# Horizontal Scaling

> A technique for adding capacity by running more stateless instances of a service behind a load balancer.

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

Listed in [Architecture principles](https://banes-lab.com/api/records/architecture.md), after [Scalability](https://banes-lab.com/records/architecture/scalability.md) and before [Vertical Scaling](https://banes-lab.com/records/architecture/vertical-scaling.md).

## Repair

- Refactored by: Externalize State, Add Load Balancer
- Detected by: local session/state coupling
- Violated by: sticky instance state required for correctness
- Measured by: scale-out efficiency
- Enforced by: deployment tests

## Requires

- [Statelessness or Shared State Strategy](https://banes-lab.com/records/lexicon/statelessness-or-shared-state-strategy.md)

## Reinforces

- [Elasticity](https://banes-lab.com/records/architecture/elasticity.md)
- [Availability](https://banes-lab.com/records/lexicon/availability.md)

## Enables

- [Scale-Out](https://banes-lab.com/records/lexicon/scale-out.md)

## Conflicts with

- [Instance-Local State](https://banes-lab.com/records/lexicon/instance-local-state.md)

## In tension with

- [Distributed Coordination](https://banes-lab.com/records/lexicon/distributed-coordination.md)

## Tensions

- [Horizontal Scaling / Distributed Coordination](https://banes-lab.com/records/tension/distributed-coordination-horizontal-scaling.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)

## Reinforced by

- [Competing Consumers](https://banes-lab.com/records/architecture/competing-consumers.md)
- [Sharding](https://banes-lab.com/records/architecture/sharding.md)
- [Statelessness](https://banes-lab.com/records/architecture/statelessness.md)
- [Read Replica](https://banes-lab.com/records/architecture/read-replica.md)
- [Space-Based Architecture](https://banes-lab.com/records/architecture/space-based-architecture.md)

## Linked from

- [A concern is a component](https://banes-lab.com/software-architecture/scale/a-concern-is-a-component.md)
- [Event / Messaging / Asynchronous Architecture](https://banes-lab.com/ontology/principles/architecture-category-event-messaging-asynchronous-architecture.md)
- [Scalability / Performance / Optimization](https://banes-lab.com/ontology/principles/architecture-category-scalability-performance-optimization.md)
- [Codebase / System Architecture Styles](https://banes-lab.com/ontology/principles/architecture-category-codebase-system-architecture-styles.md)
- [Quality Attributes](https://banes-lab.com/ontology/lexicon/lexicon-category-quality-attributes.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)