Iterating a Loop Using Lambda

Sign in to the Lambda console, and then choose Create function.

On the Create function page, choose Author from scratch.

In the Basic information section, configure your Lambda function, as follows:

1. For Function name, enter Iterator.

2. For Runtime, choose Node.js 14.x.

3. In Change default execution role, choose Create a new role with basic Lambda permissions.

4. Choose Create function.

5. After your Lambda function is created, copy the function's Amazon Resource Name (ARN) displayed in the upper-right corner of the page. To copy the ARN, click . The following is an example ARN:

   arn:aws-cn:lambda:us-east-1:123456789012:function:Iterator

Copy the following code for the Lambda function into the Code source section of the Iterator page.

exports.handler = function iterator (event, context, callback) {
  let index = event.iterator.index
  let step = event.iterator.step
  let count = event.iterator.count
 
  index = index + step
 
  callback(null, {
    index,
    step,
    count,
    continue: index < count
  })
}

This code accepts input values for count, index, and step. It increments the index by the value of step and returns these values, and the Boolean continue. The value of continue is true if index is less than count.

Choose Deploy.

Choose Test.

In the Configure test event dialog box, enter TestIterator in the Event name box.

Replace the example data with the following.

{
  "Comment": "Test my Iterator function",
  "iterator": {
    "count": 10,
    "index": 5,
    "step": 1
  }
}

These values mimic what would come from your state machine during an iteration. The Lambda function will increment the index and return continue as true. When the index isn't less than the count, it returns continue as false. For this test, the index has already incremented to 5. The results should increment the index to 6 and set continue to true.

Choose Create.

On the Iterator page, choose Test to test your Lambda function.

The results of the test are displayed in the Execution results tab.

Choose the Execution results tab to see the output.

{
  "index": 6,
  "step": 1,
  "count": 10,
  "continue": true
}

Open the Step Functions console, and then choose Create a state machine.

On the Choose authoring method page, choose Write your workflow in code.

For Type, retain the default selection, that is, Standard.

In the Definition pane, paste the following code, but replace the ARN of the Lambda function that you created earlier in the Resource field.

{
    "Comment": "Iterator State Machine Example",
    "StartAt": "ConfigureCount",
    "States": {
        
        "ConfigureCount": {
            "Type": "Pass",
            "Result": {
                "count": 10,
                "index": 0,
                "step": 1
            },
            "ResultPath": "$.iterator",
            "Next": "Iterator"
        },
        "Iterator": {
            "Type": "Task",
            "Resource": "arn:aws-cn:lambda:us-east-1:123456789012:function:Iterate",
            "ResultPath": "$.iterator",
            "Next": "IsCountReached"
        },
        "IsCountReached": {
            "Type": "Choice",
            "Choices": [
                {
                    "Variable": "$.iterator.continue",
                    "BooleanEquals": true,
                    "Next": "ExampleWork"
                }
            ],
            "Default": "Done"
        },
        "ExampleWork": {
            "Comment": "Your application logic, to run a specific number of times",
            "Type": "Pass",
            "Result": {
              "success": true
            },
            "ResultPath": "$.result",
            "Next": "Iterator"
        },
        "Done": {
            "Type": "Pass",
            "End": true
          
        }
    }
}

Be sure to update the ARN in the Iterator state above, so that it references the Lambda function that you created earlier.

Use the graph in the Visual Workflow pane to check that your Amazon States Language code describes your state machine correctly.

This graph shows the logic expressed in the previous state machine code.

For more information about the Amazon States Language, see State Machine Structure.

If you don't see the graph, choose in the Visual Workflow pane.

Choose Next.

Enter a Name for your state machine, such as IterateCount.

In Permissions, choose Create new role.

Choose Create state machine.

On the IterateCount page, choose Start execution.

The Start execution dialog box is displayed.

(Optional) To identify your execution, you can specify a name for it in the Name box. By default, Step Functions generates a unique execution name automatically.

Choose Start Execution.

A new execution of your state machine starts, showing your running execution.

The execution increments in steps, tracking the count using your Lambda function. On each iteration, it performs the example work referenced in the ExampleWork state in your state machine.

(Optional) In the Details tab, view the Execution Status and the timestamps for Started and End Time of the execution.

When the count reaches the number specified in the ConfigureCount state in your state machine, the execution quits iterating and ends.