AWS CDK Step Functions SNS Task - input path parameter injected into message

Question

I'm trying to make a step function workflow for processing transactions and broadcasting a message when the transaction has been successful. Came up with something like this:

new SnsPublish(this, 'publish-successful-transaction', {
            message: TaskInput.fromText(`transaction #${TaskInput.fromJsonPathAt('$.transactionId')} was processed successfully!`),
            resultPath: '$.message',
            topic: this.notification.topic
        })

However, predictably .fromJsonPath() returns an instance of TaskInput which results in the normal object-in-a-string behaviour ie [Object object] .

I tried getting the value property from the TaskInput instance but that results in an error:

Error: Field references must be the entire string, cannot concatenate them
(found 'transaction #${Token[transactionId.269]} was processed successfully!')

Is there a way to have a custom text message with injected input path parameter? Can't seem to find anything on this even though it seems like quite a straightforward use case. What am I missing?

Answer 1

Step Functions have a States.Format intrinsic function to interpolate strings. Because intrinsic functions aren't accepted in all fields, the solution is a bit less straightforward than you might hope.

First, the formatting. There are two equivalent CDK syntaxes for intrinsic functions:

// produce identical State Machine JSON output:
{
  // State Machine Language JSON Syntax - DON'T FORGET .$ SUFFIX IN THE KEY
  'myMessage.$' "States.Format('transaction {} was processed successfully!', $.transactionId)",
  // JsonPath helpers - CDK only
  sameMessage: sfn.JsonPath.format('transaction {} was processed successfully!', sfn.JsonPath.stringAt('$.transactionId'))
}

Next, you have two options to wire things up:

Option 1: sfn.Pass + tasks.SnsPublish

Add a Pass task to format the message and use the result in SnsPublish 's message property:

const formatMessage = new sfn.Pass(this, 'formatMessage', {
  parameters: { 'message.$': "States.Format('transaction {} was processed successfully!', $.transactionId)", },
  resultPath: '$.formatted',
});

const snsPublish = new tasks.SnsPublish(this, 'SnsPublish', {
  topic: this.notification.topic,
  message: sfn.TaskInput.fromJsonPathAt('$.formatted.message'),
});

Option 2: tasks.CallAwsService

The parameters property of the generic CallAwsService task can handle intrinsic functions. Format and publish in a single task state:

const snsPublish2 = new tasks.CallAwsService(this, 'SnsPublish2', {
  service: 'sns',
  action: 'publish',
  parameters: {
    TopicArn: this.notification.topic.topicArn,
    'Message.$': "States.Format('transaction {} was processed successfully!', $.transactionId)",
  },
  iamResources: [this.notification.topic.topicArn],
});