Source: https://frameworkrecipes.dev/laravel/atomic-reservation/

# Use a conditional Laravel update to prevent overselling

Two requests can both pass a separate availability read. The conditional decrement lets the database decide against the current row when it performs the write. A unique tenant/SKU row and a nonnegative database constraint provide additional enforcement.

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 Laravel boundary

Save this illustrative integration module as `app/Actions/ReserveStock.php`.

```
<?php
namespace App\Actions;
use App\Models\Inventory;

class ReserveStock {
    public function handle(int $tenantId, string $sku, int $quantity): void {
        if ($quantity <= 0) throw new \InvalidArgumentException('Positive quantity required');
        $changed = Inventory::where('tenant_id', $tenantId)
            ->where('sku', $sku)
            ->where('available', '>=', $quantity)
            ->decrement('available', $quantity);
        if ($changed !== 1) throw new \RuntimeException('Stock not available');
    }
}
```

The order, inventory, or resource-lifetime scenario is adapted from Perch’s public [order-service source at commit 4af67af](https://github.com/lakeday-org/perch/blob/4af67af46d76ac99398d61c7ad0a45441760d4dc/test/fixtures/order-service/inventory.py). This Laravel implementation is an illustrative adaptation.

## Integrate with your application

Use PostgreSQL or another database with the required UPDATE and isolation behavior, an Inventory model, and a unique (tenant_id, sku) constraint. Request validation should reject noninteger quantities before calling this typed service. Wrap reservation and order insertion together in DB::transaction if both must succeed atomically.

## Exercise the boundary and failure paths

| Scenario | Expected outcome |
| --- | --- |
| Two buyers reserve the last unit | Only one changes the row |
| Quantity exceeds availability | Zero affected rows and no decrement |
| Cross-tenant request | No other tenant row changes |
| Order insert fails in outer transaction | Reservation rolls back |

Add regression tests for these cases in your existing Laravel 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:

```
php artisan test --filter=ReserveStockTest
```

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:

```
->where('available', '>=', $quantity)
```

with:

```
// Remove the source span shown above.
```

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-laravel-atomic-reservation.yaml` at the repository root, keeping your existing `perch.yaml`:

```
- name: reservation-conditional-write
  where: "app/Actions/ReserveStock.php"
  ensure: >-
    The inventory decrement must be conditional on available >= quantity in the same database query.
```

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

```
npm install -g @lakeday/perch
perch login
perch check app/Actions/ReserveStock.php --rules reservation-conditional-write --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.

| Source control | Exit | Outcome |
| --- | --- | --- |
| Broken | 3 | Rule finding |
| Corrected | 0 | No rule finding |

The rule distinguished these controls in this run. [Raw Perch capture](https://frameworkrecipes.dev/laravel/atomic-reservation/capture.json) · [Exact sources and mutation](https://frameworkrecipes.dev/laravel/atomic-reservation/source.json).

## Install the rule pack and configure your agent

[Download this rule pack ZIP](https://reporules.fyi/laravel/atomic-reservation/rule-pack.zip) · [Download the YAML rules](https://reporules.fyi/laravel/atomic-reservation/rules.yaml) · [Agent task](https://reporules.fyi/laravel/atomic-reservation/agent-task.txt). 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
```

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

```
curl -fL 'https://frameworkrecipes.dev/laravel/atomic-reservation/rule-pack.zip' -o 'perch-guide-laravel-atomic-reservation.zip'
unzip -n 'perch-guide-laravel-atomic-reservation.zip'
```

The installed rule is `.perch/rules/perch-guide-laravel-atomic-reservation.yaml`. Its selector matches `app/Actions/ReserveStock.php` in this example; adapt the selector to your application file before checking an existing project.

1. 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
```

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

> Use the installed Perch skill while editing app/Actions/ReserveStock.php. Preserve reservation-conditional-write. Run perch check app/Actions/ReserveStock.php --rules reservation-conditional-write --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.

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

```
perch check app/Actions/ReserveStock.php --rules reservation-conditional-write --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

- [laravel.com reference](https://laravel.com/framework/docs/13.x/queries#increment-and-decrement)

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.

**Framework Recipes**

Implementation, failure path, check.

Published by Perch · © 2026 Verglas LLC. Framework Recipes is a Perch publication.

Perch’s MIT license covers the CLI; hosted model analysis is a paid service. Relevant source is sent to the configured model service.

[Perch CLI MIT license](https://github.com/lakeday-org/perch/blob/main/LICENSE) · [Article text](https://frameworkrecipes.dev/laravel/atomic-reservation/index.md) · [Resource index](https://frameworkrecipes.dev/llms.txt)