Book Clean Architecture Full

OBEY Clean Architecture by Robert C. Martin

Purpose

This repository must follow Clean Architecture. When writing, modifying, or reviewing code, prefer decisions that preserve:

• independent business rules
• inward-pointing dependencies
• framework independence
• database independence
• UI independence
• testability
• replaceable details

Treat this file as a binding implementation policy: MUST is binding, SHOULD is a strong default, and MUST NOT is forbidden.

Non-Negotiable Rules

1. Follow the Dependency Rule

   • Source code dependencies must point inward, toward higher-level policies.
   • Inner layers must not import or depend on outer layers.
   • Business rules must not depend on frameworks, web handlers, database drivers, UI libraries, queues, external services, or other details.

2. Keep Business Rules Pure

   • Entities and use cases must contain business policy.
   • Business rules must not read web requests, environment variables, framework context, database-bound structures, or database rows directly.
   • Pass plain data into use cases through request models or arguments.

3. Treat Frameworks as Details

   • Frameworks are tools, not the foundation of the design.
   • Keep framework annotations, decorators, controllers, routes, middleware, serializers, and database artifacts at the edges.
   • Do not let framework types leak into core policies.

4. Treat the Database as a Detail

   • Do not shape the domain model around tables.
   • Use gateways to isolate persistence.
   • Business rules must work without a real database.

5. Treat the Web as a Detail

   • Controllers and endpoints translate delivery input into input models for use cases.
   • Use cases must not know about web transport, status codes, cookies, headers, or routing.
   • Presenters or response mappers translate use case output for delivery mechanisms.

6. Use Explicit Boundaries

   • Define interfaces at architectural seams.
   • External systems, persistence, messaging, file systems, clocks, and service clients must sit behind boundaries.
   • Prefer adapters over direct calls from policy code to implementation details.

7. Organize by Use Case

   • Prefer feature and use-case oriented structure over generic technical buckets.
   • The architecture should scream the domain and application intent.
   • Avoid codebases dominated by generic technical buckets that do not reveal use cases or business purpose.

8. Use Cases Must Orchestrate

   • A use case coordinates entities and gateways.
   • A use case should not contain delivery concerns, database concerns, or presentation formatting concerns.
   • A use case should represent one application action.

9. Entities Must Guard Invariants

   • Critical domain rules belong in entities or equivalent domain objects.
   • Entities must protect invariants and consistency.
   • Do not leave core rules in controllers, jobs, handlers, or database scripts.

10. Outer Layers May Depend on Inner Layers, Never the Reverse

    • Controllers may depend on use cases.

• Gateways may implement interfaces defined by the use case or domain layer. - Presenters may implement output boundaries owned by inner layers. - Never invert this relationship accidentally.

Required Layer Responsibilities

Domain Layer

Contains:

• entities
• enterprise business rules
• domain invariants
• core business rules

These may be implemented with plain objects, functions, modules, or other structures. Clean Architecture requires independent business rules; it does not require a specific domain modeling style.

Must:

• be framework free
• be persistence ignorant
• be delivery mechanism agnostic
• avoid annotations and infrastructure imports where possible

Must not:

• import web libraries
• import database access types
• import external service clients
• perform I/O
• read configuration directly

Application Layer

Contains:

• use cases
• input models
• output models
• ports and boundaries
• orchestration logic

Must:

• depend on domain abstractions and models
• define interfaces for required external behavior
• coordinate workflows explicitly

Must not:

• contain controller logic
• contain database access details
• return framework response types
• format UI strings unless explicitly part of a presenter boundary

Interface Adapters Layer

Contains:

• controllers
• presenters
• view models
• gateway adapters
• mappers between external and internal models

Must:

• translate between external formats and internal models
• depend inward on application and domain code
• isolate framework and vendor details

Must not:

• move business policy out of the use case or domain layer
• bypass use cases to call gateways directly unless explicitly justified by architecture

Infrastructure Layer

Contains:

• framework bootstrap
• object graph and component wiring
• database access details
• external service integrations
• message bus clients
• filesystem implementations
• network clients

Must:

• remain replaceable
• implement interfaces owned by inner layers
• stay at the outermost edge

Must not:

• define business rules
• dictate domain shapes
• leak vendor types inward

Code Generation Rules

When generating code, always apply the following.

1. Define the Use Case First

For every non-trivial feature:

• identify the use case
• define the input
• define the output
• define required ports
• keep orchestration in one place

Prefer this order:

1. domain rule or entity behavior
2. use case
3. boundary interfaces
4. presenter contract
5. gateway contract
6. adapters
7. framework wiring

2. Use Plain Models at Boundaries

• Use request and response models owned by the application layer.
• Do not pass database-bound entities, web requests, or framework-bound data structures into core logic.
• Do not return framework objects from use cases.

3. Create Ports for Volatile Dependencies

Introduce interfaces for:

• gateways
• mailers
• payment providers
• message publishers
• storage providers
• clocks
• ID generators
• transaction runners if needed

Do not call volatile details directly from core use cases.

4. Keep Wiring in the Main Component

• Object construction belongs in the composition root.
• Do not instantiate infrastructure dependencies inside use cases or entities.
• Use explicit construction, factories, or composition in the outer layer.

5. Prefer Stable Dependencies

• Inner layers own the abstractions they need.
• Outer layers implement those abstractions.
• Avoid shared "common" packages that create sideways coupling.

6. Keep Boundaries Visible

• When in doubt, introduce a boundary sooner.
• Partial boundaries are acceptable if they preserve future extraction options.
• Use interfaces, request models, and output models to avoid coupling to details.

Architecture Heuristics

Dependency Direction

Always verify:

• Does this import point inward?
• Is a high-level policy depending on a low-level detail?
• Is a framework or vendor type leaking into a core layer?
• Is an adapter bypassing the intended boundary?

If yes, refactor.

Policy vs Detail

When placing code, ask:

• Is this business policy?
• Is this orchestration?
• Is this translation?
• Is this infrastructure?

Put the code in the highest-level place that matches its responsibility.

Stable Core, Replaceable Edge

Prefer designs where you can replace:

• web framework
• persistence technology
• message broker
• job runner
• cloud vendor
• serializer
• UI without rewriting business rules.

Feature First Structure

Prefer:

• feature/use-case names
• business-capability/use-case names
• names that reveal the application's use cases

Over:

• generic controller, service, or gateway buckets
• generic technical buckets

Technical subfolders are acceptable only when they do not obscure use-case ownership.

Architecture Economics and Priority

1. Treat architecture as a way to keep future change cost proportional to the scope of change.
2. Do not sacrifice important architectural work merely because urgent feature work is louder.
3. Preserve options around frameworks, databases, delivery mechanisms, and deployment topology until evidence justifies commitment.
4. Choose boundaries by volatility, policy importance, substitution value, testability, and cost.
5. Do not overbuild boundaries whose cost exceeds the option value they preserve.
6. Revisit architecture when change shape, team ownership, deployment needs, or operational constraints reveal rising cost.

Paradigm and Component Rules

1. Use structured programming to make behavior decomposable and testable.
2. Use polymorphism to invert dependencies when high-level policy must not know low-level details.
3. Use immutability or controlled mutation when it protects policy from accidental state coupling.
4. Apply SRP by separating code that changes for different actors or reasons.
5. Apply the Open-Closed Principle by protecting stable policy from volatile extension details.
6. Apply LSP by ensuring substitutable implementations preserve caller expectations.
7. Apply ISP by keeping interfaces focused on what each client actually needs.
8. Apply DIP by making source dependencies point toward stable policy and abstractions.
9. Group components by cohesion and release pressure; do not group unrelated policy just because it shares a technical layer.
10. Avoid component cycles; break cycles before they harden into deployment or test bottlenecks.
11. Balance stability and abstraction: stable components should not depend on unstable details, and abstract components should have concrete reason to exist.

Boundary Cost, Deployment, and Operations

1. A boundary may be a source boundary, deployment boundary, process boundary, service boundary, or partial boundary.
2. Choose the lightest boundary that preserves the needed independence.
3. Use partial boundaries when a full deployment/runtime split is too expensive but future separation is valuable.
4. Keep development, deployment, operation, and maintenance concerns visible without letting them own business policy.
5. Do not combine unrelated use cases just because operational wiring is easier.
6. Do not eliminate duplication when the shared code would couple use cases that change for different actors.
7. Make architectural boundaries enforceable through package structure, tests, dependency rules, or build constraints.

Services, Distribution, and Embedded Boundaries

1. A service is not automatically an architectural boundary; source dependencies and data ownership still decide coupling.
2. Remote calls must be treated as I/O boundaries, not as local method calls.
3. Keep service listeners humble: translate external messages into use case calls and return through output boundaries.
4. Keep embedded and hardware details behind interfaces so policy can be tested without the target device.
5. Do not let real-time, firmware, database, web, or framework concerns pull policy outward.

Naming Rules

• Name modules and packages after business capabilities or use cases.
• Name use cases with action verbs from the application's use cases.
• Name ports by the role they play for the use case.
• Name adapters by the external detail or delivery mechanism they adapt.
• Avoid vague technical names when a use case, policy, boundary, presenter, controller, gateway, or entity role is more precise.
• If a class is named Service, justify why it is not a use case, adapter, or domain object.

Testing Rules

Core Tests First

Prioritize tests for:

• entities
• use cases
• boundary contracts

These tests must:

• run without the real framework
• run without the real database
• run without the network
• run fast and deterministically

Adapter Tests

Test adapters separately for:

• mapping correctness
• gateway behavior
• controller translation
• presenter formatting
• integration with framework or external service

Do not use slow integration tests as a substitute for testing business rules.

Test Through Supported Boundaries

• Avoid reaching private internals when a public use case boundary exists.
• Prefer testing use cases with fakes or mocks for ports.
• Use integration tests only where architectural seams meet real details.

Forbidden Patterns

Do not generate or keep code that does any of the following unless explicitly required and justified.

Framework Leakage

• domain entities annotated with database or web framework metadata when avoidable
• use cases depending on Request, Response, controller base classes, framework sessions, or middleware objects
• application layer importing serializer or database base classes

Database Leakage

• use cases returning table rows or database-bound entities
• domain rules embedded in gateway implementations or database access
• domain objects designed primarily around persistence convenience

Controller-Centric Logic

• controllers containing branching business rules
• controllers performing validation that belongs to business policy
• controllers calling gateways directly instead of use cases

God Services

• large *Service classes that create, fetch, validate, persist, publish, and present everything
• services that own unrelated use cases
• application services that become dumping grounds

Layer Bypass

• controllers bypassing use cases to call gateways
• presenters reading directly from databases
• infrastructure code importing inward and also being imported by domain code

Direction Violations

• gateway interfaces defined in infrastructure and consumed by core policy
• entities importing adapters
• use cases depending on concrete implementations

Utility Dumping Grounds

• generic utility, shared, base, or core folders used as architecture escape hatches
• generic abstractions with no clear ownership
• convenience modules that hide bad dependency direction

Refactoring Rules

When modifying existing code:

1. Move business rules inward

   • Extract domain logic from controllers, handlers, views, gateway classes, and jobs.

2. Introduce boundaries around details

   • Wrap external services, database access, message buses, filesystem operations, and clocks.

3. Replace concrete dependencies with ports

   • Define interfaces in inner layers.
   • Implement them in outer layers.

4. Separate translation from policy

   • Request parsing, data mapping, serialization, and presentation formatting belong outside core business rules.

5. Break up god services

   • Split by use case.
   • Give each use case one clear application action.

6. Eliminate framework coupling from tests

   • Rewrite tests to target use cases and entities directly where possible.

7. Preserve behavior while improving direction

   • Refactor incrementally.
   • Prefer safe boundary extraction over large rewrites.

Output Expectations

When asked to implement a feature, default to producing:

• a domain model or entity if business invariants exist
• a focused use case
• input and output models if needed
• ports/interfaces for external dependencies
• adapters for web, persistence, or messaging details
• composition root wiring outside the use case

When asked to modify existing code:

• keep or improve dependency direction
• avoid adding framework dependencies to inner layers
• call out architectural debt explicitly if it cannot be fixed safely now

When asked to review code:

• identify boundary violations
• identify dependency rule violations
• identify framework leakage
• identify misplaced business rules
• identify god services and layer bypass
• propose concrete refactorings toward Clean Architecture

Review Checklist

Before finalizing any change, verify:

• Are business rules independent from frameworks?
• Are use cases independent from delivery and persistence details?
• Do source dependencies point inward?
• Are controllers thin?
• Are gateways just persistence adapters?
• Are entities guarding domain invariants?
• Are ports owned by inner layers?
• Is composition happening at the edge?
• Can core tests run without the web framework and database?
• Does the project structure reflect the domain and use cases?
• Did we avoid generic utility dumping grounds?
• Did we avoid creating another god service?
• Did we keep details replaceable?

If any answer is no, revise the design before shipping.

Preferred Default Shapes

Preferred feature shape

• domain
• application
• adapters
• infrastructure

Or, if feature-oriented:

• feature/domain
• feature/application
• feature/adapters
• feature/infrastructure

Preferred use case shape

• request model
• use case
• output boundary or response model
• ports
• adapter implementations outside

Preferred dependency pattern

• inner layer defines interface
• outer layer implements interface
• composition root wires them together

When Tradeoffs Are Necessary

If constraints force a compromise:

• keep the compromise at the outermost layer possible
• document the boundary violation clearly in code comments or review notes
• avoid normalizing the compromise into the core architecture
• preserve a future path to separation

Choose the design that minimizes long-term coupling, not the one that is only shortest today.

Final Instruction

When uncertain, choose the option that:

1. keeps business rules independent
2. points dependencies inward
3. isolates details behind boundaries
4. improves testability
5. makes replacement of frameworks, databases, and delivery mechanisms easier

If a proposed change conflicts with these priorities, reject it and propose a cleaner architectural alternative.