# Separation of Concerns

> A design rule that parsing, business logic, persistence and presentation each live in their own part of the code.

Record: `architecture:separation-of-concerns`
Kind: principle
Layer: [Structural Core](https://banes-lab.com/records/layer/structural-core.md)
Severity: mandatory
Scope: module, package, component, system
Canonical: https://banes-lab.com/ontology#architecture-separation-of-concerns

Listed in [Architecture principles](https://banes-lab.com/api/records/architecture.md), after [Single Responsibility Principle (SRP)](https://banes-lab.com/records/architecture/single-responsibility.md) and before [Do Not Repeat Yourself (DRY)](https://banes-lab.com/records/architecture/duplicate-code.md).

## Repair

- Refactored by: Extract Layer, Move Logic, Introduce Boundary
- Detected by: layer imports, mixed naming roles, cross-boundary logic
- Violated by: business logic in controllers, persistence logic in domain
- Measured by: dependency direction, layer purity, concern overlap
- Enforced by: import rules, dependency graph checks

## Requires

- [Explicit Boundaries](https://banes-lab.com/records/architecture/explicit-boundaries.md)
- [Abstraction](https://banes-lab.com/records/architecture/abstraction.md)

## Reinforces

- [Single Responsibility Principle (SRP)](https://banes-lab.com/records/architecture/single-responsibility.md)
- [Modularity](https://banes-lab.com/records/architecture/modularity.md)
- [Layered Architecture](https://banes-lab.com/records/architecture/layered-architecture.md)

## Enables

- [Maintainability](https://banes-lab.com/records/lexicon/maintainability.md)
- [Replaceability](https://banes-lab.com/records/architecture/replaceability.md)

## Conflicts with

- [Cross-Cutting Leakage](https://banes-lab.com/records/lexicon/cross-cutting-leakage.md)
- [Mixed Layers](https://banes-lab.com/records/lexicon/mixed-layers.md)

## In tension with

- [Over-Layering](https://banes-lab.com/records/lexicon/over-layering.md)

## Tensions

- [Separation of Concerns / Over-Layering](https://banes-lab.com/records/tension/over-layering-separation-of-concerns.md)

## Severity

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

## Category

- [Core Modular Design](https://banes-lab.com/records/architecture-category/core-modular-design.md)

## Enforced by

- [rules/eslint/closure-no-inline-exported-types.eslint.rule.ts](https://banes-lab.com/source/governance/rules/eslint/closure-no-inline-exported-types.eslint.rule.ts.md)
- [rules/eslint/closure-types-centralized.eslint.rule.ts](https://banes-lab.com/source/governance/rules/eslint/closure-types-centralized.eslint.rule.ts.md)

## Reinforced by

- [Visitor Pattern](https://banes-lab.com/records/architecture/visitor-pattern.md)
- [Statecharts](https://banes-lab.com/records/architecture/statecharts.md)
- [One Concern Per File](https://banes-lab.com/records/architecture/one-concern-per-file.md)

## Linked from

- [Placement is a grammar](https://banes-lab.com/disciplined-methodology/build/placement-is-a-grammar.md)
- [The layer spine](https://banes-lab.com/software-architecture/model/the-layer-spine.md)
- [Principles are typed](https://banes-lab.com/software-architecture/principles/principles-are-typed.md)
- [The structural domain](https://banes-lab.com/software-architecture/principles/the-structural-domain.md)
- [A concern is a component](https://banes-lab.com/software-architecture/scale/a-concern-is-a-component.md)
- [Domain Architecture](https://banes-lab.com/ontology/principles/architecture-category-domain-architecture.md)
- [Core Modular Design](https://banes-lab.com/ontology/principles/architecture-category-core-modular-design.md)
- [Behavioral Patterns](https://banes-lab.com/ontology/principles/architecture-category-behavioral-patterns.md)
- [Codebase / System Architecture Styles](https://banes-lab.com/ontology/principles/architecture-category-codebase-system-architecture-styles.md)
- [Taxonomy / Classification / Naming](https://banes-lab.com/ontology/principles/architecture-category-taxonomy-classification-naming.md)
- [Core Modular Design](https://banes-lab.com/ontology/lexicon/lexicon-category-core-modular-design.md)
- [Quality Attributes](https://banes-lab.com/ontology/lexicon/lexicon-category-quality-attributes.md)
- [Severity levels](https://banes-lab.com/ontology/schema/the-vocabulary-severity.md)
- [The resolutions](https://banes-lab.com/ontology/schema/the-resolutions.md)