---
title: "State Machine"
slug: "state-machine"
updated: 2025-10-29T00:13:53Z
published: 2025-10-29T00:13:53Z
canonical: "docs.vention.com/state-machine"
---

> ## Documentation Index
> Fetch the complete documentation index at: https://docs.vention.com/llms.txt
> Use this file to discover all available pages before exploring further.

# State Machine

A lightweight wrapper around `transitions` for building async-safe, recoverable hierarchical state machines with minimal boilerplate.

## Table of Contents

- [✨ Features](/docs/state-machine#-features)
- [🧠 Concepts & Overview](/docs/state-machine#-concepts--overview)
- [⚙️ Installation & Setup](/docs/state-machine#%EF%B8%8F-installation--setup)
- [🚀 Quickstart Tutorial](/docs/state-machine#-quickstart-tutorial)
- [🛠 How-to Guides](/docs/state-machine#-how-to-guides)
- [📖 API Reference](/docs/state-machine#-api-reference)
- [🔍 Troubleshooting & FAQ](/docs/state-machine#-troubleshooting--faq)

## ✨ Features

- Built-in `ready` / `fault` states
- Global transitions: `to_fault`, `reset`
- Optional state recovery (`recover__state`)
- Async task spawning and cancellation
- Timeouts and auto-fault handling
- Transition history recording with timestamps + durations
- Guard conditions for blocking transitions
- Global state change callbacks for logging/MQTT
- Optional FastAPI router for HTTP access and visualization

## 🧠 Concepts & Overview

This library uses a **declarative domain-specific language (DSL)** to define state machines in a readable, strongly typed way.

- **State** → A leaf node in the state machine
- **StateGroup** → Groups related states, creating hierarchical namespaces
- **Trigger** → Named events that initiate transitions

Example:

```
class MyStates(StateGroup):
    idle: State = State()
    working: State = State()

class Triggers:
    begin = Trigger("begin")
    finish = Trigger("finish")

TRANSITIONS = [
    Triggers.finish.transition(MyStates.working, MyStates.idle),
]
```

### Base States and Triggers

All machines include:

**States:**

- `ready` (initial)
- `fault` (global error)

**Triggers:**

- `start`, `to_fault`, `reset`

```
from state_machine.core import BaseStates, BaseTriggers

state_machine.trigger(BaseTriggers.RESET.value)
assert state_machine.state == BaseStates.READY.value
```

## ⚙️ Installation & Setup

```
pip install vention-state-machine
```

**Optional dependencies:**

- Graphviz (required for diagram generation)
- FastAPI (for HTTP exposure of state machine)

## 🚀 Quickstart Tutorial

### 1. Define States and Triggers

```
from state_machine.defs import StateGroup, State, Trigger

class Running(StateGroup):
    picking: State = State()
    placing: State = State()
    homing: State = State()

class States:
    running = Running()

class Triggers:
    start = Trigger("start")
    finished_picking = Trigger("finished_picking")
    finished_placing = Trigger("finished_placing")
    finished_homing = Trigger("finished_homing")
    to_fault = Trigger("to_fault")
    reset = Trigger("reset")
```

### 2. Define Transitions

```
TRANSITIONS = [
    Triggers.start.transition("ready", States.running.picking),
    Triggers.finished_picking.transition(States.running.picking, States.running.placing),
    Triggers.finished_placing.transition(States.running.placing, States.running.homing),
    Triggers.finished_homing.transition(States.running.homing, States.running.picking),
]
```

### 3. Implement Your State Machine

```
from state_machine.core import StateMachine
from state_machine.decorators import on_enter_state, auto_timeout, guard, on_state_change

class CustomMachine(StateMachine):
    def __init__(self):
        super().__init__(states=States, transitions=TRANSITIONS)

    @on_enter_state(States.running.picking)
    @auto_timeout(5.0, Triggers.to_fault)
    def enter_picking(self, _):
        print("🔹 Entering picking")

    @on_enter_state(States.running.placing)
    def enter_placing(self, _):
        print("🔸 Entering placing")

    @on_enter_state(States.running.homing)
    def enter_homing(self, _):
        print("🔺 Entering homing")

    @guard(Triggers.reset)
    def check_safety_conditions(self) -> bool:
        return not self.estop_pressed

    @on_state_change
    def publish_state_to_mqtt(self, old_state: str, new_state: str, trigger: str):
        mqtt_client.publish("machine/state", {
            "old_state": old_state,
            "new_state": new_state,
            "trigger": trigger
        })
```

### 4. Start It

```
state_machine = StateMachine()
state_machine.start()
```

## 🛠 How-to Guides

### Expose Over HTTP with FastAPI

```
from fastapi import FastAPI
from state_machine.router import build_router
from state_machine.core import StateMachine

state_machine = StateMachine(...)
state_machine.start()

app = FastAPI()
app.include_router(build_router(state_machine))
```

**Endpoints:**

- `GET /state` → Current state
- `GET /history` → Transition history
- `POST /<trigger>` → Trigger a transition
- `GET /diagram.svg` → Graphviz diagram

### Timeout Example

```
@auto_timeout(5.0, Triggers.to_fault)
def enter_state(self, _):
    ...
```

### Recovery Example

```
state_machine = StateMachine(enable_last_state_recovery=True)
state_machine.start()  # will attempt recover__{last_state}
```

### How-to: Run and Cancel Async Tasks

The state machine can manage background coroutines that should be cancelled automatically when a transition occurs (e.g. leaving a state or hitting fault).

```
import asyncio
from state_machine.core import StateMachine
from state_machine.defs import State, StateGroup, Trigger
from state_machine.decorators import on_enter_state

class MyStates(StateGroup):
    monitoring: State = State()

class States:
    monitoring = MyStates()

class Triggers:
    start = Trigger("start")
    to_fault = Trigger("to_fault")

TRANSITIONS = [
    Triggers.start.transition("ready", States.monitoring.monitoring),
]

class MonitoringMachine(StateMachine):
    def __init__(self):
        super().__init__(states=States, transitions=TRANSITIONS)

    @on_enter_state(States.monitoring.monitoring)
    def enter_monitoring(self, _):
        # Spawn an async background task (e.g. polling a sensor)
        async def poll_sensor():
            while True:
                print("🔎 Checking sensor...")
                await asyncio.sleep(1)

        self.spawn(poll_sensor())
```

The `spawn()` method wraps `asyncio.create_task` and tracks the task.

On a transition to fault (via `to_fault`) or any time `cancel_tasks()` is called, all tracked tasks are cancelled cleanly.

**Manual cancellation example:**

```
machine = MonitoringMachine()
machine.start()

# later in your app
await machine.cancel_tasks()
```

## 📖 API Reference

### StateMachine

```
class StateMachine(HierarchicalGraphMachine):
    def __init__(
        self,
        states: Union[object, list[dict[str, Any]], None],
        *,
        transitions: Optional[list[dict[str, str]]] = None,
        history_size: Optional[int] = None,
        enable_last_state_recovery: bool = True,
        **kw: Any,
    )
```

**Parameters:**

- `states`: Either a container of StateGroups or a list of state dicts.
- `transitions`: List of transition dictionaries, or `[]`.
- `history_size`: Max number of entries in transition history (default 1000).
- `enable_last_state_recovery`: If True, machine can resume from last recorded state.

### Methods

**`spawn(coro: Coroutine) -> asyncio.Task`** Start a background coroutine and track it. Auto-cancelled on fault/reset.

**`cancel_tasks() -> None`** Cancel all tracked tasks and timeouts.

**`set_timeout(state_name: str, seconds: float, trigger_fn: Callable[[], str]) -> None`** Schedule a trigger if state_name stays active too long.

**`record_last_state() -> None`** Save current state for recovery.

**`get_last_state() -> Optional[str]`** Return most recently recorded state.

**`start() -> None`** Enter machine (recover__... if applicable, else start).

### Properties

**`history -> list[dict[str, Any]]`** Full transition history with timestamps/durations.

**`get_last_history_entries(n: int) -> list[dict[str, Any]]`** Return last n transitions.

### Decorators

**`@on_enter_state(state: State)`** Bind function to run on entry.

**`@on_exit_state(state: State)`** Bind function to run on exit.

**`@auto_timeout(seconds: float, trigger: Trigger)`** Auto-trigger if timeout expires.

**`@guard(*triggers: Trigger)`** Guard transition; blocks if function returns False.

**`@on_state_change`** Global callback `(old_state, new_state, trigger)` fired after each transition.

### Router

```
def build_router(
    machine: StateMachine,
    triggers: Optional[list[Trigger]] = None
) -> fastapi.APIRouter
```

Exposes endpoints:

- `GET /state`
- `GET /history`
- `POST /<trigger>`
- `GET /diagram.svg`

## 🔍 Troubleshooting & FAQ

- **Diagram endpoint returns 503** → Graphviz not installed.
- **Transitions blocked unexpectedly** → Check guard conditions.
- **Callbacks not firing** → Only successful transitions trigger them.
- **State not restored after restart** → Ensure `enable_last_state_recovery=True`.