Learn about state machines in Step Functions - AWS Step Functions
Key conceptsState Machine DataInvoke Step FunctionsTransitions in state machinesRead Consistency

Learn about state machines in Step Functions

Step Functions is based on state machines and tasks. In Step Functions, state machines are called workflows, which are a series of event-driven steps. Each step in a workflow is called a state. For example, a Task state represents a unit of work that another AWS service performs, such as calling another AWS service or API. Instances of running workflows performing tasks are called executions in Step Functions.

Illustrative example of a Step Functions workflow diagram.

Key concepts

The following provides an overview of the key Step Functions terms for context.

Term Description
Workflow A sequence of steps that often reflect a business process.
States

Individual steps in your state machine that can make decisions based on their input, perform actions from those inputs, and pass output to other states.

For more information, see Discovering workflow states to use in Step Functions.
Workflow Studio

A visual workflow designer that helps you to prototype and build workflows faster.

For more information, see Developing workflows in Step Functions Workflow Studio.
State machine

A workflow defined using JSON text representing the individual states or steps in the workflow along with fields, such as StartAt, TimeoutSeconds, and Version.

For more information, see State machine structure in Amazon States Language for Step Functions workflows.
Amazon States Language

A JSON-based, structured language used to define your state machines. With ASL, you define a collection of states that can do work (Task state), determine which states to transition to next (Choice state), and stop an execution with an error (Fail state).

For more information, see Using Amazon States Language to define Step Functions workflows.
Input and output configuration

States in a workflow receive JSON data as input and usually pass JSON data as output to the next state. Step Functions provides filters to control the data flow between states.

For more information, see Processing input and output in Step Functions.
Service integration

You can call AWS service API actions from your workflow.

For more information, see Integrating services with Step Functions.
Service integration type

  • AWS SDK integrations – Standard way to call any of over two hundred AWS services and over nine thousand API actions directly from your state machine.

  • Optimized integrations – Custom integrations that streamline calling and exchanging data with certain services. For example, Lambda Invoke will automatically convert the Payload field of the response from an escaped JSON string into a JSON object.
Service integration pattern

When calling an AWS service, you use one of the following service integration patterns:
Execution

State machine executions are instances where you run your workflow to perform tasks.

For more information, see Starting state machine executions in Step Functions.

State Machine Data

State machine data takes the following forms:

  • The initial input into a state machine

  • Data passed between states

  • The output from a state machine

This section describes how state machine data is formatted and used in AWS Step Functions.

Data Format

State machine data is represented by JSON text. You can provide values to a state machine using any data type supported by JSON.

Note

  • Numbers in JSON text format conform to JavaScript semantics. These numbers typically correspond to double-precision IEEE-854 values.

  • The following is valid JSON text:

    • Standalone, quote-delimited strings

    • Objects

    • Arrays

    • Numbers

    • Boolean values

    • null

  • The output of a state becomes the input for the next state. However, you can restrict states to work on a subset of the input data by using Input and Output Processing.

State Machine Input/Output

You can give your initial input data to an AWS Step Functions state machine in one of two ways. You can pass the data to a StartExecution action when you start an execution. You can also pass the data to the state machine from the Step Functions console. Initial data is passed to the state machine's StartAt state. If no input is provided, the default is an empty object ({}).

The output of the execution is returned by the last state (terminal). This output appears as JSON text in the execution's result.

For Standard Workflows, you can retrieve execution results from the execution history using external callers, such as the DescribeExecution action. You can view execution results on the Step Functions console.

For Express Workflows, if you enabled logging, you can retrieve results from CloudWatch Logs, or view and debug the executions in the Step Functions console. For more information, see Using CloudWatch Logs to log execution history in Step Functions and Viewing execution details in the Step Functions console.

You should also consider quotas related to your state machine. For more information, see Step Functions service quotas

State Input/Output

Each state's input consists of JSON text from the preceding state or, for the StartAt state, the input into the execution. Certain flow-control states echo their input to their output.

In the following example, the state machine adds two numbers together.

  1. Define the AWS Lambda function.

    function Add(input) {
  var numbers = JSON.parse(input).numbers;
  var total = numbers.reduce(
    function(previousValue, currentValue, index, array) {
      return previousValue + currentValue; });
  return JSON.stringify({ result: total });
}

  2. Define the state machine.

    {
  "Comment": "An example that adds two numbers together.",
  "StartAt": "Add",
  "Version": "1.0",
  "TimeoutSeconds": 10,
  "States":
    {
        "Add": {
          "Type": "Task",
          "Resource": "arn:aws:lambda:us-east-1:123456789012:function:Add",
          "End": true
        }
    }
}

  3. Start an execution with the following JSON text.

    { "numbers": [3, 4] }

    The Add state receives the JSON text and passes it to the Lambda function.

    The Lambda function returns the result of the calculation to the state.

    The state returns the following value in its output.

    { "result": 7 }

    Because Add is also the final state in the state machine, this value is returned as the state machine's output.

    If the final state returns no output, then the state machine returns an empty object ({}).

For more information, see Processing input and output in Step Functions.

Invoke AWS Step Functions from other services

You can configure several other services to invoke state machines. Based on the state machine's workflow type, you can invoke state machines asynchronously or synchronously. To invoke state machines synchronously, use the StartSyncExecution API call or Amazon API Gateway integration with Express Workflows. With asynchronous invocation, Step Functions pauses the workflow execution until a task token is returned. However, waiting for a task token does make the workflow synchronous.

Services that you can configure to invoke Step Functions include:

Step Functions invocations are governed by the StartExecution quota. For more information, see:

Transitions in state machines

When you start a new execution of your state machine, the system begins with the state referenced in the top-level StartAt field. This field, given as a string, must exactly match, including case, the name of a state in the workflow.

After a state runs, AWS Step Functions uses the value of the Next field to determine the next state to advance to.

Next fields also specify state names as strings. This string is case-sensitive and must match the name of a state specified in the state machine description exactly

For example, the following state includes a transition to NextState.

"SomeState" : {
  ...,
  "Next" : "NextState"
}

Most states permit only a single transition rule with the Next field. However, certain flow-control states, such as a Choice state, allow you to specify multiple transition rules, each with its own Next field. The Amazon States Language provides details about each of the state types you can specify, including information about how to specify transitions.

States can have multiple incoming transitions from other states.

The process repeats until it either reaches a terminal state (a state with "Type": Succeed, "Type": Fail, or "End": true), or a runtime error occurs.

When you redrive an execution, it's considered as a state transition. In addition, all states that are rerun in a redrive are also considered as state transitions.

The following rules apply to states within a state machine:

  • States can occur in any order within the enclosing block. However, the order in which they're listed doesn't affect the order in which they're run. That order is determined by the contents of the states.

  • Within a state machine, there can be only one state designated as the start state. The start state is defined by the value of the StartAt field in the top-level structure.

  • Depending on your state machine logic — for example, if your state machine has multiple logic branches — you may have more than one end state.

  • If your state machine consists of only one state, it can be both the start and end state.

Transitions in Distributed Map state

When you use the Map state in Distributed mode, you'll be charged one state transition for each child workflow execution that the Distributed Map state starts. When you use the Map state in Inline mode, you aren't charged a state transition for each iteration of the Inline Map state.

You can optimize cost by using the Map state in Distributed mode and include a nested workflow in the Map state definition. The Distributed Map state also adds more value when you start child workflow executions of type Express. Step Functions stores the response and status of the Express child workflow executions, which reduces the need to store execution data in CloudWatch Logs. You can also get access to flow controls available with a Distributed Map state, such as defining error thresholds or batching a group of items. For information about Step Functions pricing, see AWS Step Functions pricing.

Read Consistency in Step Functions

State machine updates in AWS Step Functions are eventually consistent. All StartExecution calls within a few seconds will use the updated definition and roleArn (the Amazon Resource Name for the IAM role). Executions started immediately after calling UpdateStateMachine might use the previous state machine definition and roleArn.

For more information, see the following: