Matching events on null values and empty strings in Amazon EventBridge - Amazon EventBridge
Using null values in AWS CloudFormation templates

Matching events on null values and empty strings in Amazon EventBridge

Important

In EventBridge, it is possible to create rules that can lead to higher-than-expected charges and throttling. For example, you can inadvertently create a rule that leads to an infinite loop, where a rule is fired recursively without end. Suppose you created a rule to detect that ACLs have changed on an Amazon S3 bucket, and trigger software to change them to the desired state. If the rule is not written carefully, the subsequent change to the ACLs fires the rule again, creating an infinite loop.

For guidance on how to write precise rules and event patterns to minimize such unexpected results, see Best practices for rules and Best practices.

You can create an event pattern that matches a field in an event that has a null value or is an empty string. Consider the following example event.

See best practices to avoid higher than expected charges and throttling

{
  "version": "0",
  "id": "3e3c153a-8339-4e30-8c35-687ebef853fe",
  "detail-type": "EC2 Instance Launch Successful",
  "source": "aws.autoscaling",
  "account": "123456789012",
  "time": "2015-11-11T21:31:47Z",
  "region": "us-east-1",
  "resources": [
   ],
  "detail": {
    "eventVersion": "",
    "responseElements": null
   }
}

To match events where the value of eventVersion is an empty string, use the following event pattern, which matches the preceding event.

{
  "detail": {
     "eventVersion": [""]
  }
}

To match events where the value of responseElements is null, use the following event pattern, which matches the preceding event.

{
  "detail": {
     "responseElements": [null]
  }
}
Note

Null values and empty strings are not interchangeable in pattern matching. An event pattern that matches empty strings doesn't match values of null.

Using null values in AWS CloudFormation templates

AWS CloudFormation does not allow null values in templates. If you define an event pattern with a null value using YAML or JSON object syntax, the template validation fails with the error: 'null' values are not allowed in templates.

To work around this limitation, specify the EventPattern property as a JSON string instead of a YAML or JSON object. The following example shows how to match on null values in a AWS CloudFormation template:

MyRule:
  Type: AWS::Events::Rule
  Properties:
    EventPattern: '{"detail":{"responseElements":[null]}}'