Keep a FastAPI order update and its outbox event in one transaction

Publishing a message after a database write leaves a crash window. An outbox row is written with the order state, so a worker can deliver it later. A unique event_key prevents accidental duplicate event insertion; a delivery consumer still needs its own deduplication.

Save this requirement in Perch and check it after each agent edit. The rule gives your agent a named check to run, a finding to repair, and the same command to recheck.

Implement the FastAPI boundary

Save this illustrative integration module as confirmation.py.

import json
from sqlalchemy import update
from models import Order, OutboxEvent

async def confirm_order(session, order_id, tenant_id):
    async with session.begin():
        result = await session.execute(
            update(Order)
            .where(Order.id == order_id, Order.tenant_id == tenant_id,
                   Order.status == "pending")
            .values(status="confirmed")
            .returning(Order.id)
        )
        if result.scalar_one_or_none() is None:
            raise ValueError("No pending order in tenant")
        session.add(OutboxEvent(
            event_key=f"order-confirmed:{order_id}",
            payload=json.dumps({"order_id": order_id, "tenant_id": tenant_id}),
        ))
        await session.flush()
    return order_id

The order, inventory, or resource-lifetime scenario is adapted from Perch’s public order-service source at commit 4af67af. This FastAPI implementation is an illustrative adaptation.

Integrate with your application

Use PostgreSQL models Order and OutboxEvent and a unique constraint on event_key. Start this service with a fresh session, then have a separate worker claim committed outbox rows. Keep network publication out of this transaction. A worker crash after publishing but before marking delivery can repeat a message; the consumer must deduplicate by event_key.

Exercise the boundary and failure paths

Add regression tests for these cases in your existing FastAPI test project. Use separate connections or deliberately reordered completion where the case involves concurrency; a sequential happy-path test does not exercise that failure. Run the test command after creating the matching test file or suite:

python -m pytest tests/test_outbox_transaction.py

The scenarios above specify tests to implement. A completed Perch source check below is separate from running these application integration tests.

Introduce the specific agent regression

In a throwaway copy, replace this source span:

await session.flush()
    return order_id

with:

await session.flush()
        await session.commit()
        raise ValueError("abort after early commit")
    return order_id

Run the relevant negative test and the Perch command on that broken copy. Restore the corrected source before committing the application. The supplied mutation reproduces the exact regression for the rule check.

Preserve the requirement as a Perch rule

Save this in .perch/rules/perch-guide-fastapi-outbox-transaction.yaml at the repository root, keeping your existing perch.yaml:

- name: outbox-no-early-commit
  where: "confirmation.py"
  ensure: >-
    The order update and outbox insertion stay in the same transaction. The function must not explicitly commit inside the session.begin block before it finishes successfully.

Install and authenticate the CLI once, then check the current working file:

npm install -g @lakeday/perch
perch login
perch check confirmation.py --rules outbox-no-early-commit --json

Use Node 22 or newer. The framework can use another language; Perch’s CLI remains a Node command. A gated finding exits 3. Exits 1 or 2 mean the check did not complete and must be reported as a blocker.

Inspect the captured rule check

On 2026-10-14, Perch using jev-latest checked the exact fixed and broken source provided here. The capture records SHA-256 hashes of both sources and the requirement.

The rule distinguished these controls in this run. Raw Perch capture · Exact sources and mutation.

Install the rule pack and configure your agent

Download this rule pack ZIP · Download the YAML rules · Agent task. The pack contains the rule, its example paths and the repair/recheck instructions. Review the where selector before using it in an existing project. Keep your root perch.yaml; install this rule as a separate file under .perch/rules/.

1. Install Perch using Node 22 or newer and sign in once:

npm install -g @lakeday/perch
perch login

2. Download the pack, inspect it, and extract it from your repository root. The extraction preserves any existing files:

curl -fL 'https://frameworkrecipes.dev/fastapi/outbox-transaction/rule-pack.zip' -o 'perch-guide-fastapi-outbox-transaction.zip'
unzip -n 'perch-guide-fastapi-outbox-transaction.zip'

The installed rule is .perch/rules/perch-guide-fastapi-outbox-transaction.yaml. Its selector matches confirmation.py in this example; adapt the selector to your application file before checking an existing project.

3. Configure the coding agent you use. Run one of these commands:

Claude Code:

perch setup claude-code

Codex:

perch setup codex

Cursor:

perch setup cursor

Pi:

perch setup pi

4. Ask the agent to edit the implementation and check the saved requirement:

Use the installed Perch skill while editing confirmation.py. Preserve outbox-no-early-commit. Run perch check confirmation.py --rules outbox-no-early-commit --json after the edit. Read each finding and the affected code, fix the implementation, and run the identical check again after the fix. Keep the rule and regression tests intact. Run the application's relevant regression tests and report the commands and actual results. Exit 3 means a rule finding; exit 1 or 2 means the check did not complete. Report an unavailable or incomplete check as blocked. Do not claim the application tests ran if only the Perch check ran.

5. After repairing a finding, run the same check again and inspect the result:

perch check confirmation.py --rules outbox-no-early-commit --json

Commit the downloaded rule and the installed agent configuration with your application. The rule file adds no guarantee of correctness; keep the regression tests from this guide.

With the base reference fetched, use perch scan --since origin/main --filter type=lint in CI.

Framework references and verification scope

• docs.sqlalchemy.org reference

The dated capture records the source check shown above. Implement the adapters, schema and regression tests described in this guide in your application. Unchanged requests reuse cached answers; fresh model judgments can vary. Model analysis sends relevant source to the configured service.