etcd-fsm

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A typed, asynchronous distributed finite-state machine over etcd.

The current implementation contains the declarative type system, schema and processor compiler, signature-derived transition graph, an internal etcd contract, a deterministic MVCC fake, and the production aetcd adapter. The production runtime is under construction.

from enum import StrEnum, auto
from typing import Literal
from uuid import UUID

from etcd_fsm import BaseState, FSMProcessor, FSMSchema, Retry


class OrderStatus(StrEnum):
    CREATED = auto()
    PAYMENT_REQUIRED = auto()


class OrderState[S: OrderStatus](BaseState[S]):
    order_id: UUID

    def fsm_key(self) -> str:
        return str(self.order_id)


class Created(OrderState[Literal[OrderStatus.CREATED]]):
    state: Literal[OrderStatus.CREATED] = OrderStatus.CREATED


class PaymentRequired(OrderState[Literal[OrderStatus.PAYMENT_REQUIRED]]):
    state: Literal[OrderStatus.PAYMENT_REQUIRED] = OrderStatus.PAYMENT_REQUIRED
    attempt: int = 1


orders = FSMSchema(
    name="orders",
    namespace="/state/orders",
    state_enum=OrderStatus,
    states=(Created, PaymentRequired),
)
payments = FSMProcessor(schema=orders, name="payments")


@payments.reaction()
async def request_payment(state: Created) -> PaymentRequired | Retry:
    return PaymentRequired(order_id=state.order_id)


payments.finalize()

Function annotations are the transition definition. Concrete State Models use literal enum discriminators, models are immutable snapshots, and the schema rejects incomplete or inconsistent declarations before a runtime starts.

The public Python API and graph CLI are documented in Russian in docs/API.md. The PyPI release process is documented in docs/RELEASING.md.

Transition graph

Import modules containing schemas and processors and render their combined transition graph as Mermaid:

etcd-fsm graph my_service.orders my_service.payments

Python file paths are accepted as well:

etcd-fsm graph tests/order_domain.py

Every transition is labeled with its processor, function, and producer kind. Use explicit module:OBJECT or path/to/file.py:OBJECT references when automatic discovery is not appropriate, or render Graphviz DOT into a file:

etcd-fsm graph my_service.orders:ORDER_SCHEMA \
  my_service.payments:PAYMENTS --format dot -o orders.dot

The architectural vocabulary is documented in CONTEXT.md. Design premises and implementation order are recorded in docs/DESIGN-NOTES.md, with accepted decisions under docs/adr/.

Local etcd

Start an ephemeral etcd instance for integration tests:

docker compose up -d --wait

The client endpoint is http://127.0.0.1:12379. Stop it and discard its data with docker compose down.

Run the real-etcd integration suite explicitly:

uv run pytest --run-integration -m integration

The integration tests use a unique /etcd-fsm-tests/<uuid>/ prefix and clean it after each test. Override the endpoint with ETCD_FSM_TEST_HOST and ETCD_FSM_TEST_PORT.