AWS CDK: Environment Context Experimental Feature

This is an experiment to increase ease-of-use of cdk within CI/CD.

What does it do?

TL;DR: You can export context into environment variables and they are automatically picked up the CDK execution.

Using the here provide App will make any environment variables with the prefix CDK_CONTEXT_ available in the context. The name in the context does not contain the prefix, e.g. from CDK_CONTEXT_someVar will be available with const someVar = this.node.tryGetContext("someVar"). The prefix can be modified in the AppProps.

Limitation: CDK internal context keys like "@aws-cdk/core:bootstrapQualifier" cannot be provided due to character limitations of environment variable names, which are: [a-zA-Z_][a-zA-Z0-9_]*. Could be solved with intermediate encoding, but seems like too much complexity in usage, for cases that could (and should?) be used via cdk.json or cdk.context.json and likely also better --context.

Use Case: CustomResources for CloudFormation that is implemented as Lambda and/or ECS task that executes cdk deploy, cdk destroy etc. Context parameters are provided by environment or secrets anyway.

Usage

Note: The created NPM package is hosted on Github packages as I do not consider it production ready and do not want to contribute to accidental installs… Read up how to use Github Packages hosted NPM packages if you want to use it.

in your bin/file.ts:

#!/usr/bin/env node
import 'source-map-support/register';
import * as cdk from '@aws-cdk/core';
import * as envcontext from '@ukautz/aws-cdk-envcontext';
import { YourStack } from '../lib/your-stack';

// use app from envcontext
const app = new envcontext.App();
new YourStack(app, 'YourStack');

Assuming the following environment variablse:

CDK_CONTEXT_myVar=foo
CDK_CONTEXT_otherVar=bar

Then in lib/your-stack.ts:

import * as cdk from 'aws-cdk-lib';
import { Construct } from 'constructs';

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

    // access context injected from env
    const myVar = this.node.tryGetContext('myVar');

    new cdk.CfnOutput(this, 'Vars', {
      value: `myVar = ${myVar}`, // will contain 'myVar = foo, someVar = bar'
    });
  }
}

Then in the command line:

$ export CDK_CONTEXT_myVar=foo
$ cdk synth

The rendered CloudFormation YAML should contain:

Outputs:
  Vars:
    Value: 'myVar = foo'

Using the context object

When working with a lot of parameterized values in context, here a little syntax sugar and simplified API to work with context.

import * as cdk from 'aws-cdk-lib';
import { Construct } from 'constructs';
import { contextOf } from '@ukautz/aws-cdk-envcontext';

export class ExampleStack extends cdk.Stack {
  constructor(scope: Construct, id: string, props?: cdk.StackProps) {
    super(scope, id, props);
    const context = contextOf(this);
    
    // get optional value, with fallback to default if missing
    const mayVar = context.may('myVar', 'saneDefault'); 

    // get mandatory value or throw exception if missing
    const mustVar = context.must('otherVar');

    // ...
  }
}

Useful commands

• npm run build compile typescript to js
• npm run watch watch for changes and compile
• npm run test perform the jest unit tests