W/ CDK, how to maintain Route53 alias up-to-date when the AWS-managed CloudFront distribution for a Cognito custom domain changes?

Question

A few weeks ago, I updated my CDK stack to access Cognito via a custom URL (details in https://repost.aws/questions/QUe6Vdvu1HRtWhPgGwOx69SQ).

Last week, after a deployment, the alias `https://auth.develop.mogaland.io` stopped responding and it's only after a visit to the AWS console that I noticed the AWS-managed CloudFront distribution changed! I manually updated the Route53 record for the alias to point to the new distribution.

Once the domain is added, I can only get the `cloudFrontDomainName` from the result of the `user.PooladdDomain()` function. The `cloudFrontDistributionId` is not available... And w/o that distribution identifier, I can't expect to create A and AAAA alias later records to adjust the custom domain with the updated distribution...

```js
// Add the custom domain to the user pool
const certificate = Certificate.fromCertificateArn(this, 'domainCert', props.acmCertificateARN);
const userPoolDomain = this.userPool.addDomain('Custom Domain', {
    customDomain: {
        domainName: `auth.${props.stageName}.mogaland.io`,
        certificate,
    },
});

// Get the reference of AWS-managed CloudFront distribution
const awsManagedDistribution = Distribution.fromDistributionAttributes(this, 'AWS Managed Distribution', {
    domainName: userPoolDomain.cloudFrontDomainName,
    // distributionId: ???, // <= w/o the distributionId, I can't later update the alias records
});

// Get the Route53 hosted zone
const hostedZone = HostedZone.fromHostedZoneAttributes(this, 'Hosted Zone', {
    hostedZoneId: props.hostedZoneId, // some value like 'Z' plus 20 characters
    zoneName: `${props.stageName}.mogaland.io`,
});

// Update the A and AAAA records (update b/c of deletion before creation)
new ARecord(this, 'aliasRecord', {
    zone: hostedZone,
    recordName: 'auth',
    deleteExisting: true,
    target: RecordTarget.fromAlias(new CloudFrontTarget(awsManagedDistribution)),
});
new AaaaRecord(this, 'aaaAliasRecord', {
    zone: hostedZone,
    recordName: 'auth',
    deleteExisting: true,
    target: RecordTarget.fromAlias(new CloudFrontTarget(awsManagedDistribution)),
});
```

Maybe there's another way to get the reference of the AWS-managed CloudFront distribution?

Answer

There is a special class `UserPoolDomainTarget`[1] for creating Route53 alias records for Cognito User Pool domains. Try updating your targets for the Route53 alias records to use that class rather than `CloudFrontTarget`. As an example, an updated version of the last two resources could look like the following:

```javascript
// Update the A and AAAA records (update b/c of deletion before creation)
new ARecord(this, 'aliasRecord', {
    zone: hostedZone,
    recordName: 'auth',
    deleteExisting: true,
    target: RecordTarget.fromAlias(new UserPoolDomainTarget(userPoolDomain)),
});
new AaaaRecord(this, 'aaaAliasRecord', {
    zone: hostedZone,
    recordName: 'auth',
    deleteExisting: true,
    target: RecordTarget.fromAlias(new UserPoolDomainTarget(userPoolDomain)),
});
```

[1] https://docs.aws.amazon.com/cdk/api/v2/docs/aws-cdk-lib.aws_route53_targets.UserPoolDomainTarget.html

W/ CDK, how to maintain Route53 alias up-to-date when the AWS-managed CloudFront distribution for a Cognito custom domain changes?

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