Using JsonPath Step Functions variable inside an array using CDK

Question

I'm trying to set up an EMR step from the CDK (typescript), using a variable from the state context object as a parameter, but I can't get it to work.

Here's what I tried:

const emrTask = new EmrAddStep(
  stack,
  name,
  {
    name: name,
    jar: jar,
    args: [
      '--arg_1',
      '$$.Execution.StartTime',
    ],
    clusterId: clusterId,
  }
);

During the state run $$.Execution.StartTime does not get replaced by the actual value.

I also tried this:

const emrTask = new EmrAddStep(
  stack,
  name,
  {
    name: name,
    jar: jar,
    args: [
      '--arg_1',
      JsonPath.stringAt('$$.Execution.StartTime')
    ],
    clusterId: clusterId,
  }
);

But I get this error:

Error: Cannot use JsonPath fields in an array, they must be used in objects

user_3545219 · Answer

You can use an EvaluateExpression task to do that

new tasks.EvaluateExpression(stack, name, {
  expression: `['--arg_1', $$.Execution.StartTime]`,
  resultPath: '$.emrArgs'
});

And then you can adapt your task creation like the following

const emrTask = new EmrAddStep(
  stack,
  name,
  {
    name: name,
    jar: jar,
    args: sfn.JsonPath.listAt('$.emrArgs'),
    clusterId: clusterId,
  }
);

Could not find a way to do it in a single task though.

Using JsonPath Step Functions variable inside an array using CDK

1 Answers