aws-cdk

What this skill does

# AWS CDK TypeScript

## Overview

Use this skill to build AWS infrastructure in TypeScript with reusable constructs, safe defaults, and a validation-first delivery loop.

## When to Use

Use this skill when:

- Creating or refactoring a CDK app, stack, or reusable construct in TypeScript
- Choosing between L1, L2, and L3 constructs
- Building serverless, networking, or security-focused AWS infrastructure
- Wiring multi-stack applications and environment-aware deployments
- Validating infrastructure changes with `cdk synth`, tests, `cdk diff`, and `cdk deploy`

## Instructions

### 1. Project Initialization

```bash
# Create a new CDK app
npx cdk init app --language typescript

# Project structure
my-cdk-app/
├── bin/
│   └── my-cdk-app.ts          # App entry point (instantiates stacks)
├── lib/
│   └── my-cdk-app-stack.ts    # Stack definition
├── test/
│   └── my-cdk-app.test.ts     # Tests
├── cdk.json                    # CDK configuration
├── tsconfig.json
└── package.json
```

### 2. Core Architecture

```typescript
import { App, Stack, StackProps, CfnOutput, RemovalPolicy } from 'aws-cdk-lib';
import { Construct } from 'constructs';
import * as s3 from 'aws-cdk-lib/aws-s3';

// Define a reusable stack
class StorageStack extends Stack {
  public readonly bucketArn: string;

  constructor(scope: Construct, id: string, props?: StackProps) {
    super(scope, id, props);

    const bucket = new s3.Bucket(this, 'DataBucket', {
      versioned: true,
      encryption: s3.BucketEncryption.S3_MANAGED,
      removalPolicy: RemovalPolicy.RETAIN,
      blockPublicAccess: s3.BlockPublicAccess.BLOCK_ALL,
    });

    this.bucketArn = bucket.bucketArn;
    new CfnOutput(this, 'BucketName', { value: bucket.bucketName });
  }
}

// App entry point
const app = new App();

new StorageStack(app, 'DevStorage', {
  env: { account: process.env.CDK_DEFAULT_ACCOUNT, region: 'us-east-1' },
  tags: { Environment: 'dev' },
});

new StorageStack(app, 'ProdStorage', {
  env: { account: '123456789012', region: 'eu-west-1' },
  tags: { Environment: 'prod' },
  terminationProtection: true,
});

app.synth();
```

### 3. Construct Levels

| Level | Description | Use When |
|-------|-------------|----------|
| **L1** (`Cfn*`) | Direct CloudFormation mapping, full control | Need properties not exposed by L2 |
| **L2** | Curated with sensible defaults and helper methods | Standard resource provisioning (recommended) |
| **L3** (Patterns) | Multi-resource architectures | Common patterns like `LambdaRestApi` |

```typescript
// L1 — Raw CloudFormation
new s3.CfnBucket(this, 'L1Bucket', { bucketName: 'my-l1-bucket' });

// L2 — Sensible defaults + grant helpers
const bucket = new s3.Bucket(this, 'L2Bucket', { versioned: true });
bucket.grantRead(myLambda);

// L3 — Multi-resource pattern
new apigateway.LambdaRestApi(this, 'Api', { handler: myLambda });
```

### 4. CDK Lifecycle Commands

```bash
cdk synth          # Synthesize CloudFormation template
cdk diff           # Compare deployed vs local changes
cdk deploy         # Deploy stack(s) to AWS
cdk deploy --all   # Deploy all stacks
cdk destroy        # Tear down stack(s)
cdk ls             # List all stacks in the app
cdk doctor         # Check environment setup
```

### 5. Recommended Delivery Loop

1. **Model the stack**
   - Start with L2 constructs and extract repeated logic into custom constructs.

2. **Run `cdk synth`**
   - Checkpoint: synthesis succeeds with no missing imports, invalid props, missing context, or unresolved references.
   - If it fails: fix the construct configuration or context values, then rerun `cdk synth`.

3. **Run infrastructure tests**
   - Checkpoint: assertions cover IAM scope, stateful resources, and critical outputs.
   - If tests fail: update the stack or test expectations, then rerun the test suite.

4. **Run `cdk diff`**
   - Checkpoint: review IAM broadening, resource replacement, export changes, and deletes on stateful resources.
   - If the diff is risky: adjust names, dependencies, or `RemovalPolicy`, then rerun `cdk diff`.

5. **Run `cdk deploy`**
   - Checkpoint: the stack reaches `CREATE_COMPLETE` or `UPDATE_COMPLETE`.
   - If deploy fails: inspect CloudFormation events, fix quotas, permissions, export conflicts, or bootstrap issues, then retry `cdk deploy`.

6. **Verify runtime outcomes**
   - Confirm stack outputs, endpoints, alarms, and integrations behave as expected before moving on.

### 6. Cross-Stack References

```typescript
// Stack A exports a value
class NetworkStack extends Stack {
  public readonly vpc: ec2.Vpc;
  constructor(scope: Construct, id: string, props?: StackProps) {
    super(scope, id, props);
    this.vpc = new ec2.Vpc(this, 'Vpc', { maxAzs: 2 });
  }
}

// Stack B imports it via props
interface AppStackProps extends StackProps {
  vpc: ec2.Vpc;
}
class AppStack extends Stack {
  constructor(scope: Construct, id: string, props: AppStackProps) {
    super(scope, id, props);
    new lambda.Function(this, 'Fn', {
      runtime: lambda.Runtime.NODEJS_20_X,
      handler: 'index.handler',
      code: lambda.Code.fromAsset('lambda'),
      vpc: props.vpc,
    });
  }
}

// Wire them together
const network = new NetworkStack(app, 'Network');
new AppStack(app, 'App', { vpc: network.vpc });
```

## Examples

### Example 1: Serverless API

```typescript
import * as cdk from 'aws-cdk-lib';
import * as lambda from 'aws-cdk-lib/aws-lambda';
import * as apigateway from 'aws-cdk-lib/aws-apigateway';
import * as dynamodb from 'aws-cdk-lib/aws-dynamodb';

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

    const table = new dynamodb.Table(this, 'Items', {
      partitionKey: { name: 'id', type: dynamodb.AttributeType.STRING },
      billingMode: dynamodb.BillingMode.PAY_PER_REQUEST,
      removalPolicy: cdk.RemovalPolicy.DESTROY,
    });

    const fn = new lambda.Function(this, 'Handler', {
      runtime: lambda.Runtime.NODEJS_20_X,
      handler: 'index.handler',
      code: lambda.Code.fromAsset('lambda'),
      environment: { TABLE_NAME: table.tableName },
    });

    table.grantReadWriteData(fn);

    new apigateway.LambdaRestApi(this, 'Api', { handler: fn });
  }
}
```

### Example 2: CDK Assertion Test

```typescript
import { Template } from 'aws-cdk-lib/assertions';
import { App } from 'aws-cdk-lib';
import { ServerlessApiStack } from '../lib/serverless-api-stack';

test('creates DynamoDB table with PAY_PER_REQUEST', () => {
  const app = new App();
  const stack = new ServerlessApiStack(app, 'TestStack');
  const template = Template.fromStack(stack);

  template.hasResourceProperties('AWS::DynamoDB::Table', {
    BillingMode: 'PAY_PER_REQUEST',
  });

  template.resourceCountIs('AWS::Lambda::Function', 1);
});
```

## Best Practices

1. **One concern per stack** — Separate network, compute, storage, and monitoring.
2. **Prefer L2 constructs** — Drop to `Cfn*` only when you need unsupported properties.
3. **Set explicit environments** — Pass `env` with account and region; avoid implicit production targets.
4. **Use grant helpers** — Prefer `.grant*()` over handwritten IAM where possible.
5. **Review the diff before deploy** — Treat IAM expansion, replacement, and deletes as mandatory checkpoints.
6. **Test infrastructure** — Cover critical resources with fine-grained assertions.
7. **Avoid hardcoded values** — Use context, parameters, or environment variables.
8. **Use the right `RemovalPolicy`** — `RETAIN` for production data, `DESTROY` only for disposable environments.

## Constraints and Warnings

- **CloudFormation limits** — Max 500 resources per stack; split large apps into multiple stacks
- **Synthesis is not deployment** — `cdk synth` only generates templates; `cdk deploy` applies changes
- **Cross-stack references** create CloudFormation exports; removing them requires careful ordering
- **Stateful resources** (RDS, DynamoDB, S3 with data) — Always set `remov