---
title: "Read replica"
slug: "read-replica"
category: "glossary"
tags: ["glossary", "backend", "database", "postgres", "scaling", "replication"]
status: "stable"
last_updated: 2026-05-14
summary: "A read replica is a continuously synchronized copy of a primary database that serves SELECT queries, offloading read traffic for horizontal scaling."
related:
  [
    "[[backend/postgres]]",
    "[[glossary/write-ahead-log]]",
    "[[glossary/base]]",
    "[[glossary/materialized-view]]",
    "[[glossary/transaction-isolation]]",
    "[[glossary/acid]]",
  ]
---

## Overview

This page is the atomic definition. Postgres replication setup lives at [[backend/postgres]].

## Definition

A read replica is a secondary database instance that streams committed changes from the primary via the [[glossary/write-ahead-log]] and serves read-only queries. Writes always go to the primary; reads can go to any replica. The replica lags behind the primary by some replication delay (typically milliseconds to seconds), so queries against it may return stale data, making it a [[glossary/base]]-like system for those reads. Postgres supports physical streaming replication (byte-level WAL copy) and logical replication (row-level change streams). Cloud platforms (RDS, Neon, Supabase) provision replicas with one click. Common use cases: reporting and analytics that would otherwise contend with OLTP writes; read-heavy API workloads where stale reads are acceptable; geographic distribution of read traffic. Direct your writes and reads that require up-to-date state exclusively to the primary.

## When it applies

Add a read replica when primary CPU is dominated by read queries and write latency is suffering. Do not route writes or read-your-own-writes queries to a replica unless the application handles replication lag explicitly.

## Example

An e-commerce site routes product catalog searches to a read replica (stale by 200 ms is acceptable) and routes checkout queries to the primary (inventory must be current).

## Related concepts

- [[backend/postgres]] - Postgres streaming replication and WAL-based architecture.
- [[glossary/write-ahead-log]] - the mechanism that replicates changes to replicas.
- [[glossary/base]] - the consistency model replicas exhibit due to replication lag.
- [[glossary/materialized-view]] - materialized views provide another form of precomputed read offloading.

## Citing this term

> See [[glossary/read-replica|Read replica]] (llmbestpractices.com/glossary/read-replica).

## Related

- [[backend/postgres]]
- [[glossary/write-ahead-log]]
- [[glossary/base]]
- [[glossary/materialized-view]]
- [[glossary/transaction-isolation]]