# Read Replica

> A technique for routing read queries to replicated copies of a database, so the primary handles only writes.

Record: `architecture:read-replica`
Kind: technique
Layer: [Performance Core](https://banes-lab.com/records/layer/performance-core.md)
Severity: contextual
Scope: service, database, scalability
Canonical: https://banes-lab.com/ontology#architecture-read-replica

Listed in [Architecture principles](https://banes-lab.com/api/records/architecture.md), after [CDN / Edge Caching](https://banes-lab.com/records/architecture/cdn-edge-caching.md) and before [Queuing Theory](https://banes-lab.com/records/architecture/queuing-theory.md).

## Repair

- Refactored by: Route Reads to Replicas
- Detected by: read load saturating the write primary
- Violated by: all reads and writes hitting one primary
- Measured by: primary read/write contention ratio
- Enforced by: database design review

## Requires

- [Replication](https://banes-lab.com/records/architecture/replication.md)

## Reinforces

- [Horizontal Scaling](https://banes-lab.com/records/architecture/horizontal-scaling.md)
- [Load Balancing](https://banes-lab.com/records/architecture/load-balancing.md)

## Enables

- [Read Traffic Offload](https://banes-lab.com/records/lexicon/read-traffic-offload.md)

## Conflicts with

- [Single-Primary Read Contention](https://banes-lab.com/records/lexicon/single-primary-read-contention.md)

## In tension with

- [Read-Your-Writes Consistency](https://banes-lab.com/records/lexicon/read-your-writes-consistency.md)

## Tensions

- [Read Replica / Read-Your-Writes Consistency](https://banes-lab.com/records/tension/read-replica-read-your-writes-consistency.md)

## Severity

- [contextual](https://banes-lab.com/records/vocabulary/severity-contextual.md)

## Category

- [Scalability / Performance / Optimization](https://banes-lab.com/records/architecture-category/scalability-performance-optimization.md)

## Linked from

- [Scalability / Performance / Optimization](https://banes-lab.com/ontology/principles/architecture-category-scalability-performance-optimization.md)
- [Self-Healing / Recovery / Deployment Safety](https://banes-lab.com/ontology/principles/architecture-category-self-healing-recovery-deployment-safety.md)
- [Scalability / Performance / Optimization](https://banes-lab.com/ontology/lexicon/lexicon-category-scalability-performance-optimization.md)
- [Severity levels](https://banes-lab.com/ontology/schema/the-vocabulary-severity.md)
- [The resolutions](https://banes-lab.com/ontology/schema/the-resolutions.md)