An Introduction to AWS CDK: Infrastructure is Code

For years, Infrastructure as Code (IaC) has been dominated by declarative, domain-specific languages like HashiCorp's HCL (for Terraform) or JSON/YAML (for AWS CloudFormation). While powerful, these languages often lack the expressiveness and familiarity of general-purpose programming languages. The AWS Cloud Development Kit (CDK) is an open-source framework that changes this paradigm.

The AWS CDK allows you to define your cloud infrastructure using the programming languages you already know, such as TypeScript, Python, C#, and Java. You get to use the full power of your favorite language—loops, conditionals, object-oriented programming, and dependency management—to create your infrastructure.

How Does it Work?

The magic of the CDK is that it doesn't replace AWS CloudFormation; it embraces it. The CDK is essentially a toolkit that synthesizes your code into a standard AWS CloudFormation template. The workflow looks like this:

1. Write Code: You write a CDK application in a language like TypeScript.
2. Synthesize: You run the cdk synth command. The CDK CLI executes your code, which generates a CloudFormation template as a JSON file.
3. Deploy: You run the cdk deploy command. The CDK CLI then deploys this generated template to the AWS CloudFormation service, which in turn provisions all the specified resources.

This approach gives you the best of both worlds: the power and familiarity of a real programming language for authoring, and the reliability and predictability of CloudFormation for deployment.

Core Concepts of the CDK

There are three fundamental building blocks in a CDK application:

1. App: The root of your CDK application. It's the entry point where you define all your stacks.

2. Stack: A stack is the unit of deployment in the CDK. A stack directly corresponds to a CloudFormation stack. It's a collection of resources that are provisioned and managed together.

3. Construct: A construct is the basic building block of a CDK app. A construct represents a cloud component and can be as simple as a single resource (like an S3 bucket) or as complex as a multi-service architecture (like a load-balanced web service). This is where the power of composition comes in. You can create your own reusable constructs that encapsulate best practices and company standards.

A Simple Example: Creating an S3 Bucket

Let's look at how you would define an S3 bucket in TypeScript.

// lib/my-stack.ts
import * as cdk from 'aws-cdk-lib';
import * as s3 from 'aws-cdk-lib/aws-s3';
import { Construct } from 'constructs';

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

    // This is a Level 2 (L2) construct, which provides convenient defaults.
    new s3.Bucket(this, 'MyFirstBucket', {
      versioned: true,
      removalPolicy: cdk.RemovalPolicy.DESTROY, // Automatically delete the bucket when the stack is deleted
    });
  }
}

This code is clear, concise, and type-safe. The CDK's Construct Library provides high-level constructs (like s3.Bucket) that have sensible defaults and reduce the amount of boilerplate you need to write.

The CDK CLI: Your Main Tool

• cdk init app --language typescript: Initializes a new CDK project.
• cdk synth: Synthesizes the CloudFormation template.
• cdk diff: Compares your current stack definition with what's already deployed and shows you the differences.
• cdk deploy: Deploys your stack to your AWS account.
• cdk destroy: Destroys the stack and all its resources.

CDK vs. Terraform

• Language: This is the biggest difference. CDK uses general-purpose languages, while Terraform uses a declarative DSL (HCL).
• State Management: CDK leverages CloudFormation for state management. Terraform manages its own state file.
• Cloud Support: CDK is primarily focused on AWS, while Terraform is cloud-agnostic.

The Future: CDK v2

AWS is also preparing to release CDK v2, which will consolidate the entire AWS Construct Library into a single, stable package (aws-cdk-lib), simplifying dependency management and making the upgrade process much smoother.

Conclusion

The AWS CDK represents a major shift in how we think about Infrastructure as Code. By allowing developers to use the same languages and tools they use for their application code, it makes infrastructure more accessible, more powerful, and more fun. If you are a developer working on AWS, the CDK is a tool you absolutely need to explore.