Domain-Driven Design Study Cases: Real-World Examples 📚

Overview and Problem Statement

While the theoretical foundations of DDD are important, seeing how these concepts apply in real-world scenarios can be challenging. Software teams often struggle to translate DDD principles into practical implementations that solve actual business problems. This guide presents detailed study cases showing how different organizations have successfully applied DDD to address complex business challenges.

Each case study will explore:

• The initial business problem
• How DDD concepts were applied
• The implementation approach
• Challenges faced and solutions found
• Results and lessons learned

Let's examine several comprehensive real-world examples:

Study Case 1: E-commerce Order Management 🛍️

Business Context

A growing e-commerce company was struggling with their order management system. They had different rules for different market segments, complex promotional logic, and multiple fulfillment methods. The existing monolithic system was becoming unmaintainable.

Let's look at how they applied DDD:

// Core Domain: Order Management
public class Order {
    private final OrderId id;
    private final CustomerId customerId;
    private final MarketSegment marketSegment;
    private OrderStatus status;
    private final List<OrderLine> orderLines;
    private Money totalAmount;
    private FulfillmentMethod fulfillmentMethod;
    private final List<DomainEvent> domainEvents;

    public void applyPromotion(Promotion promotion) {
        validatePromotionEligibility(promotion);
        
        // Apply promotion rules based on market segment
        Money discount = marketSegment.calculateDiscount(
            promotion,
            this.totalAmount
        );
        
        // Record the applied promotion
        this.appliedPromotions.add(new AppliedPromotion(
            promotion.getId(),
            discount
        ));
        
        // Recalculate total
        recalculateTotal();
        
        // Raise domain event
        domainEvents.add(new PromotionAppliedEvent(
            this.id,
            promotion.getId(),
            discount
        ));
    }

    public void selectFulfillmentMethod(FulfillmentMethod method) {
        validateFulfillmentMethodAvailable(method);
        
        this.fulfillmentMethod = method;
        
        // Adjust delivery estimates based on market segment
        DeliveryEstimate estimate = marketSegment.calculateDeliveryEstimate(
            method,
            this.getShippingAddress()
        );
        
        this.estimatedDeliveryDate = estimate.getEstimatedDate();
        
        domainEvents.add(new FulfillmentMethodSelectedEvent(
            this.id,
            method,
            estimate
        ));
    }

    private void validatePromotionEligibility(Promotion promotion) {
        if (!marketSegment.isEligibleForPromotion(promotion)) {
            throw new PromotionNotApplicableException(
                "Promotion not available for market segment: " + marketSegment
            );
        }
        
        if (!promotion.isValidForOrder(this)) {
            throw new PromotionNotApplicableException(
                "Order does not meet promotion criteria"
            );
        }
    }
}

// Supporting Subdomain: Fulfillment
public class FulfillmentService {
    private final WarehouseRepository warehouseRepository;
    private final ShippingProviderGateway shippingGateway;
    
    public FulfillmentPlan createFulfillmentPlan(Order order) {
        // Find optimal warehouse based on market segment rules
        Warehouse warehouse = findOptimalWarehouse(
            order.getShippingAddress(),
            order.getMarketSegment()
        );
        
        // Create fulfillment plan
        FulfillmentPlan plan = new FulfillmentPlan(
            order.getId(),
            warehouse.getId(),
            order.getFulfillmentMethod()
        );
        
        // Add items to plan based on warehouse availability
        for (OrderLine line : order.getOrderLines()) {
            WarehouseInventory inventory = 
                warehouse.getInventoryFor(line.getProductId());
                
            if (inventory.canFulfill(line.getQuantity())) {
                plan.addItem(line, warehouse);
            } else {
                // Split order if needed
                handleSplitFulfillment(plan, line);
            }
        }
        
        return plan;
    }
}

Results and Lessons

1. Clearly separated market segment-specific rules using distinct value objects
2. Improved handling of promotions through explicit domain events
3. Better fulfillment planning through dedicated subdomain
4. Reduced complexity by breaking down the monolith into bounded contexts

Study Case 2: Banking Transaction System 🏦

Business Context

A bank needed to modernize their transaction processing system to handle multiple types of accounts, various transaction rules, and complex compliance requirements.

Here's their DDD implementation:

// Core Domain: Transaction Processing
public class Transaction {
    private final TransactionId id;
    private final AccountId sourceAccountId;
    private final AccountId destinationAccountId;
    private final Money amount;
    private TransactionStatus status;
    private final List<ComplianceCheck> complianceChecks;
    private final List<DomainEvent> domainEvents;

    public void process(
            TransactionProcessor processor,
            ComplianceService compliance) {
        // Perform initial validation
        validateTransaction();
        
        // Run compliance checks
        ComplianceResult complianceResult = 
            compliance.checkTransaction(this);
            
        if (!complianceResult.isCompliant()) {
            markAsFailed(complianceResult.getViolations());
            return;
        }
        
        try {
            // Process the transaction
            TransactionResult result = processor.process(this);
            
            // Update status based on result
            if (result.isSuccessful()) {
                markAsCompleted(result);
            } else {
                markAsFailed(result.getFailureReason());
            }
            
        } catch (TransactionProcessingException e) {
            markAsFailed(e.getMessage());
        }
    }

    private void markAsCompleted(TransactionResult result) {
        this.status = TransactionStatus.COMPLETED;
        
        domainEvents.add(new TransactionCompletedEvent(
            this.id,
            this.sourceAccountId,
            this.destinationAccountId,
            this.amount,
            result.getProcessingDetails()
        ));
    }
}

// Supporting Subdomain: Compliance
public class ComplianceService {
    private final List<ComplianceRule> rules;
    private final AuditLogger auditLogger;
    
    public ComplianceResult checkTransaction(Transaction transaction) {
        ComplianceResult result = new ComplianceResult();
        
        // Apply all compliance rules
        for (ComplianceRule rule : rules) {
            RuleEvaluation evaluation = rule.evaluate(transaction);
            result.addEvaluation(evaluation);
            
            // Log for audit purposes
            auditLogger.logRuleEvaluation(
                transaction.getId(),
                rule.getId(),
                evaluation
            );
            
            // Stop on first violation if rule requires
            if (!evaluation.isPassed() && rule.isBreaking()) {
                return result;
            }
        }
        
        return result;
    }
}

// Generic Subdomain: Audit Logging
public class AuditLogger {
    private final AuditRepository repository;
    
    public void logRuleEvaluation(
            TransactionId transactionId,
            RuleId ruleId,
            RuleEvaluation evaluation) {
        AuditEntry entry = new AuditEntry(
            AuditEntryId.generate(),
            transactionId,
            ruleId,
            evaluation.getResult(),
            evaluation.getDetails(),
            LocalDateTime.now()
        );
        
        repository.save(entry);
    }
}

Results and Lessons

1. Clear separation of transaction processing from compliance checks
2. Improved audit trail through dedicated audit logging subdomain
3. Better handling of compliance rules through explicit domain model
4. Enhanced monitoring and reporting capabilities

Study Case 3: Insurance Claims Processing 📋

Business Context

An insurance company needed to modernize their claims processing system to handle multiple insurance products, complex policy rules, and various claim types.

Here's how they implemented it using DDD:

// Core Domain: Claims Processing
public class Claim {
    private final ClaimId id;
    private final PolicyId policyId;
    private final ClaimType type;
    private ClaimStatus status;
    private Money claimedAmount;
    private final List<ClaimEvidence> evidence;
    private final List<DomainEvent> domainEvents;

    public void submit(Policy policy) {
        validateCanBeSubmitted(policy);
        
        // Check policy coverage
        Coverage coverage = policy.getCoverageFor(this.type);
        if (!coverage.covers(this.claimedAmount)) {
            throw new ClaimAmountExceedsCoverageException(
                coverage.getMaximumAmount()
            );
        }
        
        // Mark as submitted
        this.status = ClaimStatus.SUBMITTED;
        
        // Record the event
        domainEvents.add(new ClaimSubmittedEvent(
            this.id,
            this.policyId,
            this.type,
            this.claimedAmount
        ));
    }

    public void evaluate(ClaimAdjuster adjuster) {
        validateCanBeEvaluated();
        
        // Perform evaluation
        EvaluationResult result = adjuster.evaluate(this);
        
        if (result.isApproved()) {
            this.approvedAmount = result.getApprovedAmount();
            this.status = ClaimStatus.APPROVED;
            
            domainEvents.add(new ClaimApprovedEvent(
                this.id,
                this.approvedAmount,
                result.getJustification()
            ));
        } else {
            this.status = ClaimStatus.REJECTED;
            
            domainEvents.add(new ClaimRejectedEvent(
                this.id,
                result.getRejectionReason()
            ));
        }
    }
}

// Supporting Subdomain: Policy Management
public class Policy {
    private final PolicyId id;
    private final CustomerId customerId;
    private final InsuranceProduct product;
    private PolicyStatus status;
    private final Map<ClaimType, Coverage> coverages;

    public Coverage getCoverageFor(ClaimType claimType) {
        Coverage coverage = coverages.get(claimType);
        if (coverage == null) {
            throw new ClaimTypeNotCoveredException(claimType);
        }
        
        if (!status.isActive()) {
            throw new PolicyNotActiveException(this.id);
        }
        
        return coverage;
    }

    public boolean validateClaim(Claim claim) {
        // Check policy was active when incident occurred
        if (!wasActiveAt(claim.getIncidentDate())) {
            return false;
        }
        
        // Check claim is within waiting period
        if (!hasPassedWaitingPeriod(claim)) {
            return false;
        }
        
        // Check claim type is covered
        Coverage coverage = coverages.get(claim.getType());
        return coverage != null && 
               coverage.covers(claim.getClaimedAmount());
    }
}

// Generic Subdomain: Document Management
public class ClaimDocumentService {
    private final DocumentRepository documentRepository;
    private final DocumentValidator validator;
    
    public void attachDocument(
            ClaimId claimId,
            DocumentType type,
            byte[] content) {
        // Validate document
        ValidationResult validation = validator.validate(
            type,
            content
        );
        
        if (!validation.isValid()) {
            throw new InvalidDocumentException(validation.getErrors());
        }
        
        // Store document
        Document document = new Document(
            DocumentId.generate(),
            claimId,
            type,
            content,
            LocalDateTime.now()
        );
        
        documentRepository.save(document);
    }
}

Results and Lessons

1. Clear separation between claims processing and policy management
2. Improved handling of different claim types through explicit modeling
3. Better document management through dedicated subdomain
4. Enhanced policy validation and coverage checking

Key Takeaways from Study Cases 📝

These case studies demonstrate several important lessons about implementing DDD:

1. Start with Core Domain

   • Identify and focus on the most critical business operations
   • Model complex business rules explicitly
   • Use domain events to capture important state changes

2. Separate Supporting Subdomains

   • Keep specialized business logic separate from core domain
   • Use clear interfaces between subdomains
   • Allow different implementation approaches for different subdomains

3. Handle Generic Subdomains Appropriately

   • Don't over-complicate generic functionality
   • Consider using existing solutions for common problems
   • Focus development effort on core business value

4. Use Bounded Contexts Effectively

   • Define clear boundaries between different parts of the system
   • Use context mapping to manage relationships
   • Allow different models in different contexts

References and Resources 📚

Essential reading materials:

• "Domain-Driven Design" by Eric Evans
• "Implementing Domain-Driven Design" by Vaughn Vernon
• "Domain-Driven Design Distilled" by Vaughn Vernon

Community resources:

• DDD Community Discord
• Domain-Driven Design Weekly Newsletter
• DDD Exchange Conference Proceedings

These study cases demonstrate how DDD can be effectively applied to solve real-world business problems. The key is to focus on understanding the domain deeply, model it explicitly, and use the appropriate patterns and practices for different parts of the system. Remember that DDD is not about following a specific set of rules, but about creating software that effectively serves the business needs.