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step-functions.md

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AWS Step Functions

  • Step Functions allow us to model our workflows as stat machines
  • A workflow is represented as a JSON state machine
  • Features:
    • Sequence
    • Parallel execution
    • Conditions
    • Timeouts
    • Error handling
  • Can also integrate with EC2, ECS, on premise servers, API Gateway
  • Maximum execution time is 1 year
  • Possibility to implement human approval feature
  • Use cases:
    • Order fulfillment
    • Data processing
    • Web applications
    • Any workflow
  • When designing a Step Function we get an aspect of visualization (flow diagram)
  • The execution can be visually represented on this diagram

Task States

  • States in the state machine which can do some work, for example:
    • Invoke an AWS service such as Lambda function, start an AWS Batch job, run an ECS task, insert an item into DynamoDB table, etc.
    • Run an Activity:
      • Activities run on an EC2 machine, ECS container, on-premises server
      • Activities poll the Step Functions for work
      • Activities send back the result to the Step Function
  • State types:
    • Choice State: test for a condition to send to a branch
    • Fail or Succeed State: stop execution with failure or success
    • Pass State: simply pass its input to the output or inject some fixed data
    • Wait State: wait for a certain amount of time or until a specific date/time
    • Map State: dynamically iterate steps
    • Parallel State: begin parallel branches of execution
  • Any state can encounter errors:
    • State machine definition issues (example. no matching rules in choice state)
    • Task failures (example: an exception in a Lambda function)
    • Transient failures (example: network partition events)
  • By default: when a state reports an error, the execution of the Step Functions fails entirely
  • Failures can be retried:
    • Exponential back-off: IntervalSeconds, MaxAttempts, BackoffRate
    • Move on - Catch: ErrorEquals, Next
  • Best practice: include data in the error message

Standard vs Express Step Functions

  • Standard Function
    • Maximum duration: 1 year
    • Exactly-one workflow execution
    • Execution start rate: 2000 per second
    • Price per state transition: more expensive in general
  • Express Function
    • Maximum duration: 5 minutes
    • At-least-once workflow execution, an execution could run more than once
    • Execution start rate: 100_000 per second

Step Function Use Cases