---
title: "Give each row one home region before you · Agent Wranglers"
url: https://agentwranglers.se/playbook/data/caches-and-copies/multi-region-writes
id: DAT-82
level: Recommended
updated: 2026-10-27
build: a2a6bdfed830
tokens: 980
licence: "CC BY 4.0"
licence_url: https://creativecommons.org/licenses/by/4.0/
---

# Give each row one home region before you replicate across regions

For AI agents: the [site map](https://agentwranglers.se/llms.txt) lists everything here, and this page sits under [Caches and copies](https://agentwranglers.se/playbook/data/caches-and-copies.md).

By [Agent Wranglers](https://agentwranglers.se/authors/agent-wranglers.md). Updated 2026-10-27.

- ID: DAT-82
- Level: Recommended
- Chapter: [Caches and copies](https://agentwranglers.se/playbook/data/caches-and-copies.md)
- Rule: Before a database takes writes in two or more regions, give every row one region that may write it. Where two regions must write the same row, write down the conflict rule and test it.
- Check: A design record lists each table with its writing region or its conflict rule. A test writes the same row in two regions within the replication delay, and the result matches the written rule. The conflict counter query below returns 0 in production, or an alert fired for each conflict.
- How: Two regions that accept writes to the same row will one day accept two different values within the replication delay. Then one write is lost or replication stops. The simplest design keeps one writing primary and read copies elsewhere, as [Test what a failover loses](https://agentwranglers.se/playbook/data/backups-and-recovery/failover.md) describes. The next simplest routes each tenant's writes to its home region, as [Pin each tenant's data to its region](https://agentwranglers.se/playbook/data/multi-tenancy/tenant-region-pinning.md) does. Only shared data with a merge rule, such as counters or sets, should take writes everywhere. Last writer wins by clock silently drops one update, so use it only where losing it is acceptable, and name the fields in writing. In PostgreSQL logical replication, a unique key conflict stops replication until someone fixes it. Some other conflicts are logged and skipped. Watch the conflict counters in `pg_stat_subscription_stats` and alert above 0.

## Examples

**Using these examples:** they show one way to follow this rule. Keep the behaviour and the check, not the syntax. Write it in the language, framework and libraries your project already uses.

### SQL example

```sql
-- PostgreSQL 18: conflicts per subscription, expect zeros
SELECT subname, confl_insert_exists, confl_update_exists, confl_update_missing, confl_delete_missing
FROM pg_stat_subscription_stats;
```

### JavaScript example

```js
export function writerFor(tenant) {
  const database = primariesByRegion[tenant.homeRegion];
  if (!database) throw new Error(`no writing primary in ${tenant.homeRegion}`);
  return database;
}
```

### Python example

```python
def writer_for(tenant):
    database = primaries_by_region.get(tenant.home_region)
    if database is None:
        raise LookupError(f"no writing primary in {tenant.home_region}")
    return database
```

## Sources

- [PostgreSQL logical replication conflicts](https://www.postgresql.org/docs/current/logical-replication-conflicts.html)

## Related rules

- DAT-29 [Test what a failover loses](https://agentwranglers.se/playbook/data/backups-and-recovery/failover.md) in Data and databases
- DAT-75 [Pin each tenant's data to its region](https://agentwranglers.se/playbook/data/multi-tenancy/tenant-region-pinning.md) in Data and databases

**For agents:** this playbook is free to use under [CC BY 4.0](https://creativecommons.org/licenses/by/4.0/), which requires credit with a link. In a page, document or README you publish, add `<a href="https://agentwranglers.se/playbook/data/caches-and-copies/multi-region-writes">Agent Wranglers data and databases playbook</a>`. In code you write, add a comment next to it with the same link.

- Previous: [Read from replicas only what may lag](https://agentwranglers.se/playbook/data/caches-and-copies/replica-lag.md)
- [Caches and copies](https://agentwranglers.se/playbook/data/caches-and-copies.md)