AWS Step Function Lambda error catch clause custom error with new fields

Question

The setup is an AWS Step Function with Lambdas which throw errors in a catch clause and should add them to the event payload for an Error Handler Lambda at the end of the chain. This is accomplished with adding a result path like

"Catch": [ {
  "ErrorEquals": [ "States.ALL" ],
  "ResultPath": "$.error-info",
  "Next": "Error Handler"
}]

as described in the docs: https://docs.aws.amazon.com/step-functions/latest/dg/concepts-error-handling.html.

I now need to create custom errors that include new fields lets say a new field called "lambdaName".

For that I would customize an error class like this:

class SFLambdaError extends Error {
    constructor(message,lambdaName){
        super(message)
        this.lambdaName = lambdaName
    }
}

Testing the Lambda directly this outputs the desired new field and looks fine:

Invoke Error    {"errorType":"SFLambdaError","errorMessage":"someNumber.replace is not a function","lambdaName":"testLambdaName","stack": (...)}

But when implemented in the flow of a Step Function and outputed into "error-info" of the event, the new fields got cut of like following:

"error-info": {
    "Error": "SFLambdaError",
    "Cause": "{\"errorType\":\"SFLambdaError\",\"errorMessage\":\"someNumber.replace is not a function\",\"trace\":[\"SFLambdaError: someNumber.replace is not a function\",\"    at Runtime.exports.handler (/var/task/index.js:27:23)\",\"    at Runtime.handleOnce (/var/runtime/Runtime.js:66:25)\"]}"

So it seems that SF error result path just allowes for the standard error class fields to be given out.

Work arounds I tested are to pass all desired fields in errorMessage via stringifying and parsing at the other end in the error handler. But I hope there must be a better way.

So my question: is there a thing I've overseen in the docs or a cleaner workaroun?

Thanks for any help!

Answer 1

Unfortunately there is no handy way to handle such case. A workaround is using an extra state with builtin intrinsic function States.StringToJson:

"Catch": [ {
  "ErrorEquals": [ "States.ALL" ],
  "ResultPath": "$.error-info",
  "Next": "Error Handler Preprocessing"
}]

...

"Error Handler Preprocessing": {
  "Type": "Pass",
  "Parameters": {
    "Error.$": "$.error-info.Error",
    "Cause.$": "States.StringToJson($.error-info.Cause)"
  },
  "ResultPath": "$.error-info",
  "Next": "Error Handler"
},

"Error Handler": {
  "Type": "Task",
  ...
}

---

References

• https://docs.aws.amazon.com/step-functions/latest/dg/amazon-states-language-intrinsic-functions.html