name: construct-development description: CDK construct development patterns, design principles, and type-driven development. Use when building or modifying AWS CDK constructs.

Construct Development Guidelines

Construct Pattern

Use factory functions that return CDK resources:

import {Construct} from 'constructs';
import {RemovalPolicy} from 'aws-cdk-lib';
import {Bucket, BucketEncryption, BlockPublicAccess} from 'aws-cdk-lib/aws-s3';

export type BucketProps = {
    bucketName: string;
    env: {
        name: string;
        region: string;
        account: string;
    };
    enableVersioning?: boolean;
};

export const createBucket = (scope: Construct, props: BucketProps): Bucket => {
    return new Bucket(scope, `${props.bucketName}-bucket`, {
        bucketName: `${props.bucketName}-${props.env.name}-${props.env.region}`,
        enforceSSL: true,
        encryption: BucketEncryption.S3_MANAGED,
        blockPublicAccess: BlockPublicAccess.BLOCK_ALL,
        removalPolicy: props.env.name === 'prod' ? RemovalPolicy.RETAIN : RemovalPolicy.DESTROY,
        versioned: props.enableVersioning ?? props.env.name === 'prod',
    });
};

Design Principles

1. Secure by Default

• Enable encryption (S3_MANAGED or KMS)
• Enforce SSL
• Block public access
• Use private subnets

2. Environment-Aware

• Gate expensive features to production
• Use RemovalPolicy.RETAIN in prod, DESTROY in dev
• Enable enhanced monitoring only where needed

// Performance Insights only in prod
performanceInsightRetention: props.env.name === 'prod' ? 7 : undefined,

// Reader instances only in prod
const createReaders = props.env.account === Account.PROD && props.enableReaders;

3. Cost-Efficient

// Performance Insights only in prod
performanceInsightRetention: props.env.name === 'prod' ? 7 : undefined,

// Reader instances only in prod
const createReaders = props.env.account === Account.PROD && props.enableReaders;

4. Observable

• Include CloudWatch log groups
• Set appropriate retention periods
• Enable metrics where applicable

Type-Driven Development

Use types, not interfaces. This codebase follows type-driven development where we define all data structures using type declarations.

Why Types Over Interfaces?

• More flexible for unions and intersections
• Consistent pattern across the codebase
• Better for functional programming patterns
• Clearer intent for data structures

Define all props types in src/types/:

// src/types/bucket-types.ts
import {EnvironmentConfig} from '@cdk-constructs/cdk';

export type BucketProps = {
    bucketName: string;
    env: EnvironmentConfig['env'];
    kmsKeyArn?: string;
    lifecycleRules?: BucketLifecycleRule[];
};

export type BucketLifecycleRule = {
    id: string;
    expiration?: number;
    transitions?: StorageTransition[];
};

Export Pattern

Export all public APIs from src/index.ts:

// src/index.ts

// Constructs
export {createBucket} from './constructs/bucket';
export {createLambda} from './constructs/lambda';

// Types
export type {BucketProps, BucketLifecycleRule} from './types/bucket-types';
export type {LambdaProps} from './types/lambda-types';

// Enums
export {StorageClass} from './enums/storage';

// Utilities
export {getAbsoluteLambdaPath} from './util/paths';

JSDoc Requirements

All public functions, types, and enums should have JSDoc comments:

/**
 * Creates a CodeArtifact domain.
 *
 * @param scope - The parent construct
 * @param id - The construct ID
 * @param props - The domain properties
 * @returns The created CodeArtifact domain
 *
 * @public
 */
export const createCodeArtifactDomain = (scope: Construct, id: string, props: CodeArtifactDomainProps): CfnDomain => {
    // ...
};