Keep Fastify reservation and order writes on the same transaction client

A transaction callback can look atomic while a helper accidentally uses the pool. That helper’s write can commit independently. Pass the transaction client through every helper belonging to the reservation workflow, and let errors propagate to roll back.

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 Fastify boundary

Save this illustrative integration module as reservation.cjs.

async function reserveAndCreateOrder(app, tenantId, sku, quantity) {
  if (!Number.isSafeInteger(quantity) || quantity <= 0) throw new Error("Invalid quantity");
  return app.pg.transact(async client => {
    const stock = await client.query(
      "UPDATE inventory SET available=available-$1 WHERE tenant_id=$2 AND sku=$3 AND available >= $1 RETURNING sku",
      [quantity, tenantId, sku],
    );
    if (stock.rowCount !== 1) throw new Error("Insufficient stock");
    const order = await client.query(
      "INSERT INTO orders (tenant_id, sku, quantity) VALUES ($1,$2,$3) RETURNING id",
      [tenantId, sku, quantity],
    );
    return order.rows[0].id;
  });
}
module.exports = { reserveAndCreateOrder };

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

Integrate with your application

Install @fastify/postgres and configure PostgreSQL inventory and orders tables. Add unique tenant/SKU and nonnegative stock constraints. Transaction ownership belongs to pg.transact; a called helper must not issue its own COMMIT or swallow an insert failure. A transaction does not atomically charge an external payment provider: use an idempotent payment workflow and outbox for that boundary.

Exercise the boundary and failure paths

Add regression tests for these cases in your existing Fastify 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:

node --test tests/reservation.test.cjs

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:

const order = await client.query(

with:

const order = await app.pg.query(

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-fastify-shared-transaction-client.yaml at the repository root, keeping your existing perch.yaml:

- name: reservation-transaction-client
  where: "reservation.cjs"
  ensure: >-
    Both inventory and order writes inside pg.transact must use the supplied client, never app.pg.query on a separate connection.

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

npm install -g @lakeday/perch
perch login
perch check reservation.cjs --rules reservation-transaction-client --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/fastify/shared-transaction-client/rule-pack.zip' -o 'perch-guide-fastify-shared-transaction-client.zip'
unzip -n 'perch-guide-fastify-shared-transaction-client.zip'

The installed rule is .perch/rules/perch-guide-fastify-shared-transaction-client.yaml. Its selector matches reservation.cjs 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 reservation.cjs. Preserve reservation-transaction-client. Run perch check reservation.cjs --rules reservation-transaction-client --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 reservation.cjs --rules reservation-transaction-client --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

• github.com 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.