How to Create a Metric Filter with Variables in AWS CloudWatch for VPC Flow Logs

Описание: Learn how to efficiently construct metric filters in AWS CloudWatch using Terraform with dynamic variables to manage VPC flow logs.
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This video is based on the question https://stackoverflow.com/q/73172966/ asked by the user 'Malick Sow' ( https://stackoverflow.com/u/11782958/ ) and on the answer https://stackoverflow.com/a/73173090/ provided by the user 'Marcin' ( https://stackoverflow.com/u/248823/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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How to Create a Metric Filter with Variables in AWS CloudWatch for VPC Flow Logs

When dealing with AWS CloudWatch for monitoring VPC flow logs, you might encounter scenarios where you need to create a metric filter that leverages several variables. Particularly, if you are trying to define a metric filter based on multiple Elastic Network Interfaces (ENIs), a common challenge is how to construct efficient filter patterns dynamically. In this post, we will walk you through the problem and provide a straightforward solution.

The Problem

Suppose you have defined a data source in Terraform to retrieve all the ENIs from a specified set of subnets. You want to create a metric filter that can utilize these ENIs in its pattern, but when attempting to loop over the result set, you find that multiple metric filters are being created—one for each ENI. What you ideally want is to generate a single metric filter that includes all ENIs in the format:

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

This approach reduces redundancy and streamlines your monitoring configuration. Let's explore how to achieve this effectively.

The Solution

To create a single metric filter that incorporates variables representing each ENI, we will combine Terraform features like local, join, and formatlist. Here's a step-by-step breakdown of the solution.

Step 1: Define Local Values

First, we need to define a local value that constructs the filter pattern using the formatlist function and the join function. Here’s how you can implement this:

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

formatlist will create a list of strings in the format required for the filter.

join will concatenate these strings with || to form the pattern needed.

Step 2: Create the Metric Filter

With the local variable defined, you can now implement it in the aws_cloudwatch_log_metric_filter resource like this:

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

Explanation of Components

name: This is simply the identifier for your metric filter.

pattern: Here’s the crucial part where you plug in the dynamic list created earlier. Using the local variable allows for a clean and maintainable pattern definition.

log_group_name: This refers to the CloudWatch log group where your VPC flow logs are stored.

metric_transformation: Defines how the metrics should be transformed and recorded.

Conclusion

By following the steps outlined above, you can streamline your AWS CloudWatch metric filter configuration to include dynamic values rather than creating multiple metric filters for each ENI. This method not only enhances readability but also maintains efficiency in your Terraform code.

If you have any questions or need further assistance, feel free to reach out!