architecture-paradigm-layered

What this skill does

## Table of Contents

- [When to Employ This Paradigm](#when-to-employ-this-paradigm)
- [When NOT to Use This Paradigm](#when-not-to-use-this-paradigm)
- [Adoption Steps](#adoption-steps)
- [Key Deliverables](#key-deliverables)
- [Technology Guidance](#technology-guidance)
- [Risks & Mitigations](#risks-mitigations)

# The Layered (N-Tier) Architecture Paradigm

## When to Employ This Paradigm
- When teams need clear architectural boundaries and a familiar structure for moderate-sized systems.
- When compliance or operations teams require clear separation of concerns (e.g., UI vs. domain logic vs. persistence).
- When the deployment artifact remains a monolith, but code clarity and separation are degrading.

## When NOT To Use This Paradigm
- When high scalability demands require independent scaling of components
- When multiple teams need independent deployment cycles
- When complex business logic requires frequent cross-layer communication
- When microservices architecture is already planned or in place
- When real-time processing requirements make layered communication too slow

## Adoption Steps
1. **Define the Layers**: Establish a clear set of layers. A common stack includes: Presentation -> Application/Service -> Domain -> Data Access.
2. **Enforce Dependency Direction**: Code in a given layer may only depend on the layer immediately below it. Forbid any "upward" dependencies or imports.
3. **Centralize Cross-Cutting Concerns**: Implement concerns like logging, authentication, and validation as centralized middleware or policies, rather than duplicating this logic in each layer.
4. **Test Each Layer Appropriately**: Apply testing strategies suitable for each layer, such as unit tests for the domain layer, service-layer tests for orchestration logic, and integration tests for persistence adapters.
5. **Document and Enforce Interactions**: Maintain up-to-date dependency diagrams and use automated architecture tests to prevent developers from creating "shortcut" dependencies that violate the layering rules.

## Key Deliverables
- An Architecture Decision Record (ADR) that captures the responsibilities of each layer, the allowed dependencies between them, and the policy for any exceptions.
- A formal dependency diagram stored with the project documentation.
- Automated architectural checks (e.g., using ArchUnit, dep-cruise, or custom scripts) to prevent rule violations from being merged.

## Technology Guidance

**Layer Implementation Patterns**:
- **Presentation Layer**: React/Vue/Angular (Frontend), MVC Controllers (Backend)
- **Application Layer**: Service classes, Application services, Use case orchestrators
- **Domain Layer**: Business entities, Domain services, Business rules validation
- **Data Access Layer**: Repository pattern, ORM mappers, Data access objects (DAO)

**Architecture Enforcement Tools**:
- **Java**: ArchUnit for dependency rule testing
- **JavaScript/TypeScript**: ESLint rules with dependency tracking
- **C#**: NDepend for architectural analysis
- **Python**: Custom decorators and import analysis tools

**Common Layer Stacks**:
- **3-Layer**: Presentation → Business Logic → Data Access
- **4-Layer**: Presentation → Application → Domain → Infrastructure
- **5-Layer**: UI → Controller → Service → Domain → Persistence

## Real-World Examples

**Enterprise ERP Systems**: SAP and Oracle ERP use layered architecture to separate user interfaces from business logic and database operations, enabling different frontend applications to share the same business rules.

**Banking Applications**: Financial institutions employ layered architecture to maintain strict separation between customer-facing interfaces, transaction processing, and secure data storage for regulatory compliance.

**E-commerce Platforms**: Traditional e-commerce sites use layered architecture to separate product catalogs, shopping cart logic, order processing, and payment handling into distinct layers.

## Risks & Mitigations
- **Excessive Rigidity and Latency**:
  - **Mitigation**: For features that span multiple layers, strict adherence can lead to excessive "pass-through" code and increased latency. In such cases, consider using a Façade pattern to provide a more direct interface where appropriate.
- **"Leaky" Layers**:
  - **Mitigation**: Developers may be tempted to bypass architectural rules for expediency, which degrades the architecture. Treat all architectural violations as build-breaking failures or critical issues in code review.

## Concrete Components

These vocabulary items name the concrete tools and abstractions
that show up when the paradigm is implemented. They are not
required dependencies and they are not part of the skill's
``tools:`` frontmatter (which is reserved for Claude Code tool
restrictions). Use this list to disambiguate during architecture
discussions.

- ``dependency-validator``: fails the build when a layer imports above its allowed depth
- ``layer-enforcer``: static-analysis gate that checks namespaces match layer rules
- ``architecture-compliance-checker``: diffs the implemented layer graph against the documented one