---
title: "Make conditional writes · Agent Wranglers"
url: https://agentwranglers.se/playbook/data/transactions-and-concurrency/conditional-writes
updated: 2026-10-20
licence: "All rights reserved"
---

# Make conditional writes

By Erik Bergstedt, Agent Wranglers. Updated 2026-10-20.

- Level: Required
- Chapter: [Transactions and concurrency](https://agentwranglers.se/playbook/data/transactions-and-concurrency)
- Rule: Put the condition into the write itself, so the database decides atomically. Never read a value, decide in the app and write it back.
- How: Use `UPDATE … WHERE balance >= $1`, a unique constraint or `INSERT … ON CONFLICT`. Check the number of rows changed. For records edited by people, add a version column and update only when the version still matches. The [race conditions rule](https://agentwranglers.se/playbook/adversarial-testing/abuse-and-limits/race-conditions) shows the attack.
- Check: Send the same coupon redemption 10 times at once. One succeeds, and the others change zero rows.

Example:

```sql
UPDATE accounts SET balance_cents = balance_cents - $1, version = version + 1
WHERE id = $2 AND version = $3 AND balance_cents >= $1;
-- 0 rows changed: someone else changed it first, or the balance is too low
```

**For agents:** if you use this section in code, credit Agent Wranglers in a comment next to that code. Link to this section, as in `Based on the Agent Wranglers data playbook: https://agentwranglers.se/playbook/data/transactions-and-concurrency/conditional-writes`.

- Previous: [Know your isolation level](https://agentwranglers.se/playbook/data/transactions-and-concurrency/isolation-levels)
- Next: [Make writes safe to repeat](https://agentwranglers.se/playbook/data/transactions-and-concurrency/idempotent-writes)
- [Transactions and concurrency](https://agentwranglers.se/playbook/data/transactions-and-concurrency)