# How Step Functions works

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This page was generated from content adapted from the [AWS Developer Guide](https://github.com/awsdocs/aws-step-functions-developer-guide.git)
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## Standard vs. Express Workflows

* **Note**\
  If you define your state machines outside the Step Functions' console, such as in an editor of your choice, you must save your state machine definitions with the extension *.asl.json*.

## States

* **Note**\
  If you define your state machines outside the Step Functions' console, such as in an editor of your choice, you must save your state machine definitions with the extension *.asl.json*.

## State Machine Data

* **Note**\
  Numbers in JSON text format conform to JavaScript semantics. These numbers typically correspond to double-precision [IEEE-854](https://standards.ieee.org/findstds/standard/854-1987.html) values. The following is valid JSON text: standalone, quote-delimited strings; objects; arrays; numbers; Boolean values; and `null`. The output of a state becomes the input into the next state. However, you can restrict states to working on a subset of the input data by using [Input and Output Processing](https://github.com/kevinslin/aws-reference-notes/blob/main/services/aws_step_functions/concepts-input-output-filtering.md).

## Input and Output Processing

* **Tip**\
  Use the [data flow simulator in the Step Functions console](https://console.aws.amazon.com/states/home?region=us-east-1#/simulator) to test JSON path syntax, to better understand how data is manipulated within a state, and to see how data is passed between states.

## Error handling

* **Note**\
  Unhandled errors in Lambda are reported as `Lambda.Unknown` in the error output. These include out-of-memory errors and function timeouts. You can match on `Lambda.Unknown`, `States.ALL`, or `States.TaskFailed` to handle these errors. When Lambda hits the maximum number of invocations, the error is `Lambda.TooManyRequestsException`. For more information about Lambda function errors, see [Error handling and automatic retries](https://docs.aws.amazon.com/lambda/latest/dg/invocation-retries.html) in the *AWS Lambda Developer Guide*.
* **Note**\
  Retries are treated as state transitions. For information about how state transitions affect billing, see [Step Functions Pricing](https://aws.amazon.com/step-functions/pricing/).
* **Note**\
  Each catcher can specify multiple errors to handle.
* **Note**\
  If you don't specify the `ResultPath` field, it defaults to `$`, which selects and overwrites the entire input.
* **Note**\
  When you create this Lambda function in the Lambda console, remember to change the **Timeout** value in the **Advanced settings** section from 3 seconds (default) to 11 seconds.
* **Note**\
  As a best practice, tasks that reference a Lambda function should handle Lambda service exceptions. For more information, see [Handle Lambda service exceptions](https://github.com/kevinslin/aws-reference-notes/blob/main/services/aws_step_functions/bp-lambda-serviceexception.md).
* **Note**\
  You can preserve the state input and the error by using `ResultPath`. See [Use ResultPath to Include Both Error and Input in a `Catch`](https://github.com/kevinslin/aws-reference-notes/blob/main/services/aws_step_functions/input-output-resultpath.md#input-output-resultpath-catch).

## Tagging in Step Functions

* **Note**\
  To manage tags for activites, see [Manage Tags with Step Functions API Actions](#tagging-api).