rsf 3.14

Replacement for Step Functions — YAML DSL, code gen, and visual tooling for Lambda Durable Functions

RSF — Replacement for Step Functions

A complete Python toolkit for defining, visualizing, generating, deploying, and debugging AWS Lambda Durable Functions workflows — with full AWS Step Functions feature parity.

What is RSF?

RSF replaces AWS Step Functions with a local-first developer experience built on AWS Lambda Durable Functions (launched at re:Invent 2025). Define workflows in YAML, generate deployment-ready Python code, and inspect executions with a visual debugger — no hosted orchestration service required.

rsf_version: "1.0"
StartAt: ProcessOrder
States:
  ProcessOrder:
    Type: Task
    Next: NotifyCustomer
  NotifyCustomer:
    Type: Task
    End: true

Features

• YAML/JSON DSL — All 8 ASL state types, 39 comparison operators, 18 intrinsic functions, 5-stage I/O pipeline
• Python Code Generation — Lambda Durable Functions SDK code from your workflow definition, with handler stubs that are never overwritten
• Terraform Generation — Complete Lambda + IAM + CloudWatch infrastructure with auto-derived permissions
• ASL Importer — Migrate existing Step Functions workflows to RSF in minutes
• Visual Graph Editor — React-based editor with bidirectional YAML/graph sync and ELK auto-layout
• Execution Inspector — Time machine scrubbing, live SSE updates, structural JSON diffs
• CLI Toolchain — rsf init, rsf generate, rsf validate, rsf deploy, rsf import, rsf ui, rsf inspect

Screenshots

Graph Editor	Execution Inspector

Quickstart

Install

pip install rsf

Create a project

rsf init my-workflow
cd my-workflow

Created project: my-workflow/

  + my-workflow/workflow.yaml
  + my-workflow/handlers/__init__.py
  + my-workflow/handlers/example_handler.py
  + my-workflow/pyproject.toml
  + my-workflow/.gitignore
  + my-workflow/tests/__init__.py
  + my-workflow/tests/test_example.py

Edit workflow.yaml to define your states, then generate code.

Generate code

rsf generate

Generated: orchestrator.py
  Created: handlers/validate_order.py
  Created: handlers/process_payment.py
  Created: handlers/send_confirmation.py

Summary: orchestrator written, 3 handler(s) created, 0 skipped.

Deploy

rsf deploy

Code generated: orchestrator.py + 3 handler(s) (0 skipped)
Terraform generated: 6 file(s) in terraform (0 skipped)

Running terraform init...
Running terraform apply...

Deploy complete

Inspect executions

rsf inspect

Opens the execution inspector at http://localhost:8001

Architecture

┌──────────────────────────────────────────────────┐
│                    RSF CLI                        │
│  init · generate · validate · deploy · import    │
│  ui · inspect                                    │
├──────────────────────────────────────────────────┤
│                                                  │
│  ┌──────────┐  ┌──────────┐  ┌──────────┐      │
│  │ DSL Core │  │ Code Gen │  │ Terraform│      │
│  │ Pydantic │  │  Jinja2  │  │   HCL    │      │
│  └──────────┘  └──────────┘  └──────────┘      │
│                                                  │
│  ┌──────────┐  ┌──────────┐  ┌──────────┐      │
│  │   ASL    │  │  Graph   │  │Inspector │      │
│  │ Importer │  │  Editor  │  │  + SSE   │      │
│  └──────────┘  └──────────┘  └──────────┘      │
│                                                  │
├──────────────────────────────────────────────────┤
│              React UI (Vite + xyflow)            │
│        Graph Editor  ·  Execution Inspector      │
└──────────────────────────────────────────────────┘

Documentation

Full documentation is available at the docs site:

• Tutorial — From install through deploy and inspect
• DSL Reference — Complete field reference for all state types
• State Types Guide — Detailed examples with YAML + Python
• Migration Guide — Import existing Step Functions workflows

Technical Stack

Layer	Technology
DSL Models	Pydantic v2 (discriminated unions)
Code Generation	Jinja2 templates
Infrastructure	Terraform HCL generation
CLI	Typer + Rich
Web Backend	FastAPI (REST, WebSocket, SSE)
React UI	React 19 + @xyflow/react + Zustand + Monaco Editor
Graph Layout	ELK.js (Sugiyama algorithm)
Testing	pytest (441+ tests) + vitest (52 tests)

Requirements

• Python 3.12+ (SDK requires 3.13+)
• Node.js 18+ (for UI development)
• AWS account with Lambda Durable Functions enabled (for deployment)

Development

# Clone and install
git clone https://github.com/pgdad/rsf-python.git
cd rsf-python
pip install -e ".[dev]"

# Run tests
pytest

# Run UI tests
cd ui && npm test

# Start dev server (graph editor + inspector)
rsf ui --dev

License

MIT