Importing Existing VPC in AWS CDK

Managing network infrastructure efficiently is crucial for deploying scalable and secure applications on AWS. AWS Cloud Development Kit (CDK) offers a powerful way to define cloud infrastructure using familiar programming languages. This guide provides a comprehensive approach to importing an existing Virtual Private Cloud (VPC) into your AWS CDK project.

Prerequisites

• AWS CDK Installed: Ensure you have the AWS CDK installed. If not, follow the installation guide.

• Existing VPC: An existing VPC in your AWS account that you intend to import.

• Configured AWS Credentials: Ensure your AWS credentials are properly configured.

Overview

Importing an existing VPC into AWS CDK involves using the fromLookup static method provided by the Vpc construct. This method allows CDK to reference a VPC that already exists in your AWS environment. It's essential to explicitly specify the stack environment (account and region) to enable CDK to locate the VPC accurately.

Setting Up the CDK Application

Defining the Stack Environment

Start by explicitly setting the stack's environment with the AWS account and region. This step ensures that CDK performs the lookup in the correct context.

// bin/app.ts
import * as cdk from 'aws-cdk-lib';
import { ImportVpcStack } from '../lib/import-vpc-stack';

const app = new cdk.App();

new ImportVpcStack(app, 'import-vpc-stack', {
  env: {
    region: 'us-east-1', // specify your AWS region
    account: '123456789012', // replace with your AWS account ID
  },
});

Importing an Existing VPC by Name

To import a VPC by its name, utilize the fromLookup method and provide the VPC name. This approach is straightforward and leverages the VPC's unique name within your AWS environment.

// lib/import-vpc-stack.ts
import * as ec2 from 'aws-cdk-lib/aws-ec2';
import * as lambda from 'aws-cdk-lib/aws-lambda';
import { NodejsFunction } from 'aws-cdk-lib/aws-lambda-nodejs';
import * as cdk from 'aws-cdk-lib';
import * as path from 'path';

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

    // import VPC by Name
    const myVpc = ec2.Vpc.fromLookup(this, 'external-vpc', {
      vpcName: 'YOUR_VPC_NAME', // replace with your VPC name
    });

    console.log('VPC ID:', myVpc.vpcId);
    console.log('VPC CIDR Block:', myVpc.vpcCidrBlock);

    // create a Lambda function within the imported VPC
    const myFunction = new NodejsFunction(this, 'function', {
      vpc: myVpc,
      runtime: lambda.Runtime.NODEJS_20_X,
      handler: 'handler',
      entry: path.join(__dirname, '/../lambdas/lambda/index.js'),
      allowPublicSubnet: true,
    });
  }
}

Additional Lookup Options

The fromLookup method provides several options to identify the VPC:

• isDefault: Boolean value to match the default VPC.

• tags: Key-value pairs to filter the VPC based on tags.

• vpcId: Directly specify the VPC ID.

These options offer flexibility in selecting the appropriate VPC, especially in environments with multiple VPCs.

Viewing Synthesized CloudFormation

After defining your stack, you can inspect the synthesized CloudFormation template using:

npx aws-cdk synth

or

cdk synth

Initially, placeholder values appear until CDK performs the VPC lookup and fetches the actual values. This synthesis helps verify that the imported VPC is correctly referenced in your stack.

Deploying the Stack

Deploy your CDK stack to provision resources. Upon successful deployment, the Lambda function will be deployed within the imported VPC.

npx aws-cdk deploy

or

cdk deploy

You can verify the deployment by checking the Lambda console, which should indicate that the function resides within the specified VPC.

Refreshing Cached VPC Values

If the VPC's configuration changes, you need to refresh the cached values to reflect these updates in your CDK stack. Follow these steps to reset the cache:

1. Identify the Cache Key or Number:

    npx aws-cdk context
   
    or
   
    cdk context
   

   This command lists all cached context values with their corresponding keys and numbers.

2. Reset the Cache:

    npx aws-cdk context --reset KEY_OR_NUMBER
   
    or
   
    cdk context --reset KEY_OR_NUMBER
   

   Replace KEY_OR_NUMBER with the appropriate identifier from the previous step.

3. Synthesize Again:

    npx aws-cdk synth
   
    or
   
    cdk synth
   

   CDK will perform a fresh lookup for the VPC, updating the cdk.context.json file with the latest values.

Importing the Default VPC

To import the default VPC, set the isDefault option to true in the fromLookup method. Since there is only one default VPC per account and region, this configuration is sufficient for CDK to identify it.

// lib/import-vpc-stack.ts
const defaultVpc = ec2.Vpc.fromLookup(this, 'default-vpc-id', {
  isDefault: true,
});

Ensure that the stack environment is explicitly set to help CDK locate the default VPC accurately.

Importing an Existing VPC by Tags

Tag-based lookup allows you to import a VPC by specifying key-value pairs. This method is particularly useful when managing multiple VPCs with distinct tags.

1. Tag Your VPC: Add a tag to your VPC, for example:

   • Key: app

   • Value: web

2. Import the VPC Using Tags:

    // lib/import-vpc-stack.ts
    const myVpc = ec2.Vpc.fromLookup(this, 'by-tags-id', {
      tags: {
        app: 'web',
      },
    });
   

This approach ensures that CDK imports the VPC matching the specified tag criteria.

Importing an Existing VPC by VPC ID

Directly specifying the VPC ID provides a precise method to import a VPC without ambiguity.

// lib/cdk-starter-stack.ts
const myVpc = ec2.Vpc.fromLookup(this, 'external-vpc', {
  vpcId: 'vpc-091234567ee1234567', // replace with your VPC ID
});

This method is straightforward and eliminates the need for additional lookup parameters.

Considerations for Importing Existing VPCs

When importing existing VPCs into your CDK stacks, consider the following limitations and best practices:

1. Explicit Environment Specification:

   • Requirement: Always set the stack's environment (account and region).

   • Reason: CDK needs to know where to perform the VPC lookup accurately.

2. Immutable Imported VPCs:

   • Limitation: Imported VPCs cannot be modified by CDK.

   • Implication: Ensure that the VPC configuration meets your requirements before importing.

3. Context Caching:

   • Behavior: After the initial lookup, VPC details are cached in cdk.context.json.

   • Action: If the VPC's configuration changes, reset the cache using the --reset flag to perform a fresh lookup.

4. Cache Reset Procedure:

    npx aws-cdk context --reset CONTEXT_NUMBER_OR_KEY
   
    or
   
    cdk context --reset CONTEXT_NUMBER_OR_KEY
   

   Replace CONTEXT_NUMBER_OR_KEY with the appropriate identifier from the cdk.context.json file.

Summary

Importing an existing VPC into AWS CDK is a powerful feature that enables seamless integration of your existing network infrastructure with CDK-managed resources. By leveraging methods such as fromLookup with various lookup options, you can efficiently reference and utilize your VPCs within your CDK applications. Always remember to manage context caching appropriately and adhere to best practices to maintain a robust and maintainable infrastructure as code setup.