serverless-plugin-existing-cloudwatch-rule

Allows an AWS Lambda function to be triggered by pre-configured CloudWatch event rules.

In contrast to the traditional schedule event that creates a new CloudWatch rule, the plugin assumes that an existing rule is already in place, and the Lambda function is specified as one of its targets. The plugin just adds necessary permissions to the Lambda function itself to complete the "link" between CloudWatch and Lambda.

Useful for projects that setup and scale infrastructure separately from code, e.g., deploy a Lambda function triggered by multiple timers or s3 events that are configured by scripts like Terraform, a separate CloudFormation template, or even a manually created infrastructure.

Also, can be useful for subscribing to an external (cross-account) rule.

Example serverless.yml:

plugins:
  - serverless-plugin-existing-cloudwatch-rule

functions:
  key-rotation-lambda: #1
    handler: src/key-rotation-lambda.handler
    events:
      - cloudWatchRule: key-rotation-timer
  counter-lambda: #2
    handler: src/counter-lambda.handler
    events:
      - cloudWatchRuleArn: 'arn:aws:events:us-east-1:160879880353:rule/my-project-MidnightSchedule-42UGHOTBBVIET'
  scalable-lambda: #3
    handler: src/scalable-lambda.handler
    events:
      - cloudWatchRule: ANY

1. In the example, key-rotation-timer is assumed to be setup and pointing to key-rotation-lambda. The function key-rotation-lambda will be deployed on serverless deploy and, due to the plugin, with the necessary permissions to be invoked by key-rotation-timer. The timer will be shown on the AWS Lambda -> Triggers page as if it was setup manually or using the traditional schedule event. The prefix rule/ for the name is supported but optional for the plugin.

2. To attach a rule defined by its full ARN, use the cloudWatchRuleArn property, such as in the counter-lambda example.

3. The ANY keyword allow for any trigger under the current AWS account, so serverless does not need to run on every change to the project's infrastructure. In the example, a new timer can be added that triggers scalable-lambda with a different set of arguments, but developers do not need to run serverless deploy or use CLI to change the permissions on the function.