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Discover a solution to using existing SNS topics in AWS SAM templates efficiently without the need to create duplicate parameters.
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How to Use an Existing SNS Topic in AWS SAM Templates Without Duplication

When using AWS Serverless Application Model (SAM) templates to define your cloud infrastructure, you may encounter situations where you want to use an existing Amazon Simple Notification Service (SNS) topic. However, if you are not careful with your configurations, you might end up duplicating parameters, specifically when defining SNS policies. In this guide, we will explore a common challenge and provide a clear, elegant solution for utilizing existing SNS topics without unnecessary duplication.

The Challenge: Duplicating Parameters

In a typical SAM template, you might define an SNS topic to allow a Lambda function to publish messages to it. The challenge arises when using the policy SNSPublishMessagePolicy, which requires the topic name rather than the full Amazon Resource Name (ARN) of the SNS topic. This means you would need to create a separate parameter for SNSTopicName, which simply duplicates the work you've done with SNSTopicARN. Here's a snippet of the template where this issue is apparent:

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You might think of constructing the ARN using a format like arn:aws:sns:${AWS::Region}:${AWS::AccountId}..., but this isn’t feasible if your SNS topic is created in a different account. So, what’s the best way forward?

The Solution: Using AWS Functions Split and Select

Fortunately, AWS provides built-in functions that can help us extract the necessary information from the SNSTopicARN without the need for duplicating parameters. The key functions to utilize are Split and Select:

Split: This function allows you to take a string and split it into an array based on a specified delimiter. In this case, we will split the ARN using the colon (:) as a delimiter.

Select: After splitting the ARN into an array, the Select function helps us pick an element from it. Specifically, we will select the fifth element, which corresponds to the topic name.

Implementation

Here’s how you can implement a clean solution to reference the topic name directly from the ARN:

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Breakdown of the Code

!Ref SNSTopicARN: This refers to the existing SNS topic ARN defined as a parameter in your SAM template.

!Split [ ":", ... ]: This splits the ARN string into an array of elements divided by colons.

!Select [ 5, ... ]: This selects the fifth element of the array, which is the topic name.

Conclusion

By leveraging AWS's Split and Select functions within your SAM template, you can efficiently use existing SNS topics without the hassle of duplicating parameters. This not only keeps your templates clean and maintainable but also prevents potential errors in your cloud infrastructure setup.

With this knowledge in hand, you can confidently incorporate existing SNS resources into your AWS SAM applications with ease. Happy building!