCL readme

    And without user-defined functions, Terraform errs on the side of simplicity:

    The Terraform language does not support user-defined functions, and so only the functions built in to the language are available for use.

    Terraform functions documentation

    Like YAML, HCL is easier to analyze than a general-purpose programming language making it a good compilation target for a tool like CDK.