Choice, pass, fail and succeed states in AWS Step Functions

This will be the last article explaining the different states we can use in a step function. We’ll see three simple states like Pass, Fail and Succeed and finally, we’re going to a see a more complex state like Choice. And obviously, we’re going to use the http://serverless.com framework to deploy them.

Pass state

The pass state is a simple state that just passes its input to its output, without performing any work. Apart from the common fields, you can specify two optional fields:

• Result: The result to pass to the next state, filtered by the ResultPath field.
• ResultPath: Specifies where (in the input) put the output specified in the Result field.

Fail state

The fail state stops the execution of the Step Function and marks it as a failure. It only allows the Type and Comment fields from the common fields, and you can use a couple of optional fields:

• Cause: a custom failure string
• Error: an error name that can be use for error handling.

Succeed state

The succeed state stops the execution of the Step Function successfully. It’s a terminal state, so it doesn’t have a Next or End field. It’s a good target for a choice branch that you just want to stop the execution.

Choice state

The choice state allows you to declare a decision logic into your state machine. You can specify different branches with different logic to access them. In addition to the common fields it adds a couple of fields:

• Choices (required): an array of choice rules that determines the next state.
• Default (optional but recommended): the state to transition if no choice rule is satisfied.

Choice rules

Each choice rule contains a comparision and a next field. Comparisions can be composed using And or Or operators. You can check all the operators available here.

When defining a comparision you must specify two fields:

• Variable: which value are you going to compare. It will be a path of the input value of the function.
• Operator: the field name will be the operator you want to use and the value will be the value you want to compare with.

Let’s put everything together

So, let’s put all we’ve learned so far in a single step function (without any lambda this time):

stepFunctions:
    stateMachines:
        testChoiceStepFunction:
            definition:
                StartAt: DoChoice
                States:
                    DoChoice:
                        Type: Choice
                        Choices: 
                        - Variable: "$.value"
                            NumericGreaterThan: 0
                            Next: PositiveNumber
                        - Variable: "$.value"
                            NumericLessThan: 0
                            Next: NegativeNumber
                        Default: Zero
                    PositiveNumber:
                        Type: Pass
                        Result: {"result": "It's a positive number!"}
                        Next: FinalState
                    NegativeNumber:
                        Type: Pass
                        Result: {"result": "It's a negative number!"}
                        Next: FinalState
                    Zero:
                        Type: Fail
                        Cause: "It's a zero!"
                    FinalState:
                        Type: Succeed

What are we defining here is a step function with two choices (number greater than 0 and number less than zero) and a default state. Every choice has a next state. Both PositiveNumber and NegativeNumber are Pass states with a different result, the Zero state is a Fail state and the FinalState is a Succeed state.

Let’s deploy and run the function and see what happens.

sls invoke stepf --nam

e testChoiceStepFunction –data ‘{“value”: 1}’

sls invoke stepf --nam

e testChoiceStepFunction –data ‘{“value”: -1}’

sls invoke stepf --nam

e testChoiceStepFunction –data ‘{“value”: 0}’

Summary

We’ve seen how easy is to set up a choice state in State Functions, allowing us to choose different paths depending on the input of the state.