---
title: "Dependency Rule"
section: "concepts"
type: "concept"
created: "2026-08-25"
updated: "2026-08-25"
tags: ["principle"]
canonical: "https://pyweb.dev/wiki/dependency-rule"
---
# Dependency Rule

The fundamental organizing principle of [clean architecture](/wiki/clean-architecture) that governs how software components can depend on each other. The rule states that **source code dependencies can only point inwards** toward higher-level abstractions.

## Core Constraints

### Inward-Only Dependencies
- Nothing in an inner circle can know anything about an outer circle
- Names of outer circle entities (functions, classes, variables) must not be mentioned in inner circles
- Data formats from outer circles cannot be used by inner circles
- Frameworks in outer circles cannot influence inner circle design

### Abstraction Hierarchy
As you move inward through the concentric circles:
- **Software becomes more abstract** and policy-oriented
- **Level of abstraction increases** from concrete details to general principles  
- **Inner circles are more general** and stable
- **Outer circles contain mechanisms** and implementation details

## Boundary Crossing Mechanics

When control flow must cross boundaries in a direction that would violate the dependency rule, the [dependency inversion principle](/wiki/dependency-inversion-principle) resolves the contradiction:

1. **Interfaces in Inner Circles** — Define contracts that outer circles implement
2. **Dynamic Polymorphism** — Allows source dependencies to oppose control flow direction
3. **Simple Data Structures** — Cross boundaries without carrying framework dependencies

### Example Pattern
Use case needs to call presenter:
- Use case calls interface (Use Case Output Port) in its own layer
- Presenter in outer layer implements that interface
- Source dependency points inward (rule satisfied)
- Control flows outward (functionality achieved)

## Data Crossing Rules

Data crossing boundaries must be:
- **Simple structures** — DTOs, function arguments, basic objects
- **Free of dependencies** — No framework types or database rows
- **Convenient for inner circle** — Format optimized for business rule consumption

Anti-pattern: Passing database framework RowStructure inward violates the rule by forcing inner circles to know about outer circle data formats.

## Architectural Benefits

Enforcing the dependency rule creates:
- **Testable systems** — Business rules isolated from external dependencies
- **Replaceable components** — Outer layers changeable without affecting inner logic
- **Stable core** — Business rules protected from technological churn
- **Clear separation** — Policies distinct from mechanisms

The dependency rule is the key constraint that enables [clean architecture](/wiki/clean-architecture)'s independence guarantees.

 [[source: uncle-bob-clean-architecture-2012]](/wiki/raw/articles/uncle-bob-clean-architecture-2012)

---

## Agent Navigation

cluster: person (170 pages) | betweenness: 193.9

### References (outbound)
- [Clean Architecture](https://pyweb.dev/wiki/clean-architecture.md)
- [Dependency Inversion Principle](https://pyweb.dev/wiki/dependency-inversion-principle.md)

### Referenced by (inbound)
- [Agent Harness Engineering](https://pyweb.dev/wiki/agent-harness-engineering.md)
- [Clean Architecture](https://pyweb.dev/wiki/clean-architecture.md)
- [Hexagonal Architecture](https://pyweb.dev/wiki/hexagonal-architecture.md)
- [Onion Architecture](https://pyweb.dev/wiki/onion-architecture.md)
- [Robert C. Martin (Uncle Bob)](https://pyweb.dev/wiki/robert-c-martin.md)

### Evidence (verified primary sources)
- [uncle-bob-clean-architecture-2012](https://pyweb.dev/wiki/raw/articles/uncle-bob-clean-architecture-2012.md) | origin: https://blog.cleancoder.com/uncle-bob/2012/08/13/the-clean-architecture.html | ingested: 2026-08-25 | sha256: b99ae450b5981dbee264aec42701b920ce96ccedea01d03243ea48c414442379

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