AWS Step Functions

Core concept: Step Functions orchestrate multi-step workflows using state machines — coordinate Lambda, SQS, DynamoDB, ECS, and 200+ AWS services.

Standard vs Express Workflows

Feature	Standard	Express
Max duration	1 year	5 minutes
Execution model	Exactly-once	At-least-once
Execution history	Full history in console	CloudWatch Logs only
Pricing	Per state transition	Per execution + duration
Use case	Long-running business processes	High-volume, short workflows

State Types

State	Purpose
`Task`	Do work (invoke Lambda, call API, etc.)
`Choice`	Branch based on conditions
`Wait`	Pause for a duration or until a timestamp
`Parallel`	Execute branches simultaneously
`Map`	Iterate over an array
`Pass`	Pass input to output (for testing/transformation)
`Succeed`	End the workflow successfully
`Fail`	End the workflow with an error

State Machine Definition (ASL)

{
  "Comment": "Order Processing Workflow",
  "StartAt": "ValidateOrder",
  "States": {
    "ValidateOrder": {
      "Type": "Task",
      "Resource": "arn:aws:lambda:us-east-1:123:function:ValidateOrder",
      "Next": "ProcessPayment",
      "Catch": [{
        "ErrorEquals": ["ValidationError"],
        "Next": "SendFailureNotification"
      }]
    },
    "ProcessPayment": {
      "Type": "Task",
      "Resource": "arn:aws:lambda:us-east-1:123:function:ProcessPayment",
      "Retry": [{
        "ErrorEquals": ["States.TaskFailed"],
        "IntervalSeconds": 2,
        "MaxAttempts": 3,
        "BackoffRate": 2.0
      }],
      "Next": "IsPaymentApproved"
    },
    "IsPaymentApproved": {
      "Type": "Choice",
      "Choices": [{
        "Variable": "$.paymentStatus",
        "StringEquals": "APPROVED",
        "Next": "FulfillOrder"
      }],
      "Default": "SendPaymentFailed"
    },
    "FulfillOrder": {
      "Type": "Parallel",
      "Branches": [
        {
          "StartAt": "UpdateInventory",
          "States": {
            "UpdateInventory": {
              "Type": "Task",
              "Resource": "arn:aws:lambda:...:UpdateInventory",
              "End": true
            }
          }
        },
        {
          "StartAt": "SendConfirmationEmail",
          "States": {
            "SendConfirmationEmail": {
              "Type": "Task",
              "Resource": "arn:aws:lambda:...:SendEmail",
              "End": true
            }
          }
        }
      ],
      "End": true
    }
  }
}

Error Handling

Retry

"Retry": [{
  "ErrorEquals": ["Lambda.ServiceException", "Lambda.TooManyRequestsException"],
  "IntervalSeconds": 1,
  "MaxAttempts": 3,
  "BackoffRate": 2.0   // 1s, 2s, 4s
}]

Catch

"Catch": [{
  "ErrorEquals": ["PaymentDeclined"],
  "ResultPath": "$.error",   // Preserve error info
  "Next": "HandlePaymentError"
}]

Wait for Callback Pattern

For human approval or external system responses:

"WaitForHumanApproval": {
  "Type": "Task",
  "Resource": "arn:aws:states:::lambda:invoke.waitForTaskToken",
  "Parameters": {
    "FunctionName": "SendApprovalEmail",
    "Payload": {
      "taskToken.$": "$$.Task.Token",
      "orderId.$": "$.orderId"
    }
  },
  "TimeoutSeconds": 86400,
  "Next": "ProcessApproval"
}

The Lambda sends a taskToken to the approver. They call SendTaskSuccess / SendTaskFailure to resume the workflow.

Map State (Parallel Processing)

"ProcessAllOrders": {
  "Type": "Map",
  "ItemsPath": "$.orders",
  "MaxConcurrency": 10,
  "Iterator": {
    "StartAt": "ProcessSingleOrder",
    "States": {
      "ProcessSingleOrder": {
        "Type": "Task",
        "Resource": "arn:aws:lambda:...:ProcessOrder",
        "End": true
      }
    }
  },
  "Next": "SendSummary"
}

🧪 Practice Questions

Q1. A workflow needs to process each item in a list in parallel, up to 5 items at a time. Which state type achieves this?

A) Parallel state
B) Choice state with conditions
C) Map state with MaxConcurrency: 5
D) Multiple Task states

✅ Answer & Explanation

C — The Map state iterates over an array, applying the same workflow to each item. MaxConcurrency controls parallelism. Parallel runs different branches simultaneously, not the same branch for each item.

Q2. A payment workflow needs to pause and wait for a manual approval that may come hours later via an API call. Which pattern enables this?

A) Wait state with a fixed duration
B) Poll DynamoDB every minute for approval
C) Task state with .waitForTaskToken (Callback pattern)
D) Choice state polling an SQS queue

✅ Answer & Explanation

C — The Callback pattern (waitForTaskToken) pauses the workflow indefinitely. An external system calls SendTaskSuccess or SendTaskFailure with the token to resume. No polling, no fixed wait time.

Standard vs Express Workflows​

State Types​

State Machine Definition (ASL)​

Error Handling​

Retry​

Catch​

Wait for Callback Pattern​

Map State (Parallel Processing)​

🧪 Practice Questions​

🔗 Resources​