Grammar for AWS Event Rules: https://docs.aws.amazon.com/eventbridge/latest/userguide/eb-event-patterns-content-based-filtering.html

Event Rules are JSON documents that are used to filter other JSON documents.
On AWS EventBridge, they are used to filter AWS CloudWatch Events.

Comparing to other policy formats, Event Rules are limited in terms of features, but are easy to understand for non-technical folks and are an extension to JSON.

This utility is shipped with the npm package and is a small compiler that makes OPA REGO policies from AWS Event Rule patterns. It requires NodeJS version which is capable of running WASM binaries (recent version). WASM is used to parse the JSON input and generate the REGO policy.

$ npm install -g tree-sitter-eventrule
$ rule2rego --help
Compiles AWS Event Rule pattern JSON to OPA REGO policy.
Usage: rule2rego <rule>.json

For example, for the following input Rule Event:

// Effect of "source" && ("metricName" || "namespace")
{
  "source": ["aws.cloudwatch"],
  "$or": [
    { "metricName": ["CPUUtilization", "ReadLatency"] },
    { "namespace": ["AWS/EC2", "AWS/ES"] }
  ]
}

You will get the following REGO policy output:

package rule2rego
default allow := false
default allow_or_metricName := false
default allow_or_namespace := false
default allow_or := false
default allow_source := false
allow_or_metricName {
	({ "CPUUtilization", "ReadLatency" } & { input["metricName"] }) == { input["metricName"] }
}
allow_or_namespace {
	({ "AWS/EC2", "AWS/ES" } & { input["namespace"] }) == { input["namespace"] }
}
allow_or {
	allow_or_metricName
}
allow_or {
	allow_or_namespace
}
allow_source {
	({ "aws.cloudwatch" } & { input["source"] }) == { input["source"] }
}
allow {
	allow_or
	allow_source
}

If you want to run the tests locally, you need to have:

• Docker for Tree Sitter to build you a .wasm artifact
• OPA for Jest to build you .wasm policies for evaluation

Jest picks up your tests automatically if you follow these instructions:

• Add your rule JSON <rule name>.json under test/fixtures
• With the same name, create a directory <rule name>
• Create two additional directories allows and denies inside it
• Any JSON you put in either of those two folders, is picked as an event JSON

npm test output is sorted alphabetically.

Sample directory structure:

- test/fixtures
  - prefix-matching.json
  - prefix-matching
    - allows
      - event1.json
      - event2.json
    - denies
      - event1.json
      - event2.json