Amazon EventBridge event patterns - Amazon EventBridge

Amazon EventBridge event patterns

Event patterns have the same structure as the events they match. Rules use event patterns to select events and send them to targets. An event pattern either matches an event or it doesn't.


In EventBridge, it is possible to create rules that lead to infinite loops, where a rule is fired repeatedly. For example, a rule might detect that ACLs have changed on an 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.

To prevent this, write the rules so that the triggered actions do not re-fire the same rule. For example, your rule could fire only if ACLs are found to be in a bad state, instead of after any change.

An infinite loop can quickly cause higher than expected charges. We recommend that you use budgeting, which alerts you when charges exceed your specified limit. For more information, see Managing Your Costs with Budgets.

The following video goes over the basics of event patterns:

The following event shows a simple AWS event from Amazon EC2.

{ "version": "0", "id": "6a7e8feb-b491-4cf7-a9f1-bf3703467718", "detail-type": "EC2 Instance State-change Notification", "source": "aws.ec2", "account": "111122223333", "time": "2017-12-22T18:43:48Z", "region": "us-west-1", "resources": [ "arn:aws:ec2:us-west-1:123456789012:instance/i-1234567890abcdef0" ], "detail": { "instance-id": "i-1234567890abcdef0", "state": "terminated" } }

The following event pattern processes all Amazon EC2 instance-termination events.

{ "source": ["aws.ec2"], "detail-type": ["EC2 Instance State-change Notification"], "detail": { "state": ["terminated"] } }

Create event patterns

To create an event pattern, you specify the fields of an event that you want the event pattern to match. Only specify the fields that you use for matching. The previous event pattern example only provides values for three fields: the top-level fields "source" and "detail-type", and the "state" field inside the "detail" object field. EventBridge ignores all the other fields in the event when applying the rule.

For an event pattern to match an event, the event must contain all the field names listed in the event pattern. The field names must also appear in the event with the same nesting structure.

EventBridge ignores the fields in the event that aren't included in the event pattern . The effect is that there is a "*": "*" wildcard for fields that don't appear in the event pattern.

The values that event patterns match follow JSON rules. You can include strings enclosed in quotation marks ("), numbers, and the keywords true, false, and null.

For strings, EventBridge uses exact character-by-character matching without case-folding or any other string normalization.

For numbers, EventBridge uses string representation. For example, 300, 300.0, and 3.0e2 are not considered equal.

When you write event patterns to match events, you can use the TestEventPattern API or the test-event-pattern CLI command to test that your pattern matches the correct events. For more information, see TestEventPattern.

Here's a summary of all the comparison operators available in EventBridge:

Comparison Example Rule syntax


UserID is null

"UserID": [ null ]


LastName is empty

"LastName": [""]


Name is "Alice"

"Name": [ "Alice" ]


Location is "New York" and Day is "Monday"

"Location": [ "New York" ], "Day": ["Monday"]


PaymentType is "Credit" or "Debit"

"PaymentType": [ "Credit", "Debit"]


Weather is anything but "Raining"

"Weather": [ { "anything-but": [ "Raining" ] } ]

Numeric (equals)

Price is 100

"Price": [ { "numeric": [ "=", 100 ] } ]

Numeric (range)

Price is more than 10, and less than or equal to 20

"Price": [ { "numeric": [ ">", 10, "<=", 20 ] } ]


ProductName exists

"ProductName": [ { "exists": true } ]

Does not exist

ProductName does not exist

"ProductName": [ { "exists": false } ]

Begins with

Region is in the US

"Region": [ {"prefix": "us-" } ]

Match Values

In an event pattern, the value to match is in a JSON array, surrounded by square brackets ("[", "]") so that you can provide multiple values. For example, to match events from Amazon EC2 or AWS Fargate, you could use the following pattern, which matches events where the value for the "source" field is either "aws.ec2" or "aws.fargate".

{ "source": ["aws.ec2", "aws.fargate"] }

Example events and event patterns

You can use all of the JSON data types and values to match events. The following examples show events and the event patterns that match them.

Field matching

You can match on the value of a field. Consider the following Amazon EC2 Auto Scaling event.

{ "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 } }

For the preceding event, you can use the "responseElements" field to match.

{ "source": ["aws.autoscaling"], "detail-type": ["EC2 Instance Launch Successful"], "detail": { "responseElements": [null] } }

Value matching

Consider the following Amazon Macie event, which is truncated.

{ "version": "0", "id": "0948ba87-d3b8-c6d4-f2da-732a1example", "detail-type": "Macie Finding", "source": "aws.macie", "account": "123456789012", "time": "2021-04-29T23:12:15Z", "region":"us-east-1", "resources": [ ], "detail": { "schemaVersion": "1.0", "id": "64b917aa-3843-014c-91d8-937ffexample", "accountId": "123456789012", "partition": "aws", "region": "us-east-1", "type": "Policy:IAMUser/S3BucketEncryptionDisabled", "title": "Encryption is disabled for the S3 bucket", "description": "Encryption is disabled for the Amazon S3 bucket. The data in the bucket isn’t encrypted using server-side encryption.", "severity": { "score": 1, "description": "Low" }, "createdAt": "2021-04-29T15:46:02Z", "updatedAt": "2021-04-29T23:12:15Z", "count": 2, . . .

The following event pattern matches any event that has a severity score of 1 and a count of 2.

{ "source": ["aws.macie"], "detail-type": ["Macie Finding"], "detail": { "severity": { "score": [1] }, "count":[2] } }