The Hidden Consequences of Changing Your Cognito User Pool Domain: A Guide to Avoiding Common Pitfalls

Introduction:

As a developer, you’ve likely encountered the need to change the domain in your Cognito User Pool at some point. Whether it’s due to a rebranding effort, a change in your application’s architecture, or simply a desire to use a more memorable domain name, updating your Cognito User Pool domain can be a straightforward process.

However, there’s a crucial step that’s often overlooked: updating the ALB listener rule to reflect the new domain.

What happens when you change the domain in Cognito User Pool after adding it to the ALB listener rule:

When you change the domain in your Cognito User Pool after adding it to the ALB listener rule, you may encounter a range of issues, from minor annoyances to major headaches. Here are some of the potential consequences:

1. Authentication failures: When a user tries to authenticate using the updated Cognito User Pool domain, the authentication request will fail because the ALB listener rule is still pointing to the original domain
2. Error messages: You’ll see error messages in your application or in the Cognito User Pool logs indicating that the authentication request failed due to a mismatch between the expected domain and the actual domain.
3. User frustration: Your users will be unable to authenticate, leading to frustration and potential abandonment of your application.
4. Technical debt: If you don’t update the ALB listener rule, you’ll be left with a technical debt that will need to be addressed at some point in the future.

How to change the domain in Cognito User Pool:

To change the domain in your Cognito User Pool, follow these steps:

1. Go to the Cognito User Pool dashboard: Navigate to the Cognito User Pool dashboard and select the user pool that you want to update.
2. Click on Domain: Click on the “Domain” under branding section.
3. Click on Actions: Click on the actions button and delete the domain.
4. Create Domain: Create the domain that you want to use for your user pool and save the changes.

How to update the ALB listener rule:

To update the ALB listener rule to reflect the new domain, follow these steps:

1. Go to the ALB dashboard: Navigate the ALB dashboard page and select the https listener rule which have Cognito Authentication attached to it.
2. Click on “Actions”: Click on Actions for the selected listener rule and click on edit rule.
3. Remove Authentication: Uncheck the authentication checkbox (Remove authentication) and save the changes.
4. Add Authentication: After saving the changes, again select the listener rule -> actions -> edit rule. Add authentication and save the changes (By doing this the ALB will point to the new domain that we updated in the Cognito).

Conclusion:

Changing the domain in your Cognito User Pool can be a straightforward process, but it’s essential to update the ALB listener rule to reflect the new domain. Failure to do so can lead to authentication failures, error messages, and user frustration. By following the steps outlined in this post, you can avoid these issues and ensure a seamless user experience for your application.

Additional resources:

AWS documentation: *Updating the Domain in a Cognito User Pool AWS documentation: [Creating an ALB Listener Rule](https://docs.aws.amazon.com/elasticloadbalancing/latest/application/listener-update-rules.html)*