AWS Step Functions

Overview

AWS Step Functions uses Amazon States Language (ASL) to define state machines as JSON. With AWS Step Functions, you can create workflows, also called State machines, to build distributed applications, automate processes, orchestrate microservices, and create data and machine learning pipelines.

This skill provides comprehensive guidance for writing state machines in ASL, covering:

• ASL structure and JSONata expression syntax
• Details on the eight available workflow states
• The $states reserved variable
• Workflow variables with Assign
• Error handling
• AWS Service integration patterns
• Example code for data transformation and architecture
• Validation and testing of state machines
• How to migrate from JSONPath to JSONata

When to Load Reference Files

Load the appropriate reference file based on what the user is working on:

• ASL structure, state types, Task, Pass, Choice, Wait, Succeed, Fail, Parallel, Map → see references/asl-state-types.md
• Error handling, troubleshooting, Retry, Catch, fallback, error codes, States.Timeout, States.ALL → see references/error-handling.md
• Service integrations, Lambda invoke, DynamoDB, SNS, SQS, SDK integrations, Resource ARN, sync, async → see references/service-integrations.md
• Migrating from JSONPath to JSONata, migration, JSONPath to JSONata, InputPath, Parameters, ResultSelector, ResultPath, OutputPath, intrinsic functions, Iterator, payload template → see references/migrating-from-jsonpath-to-jsonata.md
• Validation, linting, testing, TestState, test state, mock, mocking, unit test, inspection level, DEBUG, TRACE, validate state, test in isolation → see references/validation-and-testing.md
• Architecture patterns, examples, polling, saga, compensation, scatter-gather, semaphore, lock, human-in-the-loop, escalation, Express to Standard → see references/architecture-patterns.md
• Data transformation, JSONata expressions, filtering, aggregation, string operations, $reduce, $lookup, $toMillis, $partition, $parse, $hash, $uuid → see references/transforming-data.md
• State input/output, $states, Assign, Output, Arguments, variable scope, variable limits, evaluation order, passing data between states → see references/processing-state-inputs-and-outputs.md
• Deployment, SAM, CloudFormation, IaC, DefinitionSubstitutions, X-Ray tracing, logging → see the aws-serverless-deployment skill or deploy-on-aws plugin

Quick Reference

Standard vs Express Workflows

Choose Standard for: payment processing, order fulfillment, compliance workflows, anything that must never execute twice.

Choose Express for: IoT data ingestion, streaming transformations, mobile backends, high-throughput short-lived processing.

Setting the State Machine Query Language

JSONata is the modern, preferred way to reference and transform data in ASL. It replaces the five JSONPath I/O fields (InputPath, Parameters, ResultSelector, ResultPath, OutputPath) with just two: Arguments (inputs) and Output.

Enable at the top level to apply to all states:

{ "QueryLanguage": "JSONata", "StartAt": "...", "States": {...} }

Or per-state to migrate from JSONPath incrementally:

{ "Type": "Task", "QueryLanguage": "JSONata", ... }

JSONPath is still supported and is the default if QueryLanguage is omitted — existing state machines do not need to be migrated.

Best Practices

• Set "QueryLanguage": "JSONata" at the top level for new state machines unless the user wants to use JSONPath
• Keep Output minimal — only include what the state immediately after the current state needs
• Use Assign to store variables needed in later states instead of threading it through Output
• Use $states.input to reference original state input
• Remember: Assign and Output are evaluated in parallel — variable assignments in Assign are NOT available in Output of the same state
• All JSONata expressions must produce a defined value — $data.nonExistentField throws States.QueryEvaluationError
• Use $states.context.Execution.Input to access the original workflow input from any state
• Save state machine definitions with .asl.json extension when working outside the console
• Prefer the optimized Lambda integration (arn:aws:states:::lambda:invoke) over the SDK integration

Troubleshooting

Common Errors

• States.QueryEvaluationError — JSONata expression failed. Check for type errors, undefined fields, or out-of-range values.
• Mixing JSONPath fields with JSONata fields in the same state.
• Using $ or $$ at the top level of a JSONata expression — use $states.input instead.
• Forgetting {% %} delimiters around JSONata expressions — the string will be treated as a literal.
• Assigning variables in Assign and expecting them in Output of the same state — new values only take effect in the next state.
• Reference references/validation-and-testing.md and references/error-handling.md for detailed troubleshooting information.

Resources

• ASL Specification
• JSONata documentation
• Step Functions Developer Guide