# Keep a doobie reservation and order insert in one transaction

doobie ConnectionIO composes statements on one connection when the combined action is transacted. Checking the affected-row count inside that action ensures insufficient stock aborts before order creation and rolls back any prior write.

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 http4s boundary

Save this illustrative integration module as `src/main/scala/Reserve.scala`.

```scala
import cats.effect.IO
import doobie._
import doobie.implicits._
import cats.syntax.all._

object Reserve {
  def apply(xa:Transactor[IO], tenant:String, sku:String, quantity:Int): IO[Int] = {
    if (quantity <= 0) IO.raiseError(new IllegalArgumentException("Positive quantity required"))
    else {
      val action = for {
        count <- sql"UPDATE inventory SET available=available-$quantity WHERE tenant_id=$tenant AND sku=$sku AND available >= $quantity".update.run
        _ <- if (count == 1) ().pure[ConnectionIO]
             else new RuntimeException("Insufficient stock").raiseError[ConnectionIO,Unit]
        created <- sql"INSERT INTO orders (tenant_id,sku,quantity) VALUES ($tenant,$sku,$quantity)".update.run
      } yield created
      action.transact(xa)
    }
  }
}
```

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 http4s implementation is an illustrative adaptation.

## Integrate with your application

Use PostgreSQL, a doobie Transactor[IO], and http4s handlers that resolve tenant identity before calling this service. Add unique tenant/SKU and nonnegative stock constraints. Keep all related statements in action before transact; separately transacting each statement loses atomicity. The SQL interpolator binds values as parameters rather than concatenating untrusted text.



## Exercise the boundary and failure paths

| Scenario | Expected outcome |
| --- | --- |
| Sufficient stock | Inventory and order commit |
| Insufficient stock | No order |
| Order INSERT fails | Stock decrement rolls back |
| Two requests for final unit | Conditional update permits one reservation |

Add regression tests for these cases in your existing http4s 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:

```sh
sbt "testOnly ReserveIntegrationSuite"
```

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:

```scala
if (count == 1)
```

with:

```scala
if (true)
```

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-http4s-doobie-conditional-reservation.yaml` at the repository root, keeping your existing `perch.yaml`:

```yaml
- name: failed-stock-check-prevents-insert
  where: "src/main/scala/Reserve.scala"
  ensure: >-
    The order INSERT must run only if the inventory UPDATE affected one row. A zero-row reservation must raise an error before insertion.
```

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

```sh
npm install -g @lakeday/perch
perch login
perch check src/main/scala/Reserve.scala --rules failed-stock-check-prevents-insert --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](/doobie-conditional-reservation/capture.json) · [Exact sources and mutation](/doobie-conditional-reservation/source.json).

## Install the rule pack and configure your agent

[Download this rule pack ZIP](/doobie-conditional-reservation/rule-pack.zip) · [Download the YAML rules](/doobie-conditional-reservation/rules.yaml) · [Agent task](/doobie-conditional-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:

```sh
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:

```sh
curl -fL 'https://reporules.fyi/http4s/doobie-conditional-reservation/rule-pack.zip' -o 'perch-guide-http4s-doobie-conditional-reservation.zip'
unzip -n 'perch-guide-http4s-doobie-conditional-reservation.zip'
```

The installed rule is `.perch/rules/perch-guide-http4s-doobie-conditional-reservation.yaml`. Its selector matches `src/main/scala/Reserve.scala` 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:

```sh
perch setup claude-code
```

Codex:

```sh
perch setup codex
```

Cursor:

```sh
perch setup cursor
```

Pi:

```sh
perch setup pi
```

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

> Use the installed Perch skill while editing src/main/scala/Reserve.scala. Preserve failed-stock-check-prevents-insert. Run perch check src/main/scala/Reserve.scala --rules failed-stock-check-prevents-insert --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:

```sh
perch check src/main/scala/Reserve.scala --rules failed-stock-check-prevents-insert --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

- [typelevel.org reference](https://typelevel.org/doobie/docs/03-Connecting.html)

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.