aws-sns-subscriptions: add support for not exists filter in a subscription filter

[jaw111]

Describe the feature

The existsFilter method of SubscriptionFilter class does not take any parameters and hard codes the value to true. I want to be able to set the value to false.

See: https://docs.aws.amazon.com/sns/latest/dg/attribute-key-matching.html

Use Case

I want to subscribe to messages on a topic that do not include the specified property.

Proposed Solution

In: https://github.com/aws/aws-cdk/blob/main/packages/aws-cdk-lib/aws-sns/lib/subscription-filter.ts

Add a default parameter on the existsFilter method that has default value true for backwards compatibility.

  /**
   * Returns a subscription filter for attribute key matching.
   */
  public static existsFilter(existsCondition: boolean = true) {
    return new SubscriptionFilter([{ exists: existsCondition }]);
  }

Other Information

No response

Acknowledgements

• [x] I may be able to implement this feature request
• [ ] This feature might incur a breaking change

AWS CDK Library version (aws-cdk-lib)

2.198.0

AWS CDK CLI version

2.1018.0 (build e629e30)

Environment details (OS name and version, etc.)

WSL Ubuntu 22.04.5

[pahud]

Hi @jaw111,

Thank you for this well-documented feature request! This is indeed a valid enhancement that would expose AWS SNS's full capability for attribute key matching.

Your analysis is correct - the current existsFilter method hardcodes exists: true and doesn't expose the exists: false functionality that AWS SNS supports.

Your Proposed Solution: Your suggested implementation is excellent and follows good API design principles:

public static existsFilter(existsCondition: boolean = true) {
 return new SubscriptionFilter([{ exists: existsCondition }]);
}

This approach:

• ✅ Maintains backward compatibility (default true)
• ✅ Enables the missing functionality (false option)
• ✅ Aligns with AWS SNS capabilities
• ✅ Follows existing CDK patterns

Implementation Notes:

• This would be a straightforward enhancement requiring minimal changes
• Should include unit tests for both true and false cases
• Documentation should explain both use cases
• No breaking changes required

Since you indicated you may be able to implement this feature, we'd welcome a pull request! This would be a great community contribution that enhances the CDK's SNS subscription filtering capabilities.

Thanks again for the detailed issue and proposed solution! 🚀

[jaw111]

@pahud whilst exploring other filter rule implementations in CDK, I found the Amazon.CDK.AWS.Lambda.FilterRule class has exists() and notExists() methods. So I was thinking to rework the approach here for consistency to add a notExistsCondition() method on the Amazon.CDK.AWS.SNS.SubscriptionFilter class, rather than change existsCondition() method.

Any guidance/preference on which approach to take?

[github-actions[bot]]

Comments on closed issues and PRs are hard for our team to see. If you need help, please open a new issue that references this one.