HowTo

Write config.json to S3 with AWS CDK

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To make prebuilt SPA installers with CDK I like to keep environment configuration outside of the minified code via a config.json file. This allows me to build the app once and move it between environments and stages.

The file contains settings like Cognito Pool ID, region, and/or branding information.

I used to write my own Custom Resource for building this file but recently discovered a simpler CDKV2 way! Credit: aws-cdk-v2-three-tier-serverless-application/

I am copy-pasting the relevant code for future reference.

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import {
  AwsCustomResource,
  AwsCustomResourcePolicy,
  PhysicalResourceId,
} from 'aws-cdk-lib/custom-resources';
import { PolicyStatement } from 'aws-cdk-lib/aws-iam';

const myConfig = {
  prefix: 'some-static-value',
  userPoolId: userPool.userPoolId,
  // Call toString() on tokens
  region: Aws.REGION.toString(),
  etc,
};

new AwsCustomResource(this, 'SpaConfig', {
  logRetention: RetentionDays.ONE_DAY,
  onUpdate: {
    action: 'putObject',
    parameters: {
      Body: JSON.stringify(myConfig),
      Bucket: websiteBucket.bucketName,
      CacheControl: 'max-age=0, no-cache, no-store, must-revalidate',
      ContentType: 'application/json',
      Key: 'config.json',
    },
    physicalResourceId: PhysicalResourceId.of('config'),
    service: 'S3',
  },
  policy: AwsCustomResourcePolicy.fromStatements([
    new PolicyStatement({
      actions: ['s3:PutObject'],
      resources: [websiteBucket.arnForObjects('config.json')],
    }),
  ]),
});

Pitfall

I have a config.json in the SPA deploy assets also, so with each rebuild and redeploy, CloudFormation was replacing and overwriting the config.json from SPA assets. I added this to the myConfig object above to force the custom resource to execute with each cdk deploy

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buildTime: new Date().toISOString();
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