Branch Flow in AWS Step Function Based on Number of Items in Array

Question

I want to branch the flow of an AWS State Machine based on how many items are in an array. If the array has 0 items, I want to end the flow. If it has more than 0 items, I want to do some stuff.

For example, I want to do something like the following:

{
  "StartAt": "IsBig",
  "States": {
    "IsBig": {
      "Type": "Choice",
      "Choices": [
        {
          "Variable": "$.things.length",
          "NumericGreaterThan": 0,
          "Next": "Big"
        }
      ],
      "Default": "Small"
    },
    "Big": {
      "Type": "Pass",
      "Result": "1",
      "End": true
    },
    "Small": {
      "Type": "Pass",
      "Result": "0",
      "End": true
    }
  }
}

I'd then pass in the following on execution:

{ "things": [1, 2, 3] }

I'd want IsBig to then call Big and end.

Is there a way to do that in AWS states language?

If I can't, I'll just create a Lambda that gets the array's length. I am just curious.

Answer 1

The answer is "no". You can't run a function from the "Variable": "$.things.length" property.

The value in the Variable field is a.... variable. It's not an expression. The docs don't show any expression evaluation syntax. So, long story short, you can't do what I was looking to do.