Breaking Down Problems into Manageable Parts: A Domain-Driven Design and C4 Model Approach
Complex software systems can quickly become unwieldy without proper architectural decomposition. This article explores how to systematically break down problems using Domain-Driven Design (DDD) principles combined with the C4 model’s visual architecture documentation approach.
The Challenge of Complexity
Modern software systems face increasing complexity through distributed architectures, multiple data sources, varied user interfaces, and intricate business logic. Without systematic decomposition, teams encounter:
- Cognitive overload when trying to understand system boundaries
- Tight coupling between components that should be independent
- Communication gaps between technical teams and domain experts
- Inconsistent abstractions across different parts of the system
The solution lies in combining strategic domain modeling with clear architectural visualization.
Domain-Driven Design: Strategic Decomposition
Domain-Driven Design provides a strategic approach to breaking down complex problem spaces into manageable, cohesive domains.
Bounded Contexts: Natural Decomposition Boundaries
A bounded context represents a logical boundary within which a particular domain model applies consistently. These contexts serve as natural decomposition points because they:
- Encapsulate a specific set of business capabilities
- Maintain their own ubiquitous language
- Have clear ownership and responsibility
- Can evolve independently
Consider an e-commerce platform. Rather than building one monolithic system, DDD guides us toward bounded contexts like:
┌─────────────────┐ ┌─────────────────┐ ┌─────────────────┐ │ Catalog │ │ Order │ │ Payment │ │ Management │ │ Processing │ │ Processing │ │ │ │ │ │ │ │ - Product Info │ │ - Order State │ │ - Transactions │ │ - Pricing │ │ - Fulfillment │ │ - Billing │ │ - Inventory │ │ - Customer Data │ │ - Refunds │ └─────────────────┘ └─────────────────┘ └─────────────────┘
Each context contains its own domain model where “Customer” or “Product” might have different attributes and behaviors specific to that context’s needs.
Context Mapping: Understanding Relationships
Once bounded contexts are identified, context mapping reveals how they interact:
- Partnership: Two contexts collaborate closely and succeed or fail together
- Customer/Supplier: One context serves another with defined contracts
- Conformist: One context adapts to another’s model without negotiation
- Anti-corruption Layer: A protective layer that prevents external models from corrupting internal designs
These relationship patterns directly inform architectural decisions about service boundaries, data flow, and integration strategies.
The C4 Model: Hierarchical Architecture Visualization
The C4 model provides a hierarchical approach to documenting software architecture through four levels of abstraction, perfectly complementing DDD’s strategic decomposition.
Level 1: Context Diagrams
Context diagrams show how your system fits within its environment, identifying external dependencies and user types. This aligns with DDD’s emphasis on understanding the problem domain’s boundaries.
┌─────────────────┐
│ Admin Users │
└─────────────────┘
│
v
┌─────────────┐ ┌─────────────────┐ ┌─────────────┐
│ Customer │--->│ E-commerce │--->│ Payment │
│ Portal │ │ Platform │ │ Gateway │
└─────────────┘ └─────────────────┘ └─────────────┘
│
v
┌─────────────────┐
│ Inventory API │
└─────────────────┘
Level 2: Container Diagrams
Container diagrams decompose the system into high-level containers (applications, databases, microservices), mapping directly to bounded contexts identified through DDD.
E-commerce Platform ┌─────────────────────────────────────────────────────────────┐ │ │ │ ┌─────────────┐ ┌─────────────┐ ┌─────────────┐ │ │ │ Web App │ │ Mobile API │ │ Admin API │ │ │ │ (React/TS) │ │(Node.js/TS) │ │(Node.js/TS) │ │ │ └─────────────┘ └─────────────┘ └─────────────┘ │ │ │ │ │ │ │ └─────────────────┼─────────────────┘ │ │ │ │ │ ┌─────────────┐ ┌─────────────┐ ┌─────────────┐ │ │ │ Catalog │ │ Order │ │ Payment │ │ │ │ Service │ │ Service │ │ Service │ │ │ └─────────────┘ └─────────────┘ └─────────────┘ │ │ │ │ │ │ │ ┌─────────────┐ ┌─────────────┐ ┌─────────────┐ │ │ │ Catalog DB │ │ Order DB │ │ Payment DB │ │ │ └─────────────┘ └─────────────┘ └─────────────┘ │ └─────────────────────────────────────────────────────────────┘
Level 3: Component Diagrams
Component diagrams show the internal structure of individual containers, revealing how domain logic is organized within bounded contexts.
Order Service ┌─────────────────────────────────────────────────────────────┐ │ │ │ ┌─────────────┐ ┌─────────────┐ ┌─────────────┐ │ │ │ Order │ │ Inventory │ │ Notification│ │ │ │ Controller │ │ Service │ │ Service │ │ │ └─────────────┘ └─────────────┘ └─────────────┘ │ │ │ │ │ │ │ ┌─────────────┐ ┌─────────────┐ ┌─────────────┐ │ │ │ Order │ │ Inventory │ │ Event │ │ │ │ Repository │ │ Repository │ │ Publisher │ │ │ └─────────────┘ └─────────────┘ └─────────────┘ │ │ │ │ │ │ │ ┌─────────────┐ ┌─────────────┐ ┌─────────────┐ │ │ │ Order │ │ Inventory │ │ Domain │ │ │ │ Entity │ │ Entity │ │ Events │ │ │ └─────────────┘ └─────────────┘ └─────────────┘ │ └─────────────────────────────────────────────────────────────┘
Level 4: Code Diagrams
At the most detailed level, code diagrams can show class relationships, though these are often better represented in IDEs and are used sparingly for complex interactions.
Integrating DDD and C4: A Systematic Approach
Step 1: Domain Discovery and Context Identification
Begin with event storming or domain modeling sessions to identify:
- Core business processes and workflows
- Key domain events and their triggers
- Natural language boundaries (different terminology usage)
- organisational boundaries (team ownership)
Step 2: Context Mapping
Map relationships between identified contexts:
interface BoundedContext name: string; ubiquitousLanguage: Map; aggregates: Aggregate[]; domainEvents: DomainEvent[]; relationships: ContextRelationship[]; interface ContextRelationship target: BoundedContext; type: 'Partnership' | 'CustomerSupplier' | 'Conformist' | 'AntiCorruptionLayer'; integrationPattern: 'SharedKernel' | 'PublishedLanguage' | 'OpenHost'
Step 3: Architecture Mapping
Translate bounded contexts into C4 containers:
- Each bounded context typically becomes one or more containers
- Context relationships inform integration patterns
- Shared kernels might become shared libraries
- Anti-corruption layers become dedicated translation services
Step 4: Iterative Refinement
As understanding deepens, refine both domain boundaries and architectural representations:
Iteration 1: Coarse-grained contexts ┌─────────────────┐ │ E-commerce │ │ System │ └─────────────────┘ Iteration 2: Business capability alignment ┌─────────────┐ ┌─────────────┐ ┌─────────────┐ │ Product │ │ Order │ │ Customer │ │ Management │ │ Process │ │ Management │ └─────────────┘ └─────────────┘ └─────────────┘ Iteration 3: Technical boundaries consideration ┌─────────────┐ ┌─────────────┐ ┌─────────────┐ ┌─────────────┐ │ Catalog │ │ Pricing │ │ Order │ │ Payment │ │ Service │ │ Service │ │ Service │ │ Service │ └─────────────┘ └─────────────┘ └─────────────┘ └─────────────┘
Advanced Diagramming Techniques
Event Flow Diagrams
Combine C4 containers with domain events to show how information flows across bounded contexts:
Customer Order Journey
┌─────────────┐ OrderCreated ┌─────────────┐ PaymentProcessed ┌─────────────┐
│ Order │ --------------->│ Payment │ ------------------->│ Fulfillment │
│ Service │ │ Service │ │ Service │
└─────────────┘ └─────────────┘ └─────────────┘
│ │ │
v v v
┌─────────────┐ ┌─────────────┐ ┌─────────────┐
│ Order DB │ │ Payment DB │ │Fulfillment DB│
└─────────────┘ └─────────────┘ └─────────────┘
Layered Architecture within Contexts
Show how hexagonal or clean architecture principles apply within bounded contexts:
Order Bounded Context ┌─────────────────────────────────────────────────────────────┐ │ Presentation Layer │ │ ┌─────────────┐ ┌─────────────┐ ┌─────────────┐ │ │ │ REST API │ │ GraphQL │ │ gRPC │ │ │ └─────────────┘ └─────────────┘ └─────────────┘ │ ├─────────────────────────────────────────────────────────────┤ │ Application Layer │ │ ┌─────────────┐ ┌─────────────┐ ┌─────────────┐ │ │ │ Command │ │ Query │ │ Domain │ │ │ │ Handlers │ │ Handlers │ │ Services │ │ │ └─────────────┘ └─────────────┘ └─────────────┘ │ ├─────────────────────────────────────────────────────────────┤ │ Domain Layer │ │ ┌─────────────┐ ┌─────────────┐ ┌─────────────┐ │ │ │ Order │ │ Customer │ │ Domain │ │ │ │ Aggregate │ │ Entity │ │ Events │ │ │ └─────────────┘ └─────────────┘ └─────────────┘ │ ├─────────────────────────────────────────────────────────────┤ │ Infrastructure Layer │ │ ┌─────────────┐ ┌─────────────┐ ┌─────────────┐ │ │ │ Repository │ │ Event Bus │ │ External │ │ │ │ Impl │ │ │ │ APIs │ │ │ └─────────────┘ └─────────────┘ └─────────────┘ │ └─────────────────────────────────────────────────────────────┘
Deployment and Runtime Views
Extend C4 diagrams to show how logical architecture maps to physical deployment:
Production Environment ┌─────────────────────────────────────────────────────────────┐ │ Kubernetes Cluster │ │ │ │ ┌─────────────┐ ┌─────────────┐ ┌─────────────┐ │ │ │ Catalog │ │ Order │ │ Payment │ │ │ │ Pod x3 │ │ Pod x5 │ │ Pod x2 │ │ │ └─────────────┘ └─────────────┘ └─────────────┘ │ │ │ │ │ │ │ ┌─────────────┐ ┌─────────────┐ ┌─────────────┐ │ │ │ PostgreSQL │ │ PostgreSQL │ │ PostgreSQL │ │ │ │ Master │ │ Master │ │ Master │ │ │ └─────────────┘ └─────────────┘ └─────────────┘ │ └─────────────────────────────────────────────────────────────┘
Practical Implementation Strategies
Team Alignment
Use C4 diagrams as communication tools between teams owning different bounded contexts. Establish clear interfaces and responsibilities:
interface OrderService {
createOrder(customerId: CustomerId, items: OrderItem[]): Promise<OrderId>;
getOrderStatus(orderId: OrderId): Promise<OrderStatus>;
cancelOrder(orderId: OrderId): Promise<void>;
}
interface PaymentService {
processPayment(orderId: OrderId, amount: Money): Promise<PaymentResult>;
refundPayment(paymentId: PaymentId): Promise<RefundResult>;
}
Evolutionary Architecture
Plan for context boundaries to evolve as domain understanding improves:
- Start with larger contexts when domain knowledge is limited
- Split contexts when they become too complex or team boundaries emerge
- Merge contexts when maintaining separate boundaries creates unnecessary complexity
- Extract shared concerns into dedicated contexts when patterns emerge
Documentation as Code
Maintain C4 diagrams as code using tools like Structurizr or PlantUML:
!include https://raw.githubusercontent.com/plantuml-stdlib/C4-PlantUML/master/C4_Container.puml
LAYOUT_WITH_LEGEND()
Person(customer, "Customer", "E-commerce platform user")
System_Boundary(platform, "E-commerce Platform") {
Container(webapp, "Web Application", "React/TypeScript", "Customer-facing web interface")
Container(catalog, "Catalog Service", "Node.js/TypeScript", "Product information and search")
Container(order, "Order Service", "Node.js/TypeScript", "Order processing and management")
Container(payment, "Payment Service", "Node.js/TypeScript", "Payment processing")
{
System_Ext(paymentGateway, "Payment Gateway", "External payment processing")
Rel(customer, webapp, "Uses", "HTTPS")
Rel(webapp, catalog, "Fetches products", "REST/JSON")
Rel(webapp, order, "Places orders", "REST/JSON")
Rel(order, payment, "Processes payments", "gRPC")
Rel(payment, paymentGateway, "Charges cards", "HTTPS/JSON")
Measuring Success
Effective decomposition using DDD and C4 should result in:
- Reduced cognitive load: Teams can understand their domain without needing to understand the entire system
- Independent deployment: Bounded contexts can be deployed separately
- Clear ownership: Each context has a dedicated team with domain expertise
- Consistent language: Domain experts and developers use the same terminology within contexts
- Minimal coupling: Changes in one context rarely require changes in others
Monitor these metrics through regular architecture reviews and team retrospectives. Adjust context boundaries and architectural decisions based on evolving understanding and changing business needs.
Conclusion
Breaking down complex software problems requires both strategic domain thinking and clear architectural communication. Domain-Driven Design provides the strategic framework for identifying natural decomposition boundaries, while the C4 model offers a systematic approach to documenting and communicating the resulting architecture.
The combination creates a powerful methodology for managing complexity, enabling teams to build systems that align with business needs and can evolve sustainably over time. Remember that both domain boundaries and architectural representations should evolve as understanding deepens - the goal is not perfect initial design, but rather a systematic approach to continuous refinement.
Success comes from applying these techniques iteratively, maintaining close collaboration between domain experts and technical teams, and treating architecture documentation as a living representation of system design rather than a static artifact.