resonate-saga-pattern-python

Resonate Saga Pattern — Python

Overview

A saga is a long-running transaction split into smaller steps, each with a compensating action. Forward steps run top-to-bottom; on failure, compensations run bottom-to-top to restore consistency. Python's native try/except expresses this cleanly when the durable function uses await ctx.run(...) for each step.

For the TS version's mental model, see resonate-saga-pattern-typescript. The Python expression differs in syntax (try/except instead of try/catch, async def + await instead of function* + yield) and in how compensation lists are collected.

When to use

• Multi-step workflow where intermediate state is visible to other consumers (charge a card → create shipment)
• Each step is idempotent or inexpensive to retry individually
• Compensation logic exists for every committed step
• You need "all or nothing" consistency without a distributed transaction

Don't use when steps are a pure in-memory pipeline (use sequential ctx.run calls without compensation), or when you have access to a single DB transaction that covers all steps.

Basic shape

from __future__ import annotations
import asyncio, os, time
from typing import TYPE_CHECKING
from resonate.resonate import Resonate
from resonate.retry import Never

if TYPE_CHECKING:
    from resonate.context import Context

r = Resonate(url=os.environ.get("RESONATE_URL", "http://localhost:8001"), retry_policy=Never())

async def place_order(ctx: Context, order_id: str) -> dict:
    # Step 1: reserve inventory
    flight = await ctx.run(reserve_flight, order_id)

    # Step 2: charge (compensate flight on failure)
    try:
        hotel = await ctx.run(reserve_hotel, order_id)
    except Exception:
        await ctx.run(release_flight, flight)
        raise

    # Step 3: charge (compensate hotel + flight on failure)
    try:
        charge = await ctx.run(charge_card, order_id)
    except Exception:
        await ctx.run(release_hotel, hotel)
        await ctx.run(release_flight, flight)
        raise

    return {"status": "success", "order_id": order_id,
            "flight": flight, "hotel": hotel, "charge": charge}