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 analytics 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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