ADR-016: Domain-Driven Design Implementation

Status

Accepted - July 2024

Context

BookWorm's complex business domain requires a structured approach to manage domain complexity and ensure alignment between business requirements and technical implementation. The domain challenges include:

• Complex Business Logic: Book catalog management, order processing, inventory tracking, and customer relationship management
• Multiple Bounded Contexts: Distinct business areas with different vocabularies and rules (catalog, ordering, finance, rating)
• Domain Expert Collaboration: Need for close collaboration between developers and business domain experts
• Business Rule Evolution: Frequently changing business rules and policies requiring flexible domain models
• Cross-Context Integration: Managing relationships and data consistency across different domain contexts
• Legacy Integration: Incorporating existing business processes and data structures
• Team Scaling: Multiple development teams working on different domain areas simultaneously
• Technical Complexity: Balancing domain purity with technical infrastructure requirements
• Model Integrity: Maintaining domain model consistency and preventing anemic domain models
• Strategic Design: Identifying bounded contexts, context mapping, and integration patterns

The architecture must support rich domain models while enabling team autonomy and system scalability.

Decision

Adopt Domain-Driven Design (DDD) as the primary architectural approach with clear bounded context separation, rich domain models, and strategic design patterns to manage complexity and align technical implementation with business domain.

DDD Implementation Strategy

Bounded Context Architecture

• Context Boundaries: Clear separation of business contexts with independent domain models
• Ubiquitous Language: Consistent vocabulary within each context, shared between developers and domain experts
• Context Integration: Well-defined integration patterns between contexts using domain events and anti-corruption layers
• Team Organization: Development teams aligned with bounded contexts for domain expertise

Domain Modeling Approach

• Rich Domain Models: Business logic encapsulated in domain entities and value objects
• Aggregate Design: Consistent transaction boundaries with aggregate roots and domain invariants
• Domain Events: Business event modeling for loose coupling and eventual consistency
• Repository Pattern: Domain-focused data access abstraction without infrastructure concerns

BookWorm Bounded Contexts

Bounded Context	Core Domain	Subdomain Type	Team Ownership	Key Aggregates
Catalog	Book Management	Core	Catalog Team	Book, Category, Author, Publisher
Ordering	Order Processing	Core	Order Team	Order, OrderItem, Customer
Finance	Payment Processing	Supporting	Finance Team	Payment, Invoice, Transaction
Rating	Book Reviews	Supporting	Content Team	Review, Rating, Feedback
Basket	Shopping Cart	Supporting	Commerce Team	Basket, BasketItem
Notification	Communication	Generic	Platform Team	Notification, Template
Chat	Customer Support	Supporting	Support Team	Conversation, Message

Rationale

Why Domain-Driven Design?

Business Alignment and Clarity

1. Ubiquitous Language: Shared vocabulary between business experts and developers reduces miscommunication
2. Business Focus: Domain logic remains independent of technical concerns and infrastructure
3. Model Integrity: Rich domain models prevent anemic data models and scattered business logic
4. Evolutionary Design: Domain models can evolve with changing business requirements
5. Domain Expert Collaboration: Structured approach for incorporating business knowledge into software design

Strategic Design Benefits

1. Bounded Context Separation: Clear boundaries prevent model contamination and enable team autonomy
2. Context Integration: Well-defined integration patterns manage complexity between contexts
3. Scalability: Teams can work independently within their bounded contexts
4. Legacy Integration: Anti-corruption layers protect domain models from legacy system contamination
5. Technical Architecture: Strategic design guides microservice boundaries and data architecture

Tactical Design Advantages

1. Aggregate Consistency: Transaction boundaries ensure business invariants and data consistency
2. Domain Events: Loose coupling between contexts through domain-driven event modeling
3. Repository Abstraction: Domain-focused data access without infrastructure dependencies
4. Value Objects: Immutable objects for domain concepts with built-in validation
5. Domain Services: Stateless services for domain operations that don't belong to specific entities

DDD Pattern Implementation

Domain Layer Architecture

1. Entities: Objects with identity and lifecycle, encapsulating business logic and invariants
2. Value Objects: Immutable objects representing domain concepts without identity
3. Aggregates: Consistency boundaries with aggregate roots managing invariants
4. Domain Events: Business events representing significant domain occurrences
5. Domain Services: Stateless services for operations spanning multiple aggregates

Application Layer Coordination

1. Application Services: Orchestrate domain operations and coordinate cross-aggregate transactions
2. Command Handlers: Process business commands and coordinate domain operations
3. Query Handlers: Handle read operations and return view models for presentation
4. Event Handlers: Process domain events and coordinate cross-context communication
5. Anti-Corruption Layer: Protect domain models from external system contamination

Infrastructure Integration

1. Repository Implementation: Infrastructure-specific implementations of domain repository contracts
2. Event Publishing: Infrastructure services for publishing domain events to message brokers
3. External Service Integration: Adapter pattern for external API integration
4. Data Persistence: Object-relational mapping with domain-focused persistence strategies
5. Cross-Cutting Concerns: Infrastructure services for logging, monitoring, and security

Implementation

Bounded Context Integration

• Domain Events: Cross-context communication through published domain events
• Integration Events: Translation between domain events and integration events for external contexts
• Context Mapping: Explicit relationships between bounded contexts (Customer-Supplier, Conformist, Anti-Corruption Layer)
• Shared Kernels: Common domain concepts shared between specific contexts where appropriate
• Published Language: Well-defined schemas for cross-context data exchange

Aggregate Design Principles

• Consistency Boundaries: Aggregates maintain business invariants within transaction boundaries
• Identity Management: Aggregate roots provide identity and control access to aggregate internals
• Business Rules: Domain logic encapsulated within aggregates, not leaked to application services
• Event Publishing: Domain events published for side effects and cross-aggregate coordination
• Performance Optimization: Aggregate design balanced between consistency and performance requirements

Context Integration Configuration

# Context mapping configuration
bounded_contexts:
  catalog:
    upstream_contexts: []
    downstream_contexts: [rating, ordering]
    integration_pattern: "published_language"

  ordering:
    upstream_contexts: [catalog, basket]
    downstream_contexts: [finance, notification]
    integration_pattern: "customer_supplier"

  finance:
    upstream_contexts: [ordering]
    downstream_contexts: [notification]
    integration_pattern: "conformist"

Consequences

Positive

• Business Alignment: Strong alignment between business domain and technical implementation
• Model Integrity: Rich domain models with encapsulated business logic and invariants
• Team Autonomy: Clear bounded contexts enable independent team development
• Evolutionary Architecture: Domain models can evolve with changing business requirements
• Complexity Management: Strategic design patterns manage complexity at scale
• Knowledge Capture: Domain expertise captured in code through ubiquitous language

Negative

• Learning Curve: DDD concepts and patterns require significant team training and expertise
• Initial Overhead: Strategic design and modeling require upfront investment
• Over-Engineering Risk: Potential for unnecessary complexity in simple business scenarios
• Context Boundaries: Incorrect bounded context identification can lead to integration challenges
• Performance Considerations: Rich domain models may have performance implications requiring optimization

Risks and Mitigation

Risk	Impact	Probability	Mitigation Strategy
Anemic Domain Models	High	Medium	Code reviews, DDD training, refactoring practices
Context Boundary Issues	High	Medium	Event storming, domain expert collaboration
Over-Engineering	Medium	High	Pragmatic approach, iterative refinement
Team Knowledge Gap	Medium	High	Training programs, mentoring, documentation
Integration Complexity	Medium	Medium	Clear context mapping, integration patterns

References

• Domain-Driven Design by Eric Evans
• .NET Microservices: Architecture for Containerized .NET Applications
• Domain-Driven Design Reference by Eric Evans
• Event Storming