This document describes the basic concept of replicas in the PlanetScale CNDb context.

What is a replica?

A replica is a copy of your database that can serve data in response to queries and which can be promoted to master when necessary.

Replicas are distinct from read-only instances, which are like replicas, but which cannot be promoted to master.

Why have replicas?

Replicas have two main benefits:

  • Replicas let you scale up the rate of read queries your database can handle for a given number of shards. Using a replica results in eventually-consistent reads.
  • Replicas also prevent disruption to writes during planned maintenance.

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