Last time, I talked about a project where I’m making AWS email me and my (future) wife in the morning if we’ve been married a prime number of days. In this post, I’ll talk about the very simple state machine that accomplishes this.

State machines can be built via a low-code web-ui, and that gives use something that looks like this:

In this control flow we start, then run a step called “InvokeIsPrime.” Then we have a choice that either goes straight to “success” and then “end”, or else goes to “SNS Publish” and then “end.”

Let’s take a look at the JSON representation of this state machine:

{
  "Comment": "Email us if it's been a prime number of days since the big day!",
  "StartAt": "InvokeIsPrime",
  "States": {
    "InvokeIsPrime": {
      "Type": "Task",
      "Resource": "arn:aws:states:::lambda:invoke",
      "Parameters": {
        "FunctionName": "primeChecker"
      },
      "Next": "Choice"
    },
    "Choice": {
      "Type": "Choice",
      "Choices": [
        {
          "Variable": "$.Payload.isPrime",
          "BooleanEquals": true,
          "Next": "SNS Publish"
        }
      ],
      "Default": "Success"
    },
    "Success": {
      "Type": "Succeed"
    },
    "SNS Publish": {
      "Type": "Task",
      "Resource": "arn:aws:states:::sns:publish",
      "Parameters": {
        "Message.$": "States.Format('It has been {} days, a prime number!', $.Payload.days)",
        "Subject": "Happy primeversary!",
        "TopicArn": "arn:aws:sns:<some region>:<some-account-number>:prime-day"
      },
      "End": true
    }
  }
}

I think the JSON is a bit harder to create than the image, but it has two big advantages:

1. You can see all sorts of details in it, like exactly what lambda function gets invoked
2. You can commit it to source control

To be honest, when I was working on this state machine, because this project is a hobby project and not a mission-critical workload, I mostly edited the state machine directly in the AWS Console workflow editor, then used the AWS CLI to record this definition.json and commit it to source control, so that I could have revision control on it.

Overall, I’d say it a bit easier to do the high level, flow-charty code in, well, a flow chart, and leave the actual maths to the lambda function. It’s definitely possible to build the whole thing as a single lambda, but it was fun to learn a new tool, and I think this approach was a reasonable one for the project.

Next time, we’ll take a look at how the lambda actually works!

Till then, happy learning!
– Will

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