Domain Services - Learning Module

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When Logic Doesn't Belong to Entities

The Uncomfortable Truth About Object-Oriented Design

In the world of Domain-Driven Design, we strive to build rich domain models—entities that encapsulate both data and behavior, value objects that express domain concepts with clarity. The mantra is clear: "Put the logic where the data lives."

But there's an uncomfortable truth every experienced developer eventually encounters: not all business logic belongs to a single entity.

Consider this scenario: A customer wants to transfer money from one bank account to another. Where does this logic live? The source account? The destination account? Neither? This simple question exposes a fundamental gap in our object-oriented toolkit—operations that involve multiple domain objects without naturally belonging to any single one.

What You Will Learn

By the end of this page, you will understand when business logic genuinely cannot reside within entities or value objects, recognize the telltale signs of misplaced logic, and appreciate why Domain Services exist as a first-class DDD building block—not as a refuge for lazy design.

The Problem with Forcing Entity Logic

Object-oriented principles teach us that objects should encapsulate behavior. This is excellent advice—but taken to an extreme, it leads developers into awkward contortions when business operations genuinely span multiple objects.

The money transfer revisited:

Let's examine what happens when we try to force the transfer logic into an entity:

account-with-transfer.ts
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// Attempt 1: Logic in source account
class BankAccount {
    private balance: Money;
    private id: AccountId;
 
    // This feels wrong - why does BankAccount know about other accounts?
    transferTo(destination: BankAccount, amount: Money): void {
        if (this.balance.isLessThan(amount)) {
            throw new InsufficientFundsError(this.id, amount);
        }
        
        // Direct manipulation of another object's state
        this.balance = this.balance.subtract(amount);
        destination.credit(amount);  // Calling method on foreign object
    }
    
    credit(amount: Money): void {
        this.balance = this.balance.add(amount);
    }
}

What's wrong with this approach?

At first glance, this code works. But it violates several core principles:

Design Problems with Entity-Forced Logic

•Artificial Responsibility — Why does this account control that account? The source account has no natural authority over the destination. The responsibility assignment is arbitrary.
•Bidirectional Coupling — The BankAccount now knows about other BankAccount instances. This creates coupling that makes testing, refactoring, and reasoning about the domain harder.
•Transaction Boundary Ambiguity — Who ensures both sides of the transfer succeed or fail together? The entity cannot manage its own transactional consistency with external objects.
•Business Rule Dispersion — Transfer limits, regulatory compliance, fraud detection—where do these cross-cutting rules live? Stuffing them into an entity bloats its responsibility.
•Semantic Incorrectness — In the real banking domain, a transfer is not an operation of an account—it's an operation between accounts. The domain language itself tells us this logic doesn't belong to either entity.

The Entity Bloat Anti-Pattern

When you keep adding methods to entities to 'keep the logic close to the data,' you eventually create God Objects—entities with dozens of methods, many of which have little to do with the entity's core identity. This is the opposite of good design.

Categories of Homeless Logic

Through years of DDD practice, patterns have emerged that identify when logic genuinely doesn't belong to entities. Understanding these categories helps you recognize domain service candidates in your own domains.

Category 1: Operations Spanning Multiple Aggregates

When business logic requires coordination between multiple aggregates (not just entities), no single aggregate can own the operation without violating aggregate boundaries.

Multi-Aggregate Operations
Operation	Aggregates Involved	Why No Single Owner
Money Transfer	Source Account, Destination Account	Neither account has authority over the other; transfer is a symmetric operation
Order Fulfillment	Order, Inventory, Shipment	Fulfillment orchestrates multiple bounded concepts
Trade Execution	Buy Order, Sell Order, Market	Matching happens between orders, not within either
Meeting Scheduling	Multiple Calendars, Room Resource	Availability must be checked across all participants

Category 2: Stateless Business Calculations

Some business logic is pure calculation—it takes inputs, applies domain rules, and produces outputs without modifying any existing state. These calculations often don't belong to any entity because they operate on data from multiple sources or represent standalone domain operations.

pricing-calculation.ts
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// Pure domain calculation - no natural entity owner
interface PricingResult {
    basePrice: Money;
    discounts: DiscountDetail[];
    taxes: TaxDetail[];
    finalPrice: Money;
}
 
// This calculates price using data from multiple sources:
// - Product (base price, category)
// - Customer (membership tier, loyalty points)
// - Promotions (current campaigns)
// - Tax jurisdiction (location-based rules)
 
// No single entity owns this calculation
function calculateOrderPrice(
    products: OrderLine[],
    customer: Customer,
    activePromotions: Promotion[],
    taxJurisdiction: TaxJurisdiction
): PricingResult {
    // Complex business calculation involving multiple domain objects
    // but modifying none of them
}

Category 3: External System Integration with Domain Rules

When your domain needs to interact with external systems while still enforcing domain invariants, this logic often can't live in entities because:

Entities shouldn't know about infrastructure concerns
The operation spans the boundary between domain and external world
Domain rules must be applied to external data

Category 4: Domain Process Coordination

Some business processes involve a sequence of domain operations that must be coordinated according to business rules. While an Application Service might orchestrate the overall workflow, the domain rules governing that coordination belong in a Domain Service.

The 'But Where Does It Live?' Test

When you discover business logic and find yourself asking 'But where does this live?' while mentally cycling through your entities and finding none that fit—you've likely found a Domain Service candidate. Trust your instinct; the domain is telling you something.

The Anemic Domain Model Trap

Before we fully embrace Domain Services, we must acknowledge a significant danger: the Anemic Domain Model anti-pattern.

In an anemic model, entities become mere data containers with getters and setters, while all business logic migrates to services. This is the opposite of DDD's intent.

Anemic Model (Anti-Pattern)

•Entities are data bags with getters/setters
•All logic lives in 'service' classes
•Business rules scattered across services
•No encapsulation of domain invariants
•Easy to violate entity constraints
•Procedural code disguised as OO

Rich Domain Model (Target)

•Entities encapsulate behavior
•Domain Services only for cross-entity logic
•Business rules centralized in domain
•Invariants enforced by entities themselves
•Entities protect their own consistency
•True object-oriented domain modeling

anemic-vs-rich.ts
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// ❌ ANEMIC MODEL - Logic extracted to services incorrectly
class Order {
    id: OrderId;
    lines: OrderLine[];
    status: OrderStatus;
    customerId: CustomerId;
    
    // Just getters and setters - no behavior
    setStatus(status: OrderStatus) { this.status = status; }
    setLines(lines: OrderLine[]) { this.lines = lines; }
}
 
class OrderService {
    // ALL logic lives here - even logic that belongs to Order
    confirmOrder(order: Order): void {
        if (order.lines.length === 0) {
            throw new Error("Cannot confirm empty order");
        }
        if (order.status !== OrderStatus.DRAFT) {
            throw new Error("Only draft orders can be confirmed");
        }
        order.setStatus(OrderStatus.CONFIRMED);  // Bypasses invariants!
    }
}
 
// ✅ RICH MODEL - Entity owns its behavior
class Order {
    private readonly id: OrderId;
    private lines: OrderLine[];
    private status: OrderStatus;
    private readonly customerId: CustomerId;
    
    // Entity enforces its own rules
    confirm(): void {
        if (this.lines.length === 0) {
            throw new EmptyOrderError(this.id);
        }
        if (this.status !== OrderStatus.DRAFT) {
            throw new InvalidStateTransitionError(this.status, OrderStatus.CONFIRMED);
        }
        this.status = OrderStatus.CONFIRMED;
        // Possibly raise OrderConfirmed domain event
    }
    
    addLine(product: Product, quantity: number): void {
        this.ensureModifiable();
        // Entity validates and adds the line
    }
    
    private ensureModifiable(): void {
        if (this.status !== OrderStatus.DRAFT) {
            throw new OrderNotModifiableError(this.id);
        }
    }
}

The Cardinal Rule

Domain Services are NOT a dumping ground for logic you're too lazy to put in entities. If an operation naturally belongs to a single entity and only manipulates that entity's state, it MUST live in the entity. Domain Services exist for logic that genuinely has no single entity home.

Recognizing Domain Service Candidates

How do you distinguish between logic that should be in an entity and logic that genuinely needs a Domain Service? Here's a systematic evaluation framework:

Domain Service Recognition Criteria

•The operation represents a domain concept — The operation itself is something domain experts talk about. 'Transfer funds,' 'match orders,' 'calculate premium'—these are verbs in the Ubiquitous Language, not just implementation details.
•The operation is stateless — It doesn't maintain state between invocations. It takes inputs, performs domain logic, and produces outputs. Any state it needs comes from the participating entities.
•The operation involves multiple domain objects — There's no single entity that logically 'owns' this operation. Forcing it into one entity would create artificial coupling.
•Placing it in an entity would require references it shouldn't have — If putting this logic in EntityA requires EntityA to know about EntityB's internals, that's a code smell.
•The operation is defined by the domain, not infrastructure — This is domain logic, not application orchestration, not technical integration. The business cares about this operation.

Quick Reference: Where Does This Logic Belong?
Scenario	Where It Belongs	Reasoning
Updating customer's email address	Customer Entity	Single entity, its own data
Calculating order total from order lines	Order Entity	Derived from entity's own state
Transferring money between accounts	Domain Service	Spans multiple aggregates
Validating unique email across customers	Domain Service	Requires access to collection, not single entity
Matching buy/sell orders in trading	Domain Service	Operates on multiple aggregates of same type
Sending confirmation email	Application Service	Infrastructure concern, not domain logic
Logging user activity	Application Service or Infra	Cross-cutting concern
Calculating insurance premium	Domain Service	Complex calculation using multiple inputs

Listen to the Domain Language

Domain experts often reveal domain services through their language. When they say 'The system transfers funds' rather than 'The account transfers funds,' they're telling you the operation is a first-class domain concept that exists independent of any single entity.

Real-World Examples of Homeless Logic

Let's examine concrete examples from various domains to solidify your understanding of when logic doesn't belong to entities.

Example 1: E-Commerce — Inventory Reservation

When a customer adds items to their cart and proceeds to checkout, inventory must be reserved. But where does this logic live?

inventory-reservation.ts
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// The problem: Inventory reservation involves:
// - Order (who wants the items)
// - InventoryItem (what's being reserved)
// - Warehouse (where items are located)
// - Reservation rules (how long reservations last, priority)
 
// ❌ Attempt to put in Order - awkward
class Order {
    reserveInventory(warehouse: Warehouse): void {
        // Order shouldn't know about warehouses
    }
}
 
// ❌ Attempt to put in InventoryItem - also awkward
class InventoryItem {
    reserveFor(order: Order): void {
        // InventoryItem shouldn't know about orders
    }
}
 
// ✅ Domain Service - natural home
interface InventoryReservationService {
    /**
     * Attempts to reserve inventory for an order.
     * Applies domain rules for reservation priority, duration, and availability.
     */
    reserveForOrder(
        order: Order,
        reservationDuration: Duration
    ): ReservationResult;
    
    /**
     * Releases reservations when order is cancelled or expires.
     */
    releaseReservation(reservationId: ReservationId): void;
}
 
// The service encapsulates:
// - Which warehouses to check (nearest, fastest shipping)
// - Reservation priority rules (premium customers first)
// - Split-shipment decisions
// - Rollback logic if partial reservation fails
class InventoryReservationServiceImpl implements InventoryReservationService {
    constructor(
        private inventoryRepository: InventoryRepository,
        private warehouseLocator: WarehouseLocator,
        private reservationPolicyProvider: ReservationPolicyProvider
    ) {}
    
    reserveForOrder(order: Order, duration: Duration): ReservationResult {
        const policy = this.reservationPolicyProvider.getPolicyFor(order);
        const warehouses = this.warehouseLocator.findOptimalFor(order);
        
        // Complex domain logic coordinating multiple aggregates
        // This BELONGS in a domain service
    }
}

Example 2: Insurance — Premium Calculation

Insurance premium calculation is a classic domain service candidate:

premium-calculation.ts
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// Premium calculation requires:
// - Policy details (coverage, limits)
// - Customer risk profile (age, health, history)
// - Actuarial tables (statistical risk data)
// - Regulatory adjustments (state-specific rules)
// - Promotional adjustments (discounts, offers)
 
interface PremiumCalculationService {
    calculatePremium(
        policyApplication: PolicyApplication,
        customerProfile: CustomerRiskProfile,
        effectiveDate: Date
    ): PremiumQuote;
}
 
// Why this is a Domain Service:
// 1. It's a recognized domain concept ("calculating the premium")
// 2. It's stateless (same inputs → same outputs)
// 3. It involves multiple domain objects (policy, customer, tables)
// 4. Neither Policy nor Customer owns this calculation
// 5. It's pure domain logic, not infrastructure
 
class PremiumCalculationServiceImpl implements PremiumCalculationService {
    constructor(
        private actuarialTableProvider: ActuarialTableProvider,
        private regulatoryRuleEngine: RegulatoryRuleEngine,
        private promotionEvaluator: PromotionEvaluator
    ) {}
    
    calculatePremium(
        application: PolicyApplication,
        profile: CustomerRiskProfile,
        effectiveDate: Date
    ): PremiumQuote {
        // 1. Get base rate from actuarial tables
        const baseRate = this.actuarialTableProvider
            .getRateFor(application.coverageType, profile.riskClass);
        
        // 2. Apply risk adjustments based on customer profile
        const riskAdjustedRate = this.applyRiskAdjustments(baseRate, profile);
        
        // 3. Apply regulatory requirements
        const regulatoryRate = this.regulatoryRuleEngine
            .adjustForJurisdiction(riskAdjustedRate, application.state);
        
        // 4. Apply promotional discounts
        const finalRate = this.promotionEvaluator
            .applyApplicableOffers(regulatoryRate, application);
        
        return new PremiumQuote(finalRate, this.breakdownComponents());
    }
}

Example 3: Fintech — Trade Matching

In a trading system, matching buy and sell orders is a core domain operation:

trade-matching.ts
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// Trade matching clearly cannot belong to an individual Order:
// - Neither buy nor sell order 'owns' the matching
// - The matching algorithm is a domain concept itself
// - Multiple orders may participate in producing trades
 
interface TradeMatchingService {
    /**
     * Matches incoming order against existing orders in the book.
     * Returns executed trades and any unmatched remainder.
     */
    matchOrder(
        incomingOrder: Order,
        orderBook: OrderBook
    ): MatchingResult;
}
 
interface MatchingResult {
    executedTrades: Trade[];
    remainingOrder: Order | null;  // null if fully matched
    matchingEvents: MatchingEvent[];
}
 
class PriceTimeMatchingService implements TradeMatchingService {
    // Implements price-time priority matching algorithm
    // This is pure domain logic about HOW orders match
    
    matchOrder(incoming: Order, book: OrderBook): MatchingResult {
        const trades: Trade[] = [];
        let remainingQuantity = incoming.quantity;
        
        const oppositeSide = incoming.side === 'BUY' 
            ? book.getSellOrders() 
            : book.getBuyOrders();
        
        for (const restingOrder of oppositeSide) {
            if (!this.priceMatches(incoming, restingOrder)) break;
            
            const fillQuantity = Math.min(remainingQuantity, restingOrder.quantity);
            trades.push(Trade.execute(incoming, restingOrder, fillQuantity));
            remainingQuantity -= fillQuantity;
            
            if (remainingQuantity === 0) break;
        }
        
        return {
            executedTrades: trades,
            remainingOrder: remainingQuantity > 0 
                ? incoming.withReducedQuantity(remainingQuantity) 
                : null,
            matchingEvents: this.generateEvents(trades)
        };
    }
}

Pattern Recognition

Notice how in each example, the Domain Service has a name that's a noun or verb phrase from the Ubiquitous Language: 'Inventory Reservation,' 'Premium Calculation,' 'Trade Matching.' If you can't name your service using domain terminology, reconsider whether it's truly a domain service.

Summary: Recognizing Homeless Logic

We've explored the fundamental tension that gives rise to Domain Services: the gap between object-oriented design's entity-centric approach and business operations that genuinely span multiple domain objects.

Key takeaways:

Key Takeaways

•Not all logic belongs to entities — Some operations genuinely span multiple aggregates or represent standalone domain concepts.
•Forcing logic into entities causes problems — Artificial responsibility assignment, bidirectional coupling, and semantic incorrectness all signal misplaced logic.
•Domain Services are NOT an excuse for anemic models — Entity logic stays in entities. Domain Services exist only for genuinely homeless operations.
•Categories of homeless logic — Multi-aggregate operations, stateless calculations, external integration with domain rules, and process coordination.
•Listen to the domain language — If domain experts describe an operation independent of any single entity, that's a strong signal for a Domain Service.
•Named from the Ubiquitous Language — Domain Services represent concepts the business cares about, named in terms they use.

What's next:

Now that we understand when logic doesn't belong to entities, we need to understand what makes a Domain Service a Domain Service. The next page explores the essential characteristics that distinguish Domain Services from other service types and ensure they remain true to DDD principles.

Page Complete

You now understand the problem that Domain Services solve—business logic that genuinely has no single entity home. Next, we'll examine the defining characteristics of well-designed Domain Services.