A demo application illustrating how to create a CloudWatch metric alarm that triggers when a Lambda function fails using LocalStack. The alarm sends an SNS notification via email when the Lambda error count exceeds the threshold.

To receive email notifications locally, you need a mock SMTP server such as smtp4dev or Papercut SMTP. Configure the following environment variables before starting LocalStack:

• SMTP_HOST: hostname and port of your mock SMTP server (e.g., host.docker.internal:2525)
• SMTP_USER: SMTP username (optional)
• SMTP_PASS: SMTP password (optional)

• A valid LocalStack for AWS license. Your license provides a LOCALSTACK_AUTH_TOKEN to activate LocalStack.
• Docker
• localstack CLI
• awslocal CLI
• jq

make check

make install

To receive email notifications locally, start a mock SMTP server such as smtp4dev first:

docker run --rm -it -p 3000:80 -p 2525:25 rnwood/smtp4dev

Navigating to http://localhost:3000 will open a UI to view email notifications.

Then set SMTP_HOST to the SMTP server address and start LocalStack:

export LOCALSTACK_AUTH_TOKEN=<your-auth-token>
export SMTP_HOST=host.docker.internal:2525
make start

Alternatively, you can use your real SMTP server by setting SMTP_HOST, SMTP_USER, and SMTP_PASS.

make run

The script:

• Creates a failing Lambda function.
• Creates an SNS topic and subscribes an email address.
• Creates a CloudWatch alarm based on Lambda error metrics.
• Invokes the Lambda function to trigger the alarm.
• Waits for the alarm state to change and confirms the notification.

This code is available under the Apache 2.0 license.