Exporting Cloudwatch Logs automatically to S3 with a Lambda function

The missing feature of Cloudwatch Logs.

Your Cloudwatch Log Groups could look something like this:

 

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Log groups with Retention

As you might guess, after the retention time, logs are deleted.

And that’s a good approach for keeping the costs under control, but sometimes regulation mandates that logs are stored longer than this period.

Luckly, AWS allows you to export logs to S3.

 

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But this is a manual operation, there’s no option to export logs periodically and automatically.

 

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So what we want to achieve here is:

  • Every day, export logs of Log Groups to an S3 bucket
  • Only export logs that were not exported before
  • Archive the logs with S3 Glacier for even lower costs

Exporting logs of Log Groups to an S3 bucket

Let’s get to the reason you’re here. Here’s the lambda function:

At a glance:

  • Env variable S3_BUCKET needs to be set. It’s the bucket to export the logs.
  • Creates a Cloudwatch Logs Export Task
  • It only exports logs from Log Groups that have a tag ExportToS3=true
  • It will use the log group name as the prefix folder when exporting
  • Saves a checkpoint in SSM so it exports from that timestamp next time
  • Only exports if 24 hours have passed from the last checkpoint

One caveat I found was that AWS only allows one Export Task running per account. This means when the lambda function was trying to export multiple log groups at once, I got a LimitExceededException error.

To mitigate that, I added a time.sleep(5) that seems to be enough to allow an Export Task to finish and next one can start.

But, there might be cases where the Export Task takes more than 5 seconds or you have too many logs groups and the lambda execution time is not enough to export all of them.

In this case I recommend running the lambda every 4 hours. It will only export log groups that haven’t been exported for 24 hours, so it’s safe to do without causing overlapping logs to be exported.

 

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Where are the tags?

That’s right. The AWS Console show no option to list or edit tags.

But they are there, I promise!

 

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You can set a tag to a log group using the command:

aws --region ap-southeast-2 logs tag-log-group --log-group-name /ecs/dev/hello --tags ExportToS3=true

To avoid this blog running for more 10 pages (or should I say, screens), here’s a link to the Terraform file that builds the policies for the Lambda function:

Check the following docs from AWS: https://docs.aws.amazon.com/AmazonCloudWatch/latest/logs/S3ExportTasksConsole.html

Alternatively (another shameless plug), you can look at our Terraform module that builds the S3 bucket and set the right policies (on a multi-account environment): https://github.com/DNXLabs/terraform-aws-audit-buckets

Our final policy for the bucket looks like this:

 

Replace <NAME OF THIS BUCKET> with the bucket name created and <EXTERNAL ACCOUNT> with any external AWS accounts you with to allow to export logs from.

And for archiving it to Glacier, the Lifecycle rule is:

 

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We hope this article helped you setting up a more compliant, automated and cost-efficient infrastructure.

If you are interested in infrastructure-as-code, please check out our plethora of Terraform modules, Docker images and CLIs that help our clients to automate their infrastructure at https://modules.dnx.one

We work at DNX Solutions and help to bring a better cloud and application experience for digital native startups in Australia.

Our current focus areas are: AWS, Well-Architected Solutions, Containers, ECS, Kubernetes, Continuous Integration/Continuous Delivery and Service Mesh.

We are constantly hiring cloud engineers for our Sydney office, focusing on cloud-native concepts.

Check our open source projects at https://github.com/DNXLabs and follow us on our Twitter or Linkedin

At DNX Solutions, we work to bring a better cloud and application experience for digital-native companies in Australia.

Our current focus areas are AWS, Well-Architected Solutions, Containers, ECS, Kubernetes, Continuous Integration/Continuous Delivery, and Service Mesh and Data Solutions (movement, transformation, lakes, warehouses and analytics).

We are always hiring cloud engineers for our Sydney office, focusing on cloud-native concepts.

Check our open-source projects at https://github.com/DNXLabs and follow us on Linkedin or Facebook.

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