Transforming Target Input - Amazon EventBridge

Transforming Target Input

The Input transformer feature of EventBridge customizes the text from an event before it is passed to the target of a rule. You can define variables that use JSON path to reference values in the original event source. You can define up to 10 variables, assigning each a value from the input. Then you can use those variables in the Input Template as <variable-name>. The characters "<" and ">" cannot be escaped.

There are three pre-defined variables you can use without defining a JSON path. These variables are reserved, and you cannot create variables with these names.

  • — The Amazon Resource Name (ARN) of the EventBridge rule.

  • — The Name of the EventBridge rule.

  • — A copy of the original event.

Input Transform Examples

The following is an example Amazon EC2 event.

{ "version": "0", "id": "7bf73129-1428-4cd3-a780-95db273d1602", "detail-type": "EC2 Instance State-change Notification", "source": "aws.ec2", "account": "123456789012", "time": "2015-11-11T21:29:54Z", "region": "us-east-1", "resources": [ "arn:aws:ec2:us-east-1:123456789012:instance/i-abcd1111" ], "detail": { "instance-id": "i-0123456789", "state": "RUNNING" } }

When defining a rule in the console, select the Input Transformer option under Configure input. This option displays two text boxes: one for the Input Path and one for the Input Template.

				Input Transformer

The Input Path is used to define variables. You use JSON path to reference items in your event and store those values in variables. For instance, you could create an Input Path to reference values in the example event by entering the following in the first text box.

{ "instance" : "$.detail.instance", "state" : "$.detail.state" }

This defines two variables, <instance> and <state>. You can reference these variables as you create your Input Template.

The Input Template is a template for the information you want to pass to your target. You can create a template that passes either a string or JSON to the target. Using the previous event and Input Path, the following Input Template examples will transform the event to the example output before routing it to a target.

Description Template Output
Simple string
"instance <instance> is in <state>"
"instance i-0123456789 is in RUNNING"

String with escaped quotes

"instance \"<instance>\" is in <state>"
"instance \"i-0123456789\" is in RUNNING"

Note that this is the behavior in the EventBridge console. The AWS CLI escapes the slash characters and the result is "instance "i-0123456789" is in RUNNING".

Simple JSON

{ "instance" : <instance>, "state": <state> }
{ "instance" : "i-0123456789", "state": "RUNNING" }

JSON with a mix of variables and static information

{ "instance" : <instance>, "state": [ 9, <state>, true ], "Transformed" : "Yes" }
{ "instance" : "i-0123456789", "state": [ 9, "RUNNING", true ], "Transformed" : "Yes" }

Including reserved variables in JSON

{ "instance" : <instance>, "state": <state>, "ruleArn" : <>, "ruleName" : <>, "originalEvent" : <> }
{ "instance" : "i-0123456789", "state": "RUNNING", "ruleArn" : "arn:aws:events:us-east-2:123456789012:rule/example", "ruleName" : "example", "originalEvent" : { ... // commented for brevity } }

Including reserved variables in a string

"<> triggered"
"example triggered"

Transforming Input Using the EventBridge API

For information and examples on using input transformations with EventBridge API, see Use Input Transformer to extract data from an event and input that data to the target.

Common Issues with Transforming Input

These are some common issues when transforming input in EventBridge.

  • For Strings, quotes are required.

  • There is no validation when creating JSON path for your template.

  • If you specify a variable to match a JSON path that doesn't exist in the event, that variable isn't created and will not appear in the output.

  • The JSON that is passed to the target is minified and escaped.

  • EventBridge does not escape values extracted by Input Path, when populating the Input Template for a target.

  • EventBridge does not support variables inside of quotes within JSON, such as: {"value" : "instance <instance> is in <state>"}.