105
read replica
Copies that serve reads. They lag. A read-your-writes screen must not use them.
What is read replica?
A read replica follows the primary and answers SELECTs. It takes read load off the primary. It is seconds or milliseconds behind. Writes still go to the primary. Replication lag is normal, not an incident, until a user can see it.
Why does read replica matter when vibe coding?
Models send every query to the replica “to scale reads,” including the SELECT right after INSERT. The UI says the save failed. Name lag and the exception path.
How do you do read replica?
Lists and reports can use the replica. The request that just wrote reads the primary, or waits for the replica to catch that id. Show lag in metrics.
How do you ask a model for read replica?
Send read-heavy (lists) to a read replica. Anything that just wrote, or must be read-your-writes, hits the primary. Do not treat replica lag as a bug to hide. Measure it.
What goes wrong with read replica?
A replica in another region used for a checkout balance. The user spends money the primary already knows they do not have.