Скачать или смотреть Using !Sub to Concatenate Environment Variables in SAM CLI Templates

Discover how to effectively concatenate environment variables in AWS SAM CLI templates using the `!Sub` function. Learn step-by-step solutions to enhance your Lambda function configurations.
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Understanding Environment Variable Concatenation in SAM CLI Templates

When working with AWS Lambda functions, defining environment variables properly is essential for accessing configuration details, especially when using AWS Systems Manager Parameter Store. A common question that arises among developers using AWS SAM CLI templates is whether it’s possible to concatenate one environment variable with another using the !Sub function. This feature is crucial for streamlining configuration management, particularly when various parameters are organized by environment (e.g., development or production).

In this guide, let’s dive deep into this question and explore the solution to effectively manage your environment variables within your AWS SAM templates.

The Problem

You have defined multiple environment parameters in your AWS Parameter Store, structured by the environment with paths similar to:

[[See Video to Reveal this Text or Code Snippet]]

Your goal is to reference these paths within the environment variables of your template.yml for a Lambda function using AWS SAM CLI. However, upon trying to use the !Sub command, the expected output is not being generated. For instance, instead of seeing /dev/type/app1/ from your variables, the output appears as /ENV/type/app1, which is incorrect.

[[See Video to Reveal this Text or Code Snippet]]

The Limitation of !Sub in This Context

Unfortunately, !Sub does not allow the direct referencing of one environment variable within another in the way you might expect. When you attempt to use !Sub for concatenation, CloudFormation does not interpolate it as a variable reference but rather treats it as a string literal. Thus, the output becomes problematic when trying to refer to ENV directly.

The Solution

To accomplish the desired environment variable concatenation, you will need to restructure how you're defining your variables in your template. Here’s how to do it effectively.

Step 1: Define ENV as a CloudFormation Parameter

Begin by defining the ENV variable within your parameters section. This requires treating ENV as a CloudFormation parameter instead of a simple static string.

[[See Video to Reveal this Text or Code Snippet]]

This sets up ENV as a configurable parameter, which allows you to reference it properly in your environment variables.

Step 2: Update Environment Variables Configuration

Next, update the Environment section of your template to use the !Ref intrinsic function to get the value of ENV and concatenate it with your path.

Here’s the revised configuration for your Lambda function:

[[See Video to Reveal this Text or Code Snippet]]

In this setup:

ENV is now referenced as a CloudFormation parameter, ensuring that it can be used dynamically.

SSM_PS_APP1_PATH will correctly output /dev/type/app1/ based on the value of ENV.

Benefits of This Approach

Flexibility: You can easily change the environment between development, production, and other stages by modifying the ENV parameter.

Organized Configuration: Storing your parameters in Parameter Store while referencing them in your Lambda functions keeps your configurations clean and manageable.

Conclusion

While it might initially seem limiting that you cannot directly concatenate environment variables using !Sub, restructuring your approach allows for a clean and effective solution. By defining ENV as a CloudFormation parameter and referencing it using !Ref, you can successfully achieve the desired environment variable setup in your AWS SAM CLI templates.

By following these guidelines, you can create flexible and organized environment variable configurations for your AWS