Step Functions

An Introduction to AWS Step Functions – Part 2

In Part 1, I went through a whirlwind tour and left example code on how to create a Lambda function and a Step Function. The thing is, I did not explain how the code works. That is what this article is about. I am going to be using the same code from the previous post, but will be going through the code and explaining it. We shall see how well the formatting works.

Jumping right into it. This is the entire code for the StepFunction from the previous post. As of right now, this code has to be json. I did not find any information about supporting yaml. Here is the documentation provided by AWS. It is pretty thorough, but it could be a bit clearer. My explanation could be better or worse.

{
  "Comment": "A Retry example of the Amazon States Language using an AWS Lambda Function",
  "StartAt": "LambdaFunction",
  "States": {
    "LambdaFunction": {
      "Type": "Task",
      "Resource": "arn:aws:lambda:us-east-1:123456789012:function:aws-serverless-repository-hello-w-helloworldpython-1JQ8TEEDUAHCE",
      "ResultPath": "$.taskresult",
      "Retry": [
        {
          "ErrorEquals": ["CustomError"],
          "IntervalSeconds": 1,
          "MaxAttempts": 2,
          "BackoffRate": 2.0
        },
        {
          "ErrorEquals": ["States.TaskFailed"],
          "IntervalSeconds": 30,
          "MaxAttempts": 2,
          "BackoffRate": 2.0
        },
        {
          "ErrorEquals": ["States.ALL"],
          "IntervalSeconds": 5,
          "MaxAttempts": 5,
          "BackoffRate": 2.0
        }
      ],
      "Next": "ChoiceState"
    },
    "ChoiceState": {
      "Type": "Choice",
      "Choices": [
        {
          "Variable": "$.taskresult.value1",
          "StringEquals": "value1",
          "Next": "SuccessState"
        },
        {
          "Variable": "$.taskresult.count",
          "NumericLessThan": 5,
          "Next": "LambdaFunction"
        }
      ],
      "Default": "FailState"
    },
    "SuccessState": {
      "Type": "Succeed"
    },
    "FailState": {
      "Type": "Fail",
      "Cause": "Invalid response.",
      "Error": "ErrorA"
    }
  }
}

Looking at this for the first time, it all seems a bit overwhelming. However, after breaking it down, you will see that while you have to be precise with it, Step Functions are fairly easy to read. (Writing them is still a PITA, but that is just what it is) When you get down to it, the Step Function configuration file only contains 3 sections: ‘Comment‘, ‘StartsAt‘, and ‘States‘. That is it.

Let us take a quick look at the the first two mentioned above, Comment and StartsAt. The former of these is just a comment so you know what the purpose of the Step Function is. On the other hand, you could use this for whatever comment you like. Then, there is StartsAt. StartsAt, actually links to a State. And the States section is what contains all the real meat of the Step Function. The value that is used for StartsAt is the name of one of the States.

States

States are where everything really happens when it comes to Step Functions. They are the individual units that when strung together, make the magic happen. When you look at it from this perspective, it sounds ridiculously easy. But, just like everything else, the devil is in the details. The key to understanding States is to know what the various components that make up a State, and the type of states that are available.

Since this is an introduction, and based off of my code here, I am not going to go through all the options. However, there are docs from Amazon on it. The think about it is, it is not simple. Based on the docs, the only required field is Type. Great, but what are the various types? How do I link them together to make a cohesive whole? What are these other options and how do I pass variables from one section to the next and use them over and over again in a loop? This information is available, but it is scatter all over the documentation and you have to piecemeal it all together to figure out what is available.

What are the State types?

  • Pass – used to take input and pass to output. Basically, used to debug step functions, so you know what the heck is happening between States.
  • Task – this is where the work gets done. It can be a Lambda function or any other supported service. This will take your input and from there provide output to another State to be consumed
  • Choice – handles branching in your Step Function by looking at the output from the previous State and sending the Step Function to perform the next decided upon State in the workflow.
  • Wait – allows you to put a pause in your Step Function to wait for an action to finish, like deploying CloudFormation or some other long running task.
  • Succeed – the state machine has finished with success. This one is actually simple and does just what it says.
  • Fail – the end state where all has not gone according to plan. This is the opposite of Succeed, and allows for some reporting on what went wrong.
  • Parallel – run States at the same time. I have not yet worked with this, so I do not have as much information as I would like on this type of State

In the example above, I use only 4 of the States from above, Task, Choice, Succeed, and Fail. Mainly because I wanted to touch on a good bit of the basics without going to far down the rabbit hole. For my example, my Task was a Lambda function. To me this is the most logic way to use step functions, but there are probably many more that I have not considered. I also wanted to employ a loop. This being that I wanted to test flow control, and passing variable to and from Lambda functions. I could have strung the Lambda functions together in a straight line, but I wanted to test out branching as well.

The big factor to remember is that the order of the States does not matter. What matters is linking which State comes next. You could use a single Choice State to manage the flow through the entire Step Function, and based upon the input move onto any number of other States or back to a previous one. 

      ],
      "Next": "ChoiceState"
    },

This little block of code is a great example. The Next keyword is used, and the following word is the name of the next State block that is going to be executed. The name of each State is identified by the start of the json template block. In the example above, I am not going to a State of type Choice, but have created a State that is called ChoiceState. In retrospect, much of this could have been done cleaner, but it was an example that I put together rather quickly. This is the declaration:

    },
    "ChoiceState": {
      "Type": "Choice",
      "Choices": [

I am going to end here for today. The next section is going to be about passing data from the Step Function to Lambda, and how to reuse it. But, I want to work on a smaller block of code and focus on just the one thing.

Getting Started with AWS Step Functions Part I

AWS came out with Step Functions a few years ago, and up until recently, I have not had the opportunity to dive in and give them a try. Yes, I could build my own pipeline or state machine, but the idea behind Step Functions is that it does most of the heavy lifting for you. That, and it ties into other AWS services. As such, I decided to dive into getting started, and looked at the demo options and walkthroughs that were available. None of them met my needs, so I rolled my own.

The idea is to see how I can create a Step Function that will run multiple loops, and call a Lambda function multiple times. What I wanted to test was the following:

  • Pass Variables into the Step Function and see how they are handled
  • Call a Lambda function multiple times
  • Create a loop using the Step Function DSL
  • Test output from Lambda and make a decision based upon it
  • Figure out any gotchas and how to trigger Step Functions

Let’s dive in. Now, this is the final result. It took me a few iterations to actually get to this point. Smarter people than I might be able to get it done on one go, but not I.

Lambda Code

I came up with a simple Lambda function written in Python 3.6. All that I wanted to do was to perform a loop with Step Functions, and then get output the values. Simple. And as you can see, this code is pretty simple. It could be streamlined, but it was quick and easy to write.

def lambda_handler(event, context):
    print('value1 = ' + event['key1'])
    print('value2 = ' + event['key2'])
    print('value3 = ' + event['key3'])
    taskresult = event.get('taskresult', None)
    if taskresult is None:
        count = 0
    else:
        count = taskresult.get('count', None)
    
    if count is None:
        count = 0
    else:
        count = count +1
        
    if count < 5:
        output = {
            'count' : count,
            'value1' : 'ThereIsNoSpoon';
        }
    else:
        output = { 'value1' : event['key1'],
                    'value2': event['key2'],
                    'count' : count }
    return output

Now we need to move onto the body of what we are working on, and that would be the Step Function. Step Functions have their own language or domain specific language (DSL) that is used to define the state machine. I wanted more than just a “Hello World” example. The idea was to loop through a step functions. Make sure that I could call it multiple times, and then either go to a success or failed state

AWS Step Function Code

{
  "Comment": "A Retry example of the Amazon States Language using an AWS Lambda Function",
  "StartAt": "LambdaFunction",
  "States": {
    "LambdaFunction": {
      "Type": "Task",
      "Resource": "arn:aws:lambda:us-east-1:123456789012:function:aws-serverless-repository-hello-w-helloworldpython-1JQ8TEEDUAHCE",
      "ResultPath": "$.taskresult",
      "Retry": [
        {
          "ErrorEquals": ["CustomError"],
          "IntervalSeconds": 1,
          "MaxAttempts": 2,
          "BackoffRate": 2.0
        },
        {
          "ErrorEquals": ["States.TaskFailed"],
          "IntervalSeconds": 30,
          "MaxAttempts": 2,
          "BackoffRate": 2.0
        },
        {
          "ErrorEquals": ["States.ALL"],
          "IntervalSeconds": 5,
          "MaxAttempts": 5,
          "BackoffRate": 2.0
        }
      ],
      "Next": "ChoiceState"
    },
    "ChoiceState": {
      "Type": "Choice",
      "Choices": [
        {
          "Variable": "$.taskresult.value1",
          "StringEquals": "value1",
          "Next": "SuccessState"
        },
        {
          "Variable": "$.taskresult.count",
          "NumericLessThan": 5,
          "Next": "LambdaFunction"
        }
      ],
      "Default": "FailState"
    },
    "SuccessState": {
      "Type": "Succeed"
    },
    "FailState": {
      "Type": "Fail",
      "Cause": "Invalid response.",
      "Error": "ErrorA"
    }
  }
}

The way that this code works is as follows. Everything works around States. So, you have to move from State to State. This is a key concept when it comes to Step Functions. Now, there are multiple State types, but I am not going to go into that now. The key factor is that you will go through and loops if a proper return value is not returned. Looking at it now, it looks like a bunch of gobbledygook. I am going to have to come back and write up how this works later.

This is what the visual representation looks like when viewed in the AWS Step Function page. There is a defined ‘Start’ and ‘Stop’. The other stages match what was named in the previous section. The code works to present a model that you can follow.

The cool think about AWS Step Functions is that they guarantee a run. And in a situation where you need to ensure that the code is run, and you need a guarantee. This is mostly due to the cost that is associated with it. Running Lambda that Triggers on SQS would be cheaper, but not as easy to ensure.

Back on with our stuff now. We are looking at how we execute the AWS Step Function. Now we need to execute it. Right now, I am not going to go into the logic around passing variables around. Needless, you will need to understand that when writing your own, and I am going to have to revisit it.

Execution. A couple of items to note.

  • Each execution has to have a unique name.
    • Note, this will bite you when you are testing, and think about this when executing it via automation.
  • It takes in an action just like Lambda, via json
  • Making a small change in the inputs can cause madness
{
  "key1": "value1",
  "key2": "value2",
  "key3": "value3"
}

The output will also be in json, and you can see the results in the visual display.

{
  "key1": "value1",
  "key2": "value2",
  "key3": "value3",
  "taskresult": {
    "value1": "value1",
    "value2": "value2",
    "count": 5
  }
}

This is what the output looks like.

I will go into more detail on the breakdown of the Step Function in the next post. There is a lot to be covered, and this just scratches at the surface.