domain-driven-design

# Domain-Driven Design (DDD) This skill applies only to projects that have opted in to DDD. Do not apply these patterns to projects that use a different approach. For hexagonal architecture (ports and adapters), load the `hexagonal-architecture` skill. DDD and hexagonal architecture are complementary but independent — a project may use one without the other. **Deep-dive resources** are in the `resources/` directory. Load them on demand: | Resource | Load when... | |----------|-------------| | `aggregate-design.md` | Designing or splitting aggregates, sizing questions, optimistic locking | | `domain-services.md` | Unsure if logic is a domain service vs use case, naming conventions | | `domain-events.md` | Cross-aggregate coordination, Decider pattern, event dispatch (outbox), process managers | | `bounded-contexts.md` | Drawing context boundaries, integrating with external systems (ACL), context mapping | | `error-modeling.md` | Deciding between result types and exceptions, error propagation | | `testing-by-layer.md` | Writing tests for DDD code, property-based testing for invariants | For authoritative sources, see `../REFERENCES.md`. --- ## When to Use DDD DDD adds value for **complex domains** with rich business rules. Not every project needs it. **Use DDD when:** - Domain has complex business rules and invariants - Multiple stakeholders with domain expertise - Business logic is the core differentiator - Terms have specific, important meanings **Don't use DDD when:** - Simple CRUD with no business rules - Technical/infrastructure-focused projects - No domain expert to consult **Start simple and evolve:** Begin with ubiquitous language (glossary) and value objects. Add aggregates, domain events, and bounded contexts only when the domain demands it. Your first model will be wrong — that's fine. The goal is to learn quickly and refactor toward deeper insight. --- ## Core Principle **The code must speak the language of the domain.** Every type, function, variable, and test name must use terms from the project's ubiquitous language (glossary). If a concept doesn't have a domain term, that's a modeling gap to discuss with stakeholders — not something to paper over with technical jargon. **Domain models evolve.** The first model is never the final model. As understanding deepens through conversations with domain experts and building working software, the model should change — types get renamed, aggregates get split or merged, new concepts emerge. This is expected and ideal. A model that never changes is either perfect (unlikely) or stagnant (the team stopped learning). TDD and behavioral tests make this evolution safe — rename a concept, update the glossary, and the tests tell you what needs to change. --- ## Where Does This Code Belong? This is the most common decision in DDD. When unsure, use this framework: | Question | If yes → | If no ↓ | |----------|----------|---------| | Does it enforce a business rule or compute a business value? | `domain/` (entity function, value object, or domain service) | ↓ | | Does it orchestrate multiple domain operations without owning logic? | Use case / application service | ↓ | | Does it format, transform, or prepare data for display? | `lib/` or inline in the view | ↓ | | Does it talk to an external system (DB, API, file system)? | Adapter (implements a port defined in domain) | ↓ | | Is it framework-specific glue (route handler, middleware)? | Delivery layer (`app/`) | — | **The purity test is necessary but not sufficient.** A pure function that formats a date for display does not belong in `domain/` just because it's pure. The question is always: "Is this a business rule?" ```typescript // ❌ Pure but NOT domain — formats for human display export const formatEventDate = (date: string | null) => date ? format(parseISO(date), "MMMM d, yyyy") : undefined; // → Belongs in lib/format.ts // ✅ Pure AND domain — business rule that affects behavior export const isPastEvent = (eventDate: string | null, now: Date) => eventDate ? parseISO(eventDate) < now : false; // → Belongs in domain/event/ // ✅ Pure AND domain — business calculation export const calculateCommittedTotal = (items: readonly GiftItem[]) => items.filter(i => i.status !== "idea").reduce((sum, i) => sum + i.pricePence, 0); // → Belongs in domain/budget/ ``` **Why placement matters:** `domain/` files typically have strict coverage requirements and zero infrastructure imports. Putting code in the wrong layer creates unnecessary testing obligations and architectural violations. --- ## Ubiquitous Language & Glossary DDD projects must maintain a glossary file that defines all domain terms. This is the single source of truth for naming. The glossary evolves as the model evolves — when the team discovers a better name or splits a concept, update the glossary first and let the code follow. ### The Glossary File For projects with multiple bounded contexts, organize by context. The same term may have different definitions in different contexts — this is correct, not a bug. ```markdown ## Gifting Context | Term | Definition | Examples | |------|-----------|----------| | Occasion | A gift-giving event (birthday, holiday) | "Mum's Birthday", "Christmas 2026" | | Gift Idea | A potential gift for an occasion | "Cookbook", "Scarf" | | Contribution | Money pledged toward a gift | "£25 from Dad" | ## Notifications Context | Term | Definition | Examples | |------|-----------|----------| | Occasion | An upcoming event that may trigger reminders | (same events, different concern) | | Recipient | The person being gifted — target of reminder scheduling | "Mum" | ``` ### Enforcement Rules - All `type` and `interface` names must use glossary terms - All function names must use glossary verbs and nouns - All test descriptions must use domain language - If you need a new term, add it to the glossary first ```typescript // ✅ Uses domain language type GiftIdea = { readonly id: GiftIdeaId; readonly description: string; readonly occasion: OccasionId; readonly estimatedCost: Money; }; // ❌ Technical jargon type Item = { readonly id: string; readonly text: string; readonly parentId: string; }; ``` --- ## Building Blocks ### Value Objects Immutable, identity-less, compared by their attributes (not by reference). Represent domain concepts defined by their attributes. Two `Money` values with the same amount and currency are equal — value objects have no identity. ```typescript type Currency = 'GBP' | 'USD' | 'EUR'; type Money = { readonly amount: number; readonly currency: Currency }; const createMoney = (amount: number, currency: Currency): Money => { if (amount < 0) throw new Error('Money cannot be negative'); return { amount, currency }; }; // Factory throws = invariant violation (a bug in calling code). // Schemas catch invalid user input at trust boundaries BEFORE // the factory is called. If the factory throws, something // bypassed the schema. ``` For value objects crossing trust boundaries (API input, form data), use Zod schemas. For domain-internal value objects, plain types + factory functions suffice. See the `typescript-strict` skill for schema-first patterns. **Zod-to-branded-type bridging** — parse raw input into branded domain types at trust boundaries: ```typescript // Schema at trust boundary — parses raw strings into branded types const PledgeInputSchema = z.object({ occasionId: z.string().min(1).transform(createOccasionId), contributorId: z.string().min(1).transform(createContributorId), amount: z.object({ amount: z.number().positive(), currency: CurrencySchema }), }); // Reconstitution from persistence — same pattern, used in driven adapters const toOccasion = (row: OccasionRow): Occasion => ({ id: createOccasionId(row.id), name: row.name, budget: createMoney(row.budgetAmount, parseCurrency(row.budgetCurrency)), totalPledged: createMoney(row.pledgedAmount, parseCurrency(row.budgetCurrency)), isFundingClosed: row.isFundingClosed, }); ``` Reconstitution (rebuilding domain objects from DB rows) uses the same factory functions as creation. The factory validates, so invalid persisted data is caught on read rather than silently corrupting the domain. ### Branded Types Prevent accidental swapping of primitives at compile time. Use for entity IDs and single-value value objects. Always provide a factory function — raw strings become branded types only through validation: ```typescript type OccasionId = string & { readonly __brand: 'OccasionId' }; type GiftIdeaId = string & { readonly __brand: 'GiftIdeaId' }; type EmailAddress = string & { readonly __brand: 'EmailAddress' }; // Factory functions — the only way to create branded values const createOccasionId = (raw: string): OccasionId => { if (!raw.trim()) throw new Error('OccasionId cannot be empty'); return raw as OccasionId; // justified: factory validates, then brands }; const createEmailAddress = (raw: string): EmailAddress => { if (!raw.includes('@')) throw new Error('Invalid email'); return raw as EmailAddress; // justified: factory validates, then brands }; ``` The `as` assertion is the one justified exception in branded type factories — the factory validates first, then brands. This is the standard TypeScript pattern for nominal typing. Everywhere else, the compiler prevents mixing up `OccasionId` and `GiftIdeaId`. ### Entities Have identity and a lifecycle. Always valid after construction or state transition. ```typescript type Occasion = { readonly id: OccasionId; readonly name: string; readonly date: Date; readonly giftIdeas: ReadonlyArray<GiftIdea>; readonly budget: Money; }; // Immutable update — returns new valid state const renameOccasion = (occasion: Occasion, newName: string): Occasion => ({ ...occasion, name: newName, }); ``` **Always-valid principle:** An entity must satisfy its invariants at all times. Validate on construction (factory functions or schema parsing) and on every state transition. Never allow an entity to exist in an invalid state, even temporarily. ### Make Illegal States Unrepresentable Use the type system to make invalid states impossible. Replace boolean flags and optional fields with discriminated unions where each variant carries only the data valid for that state: ```typescript // WRONG — boolean + optional allows { isVerified: true, verifiedAt: undefined } type User = { readonly isVerified: boolean; readonly verifiedAt?: Date }; // RIGHT — impossible to be verified without a date type User = | { readonly status: 'unverified' } | { readonly status: 'verified'; readonly verifiedAt: Date }; ``` Apply the same principle to entity lifecycles: ```typescript type Order = | { readonly status: 'draft'; readonly items: ReadonlyArray<OrderItem> } | { readonly status: 'placed'; readonly items: ReadonlyArray<OrderItem>; readonly placedAt: Date } | { readonly status: 'shipped'; readonly items: ReadonlyArray<OrderItem>; readonly placedAt: Date; readonly shippedAt: Date; readonly trackingNumber: string }; ``` **Always handle all variants exhaustively.** The `never` type ensures the compiler catches unhandled states when you add a new variant: ```typescript const describeOrder = (order: Order): string => { switch (order.status) { case 'draft': return `Draft with ${order.items.length} items`; case 'placed': return `Placed at ${order.placedAt.toISOString()}`; case 'shipped': return `Shipped: ${order.trackingNumber}`; default: { const _exhaustive: never = order; return _exhaustive; } } }; ``` ### Aggregates Clusters of entities and value objects with a single root. All modifications go through the root. 1. **One aggregate root per transaction** 2. **Reference other aggregates by ID** — never embed 3. **All invariants enforced by the root** 4. **Keep aggregates small** — only what's needed for consistency For detailed aggregate design guidance, see `resources/aggregate-design.md`. ### Specifications (Predicate Functions) Complex business rules for filtering, eligibility, or validation are expressed as predicate functions in the domain layer. Evans calls these "specifications." ```typescript // Specification: "can this contributor pledge to this occasion?" const canPledge = (occasion: Occasion, contributor: Contributor, amount: Money): boolean => !occasion.isFundingClosed && amount.amount <= contributor.walletBalance.amount && amount.currency === occasion.budget.currency; // Compose specifications for complex eligibility const isGiftReady = (occasion: Occasion): boolean => occasion.totalPledged.amount >= occasion.budget.amount && occasion.giftIdeas.some(idea => idea.status === 'selected'); ``` Specifications are pure predicate functions — they return `boolean` and have no side effects. Use them in domain services, use cases, and query filters. Name them with `is`, `can`, or `has` prefixes. ### Domain Events Domain events represent something meaningful that happened in the domain ("OrderPlaced", "ContributionPledged"). They coordinate side effects across aggregates and bounded contexts. **Domain events earn their complexity when:** - Side effects cross aggregate boundaries - Other bounded contexts need to react to changes - You need an audit trail or event-driven workflows **Don't add domain events when:** - All logic is within a single aggregate - Side effects are within the same transaction - Explicit return values from domain functions suffice For most projects, start without domain events and add them when the domain demands coordination. See `resources/domain-events.md` for the Decider pattern and detailed guidance. --- ## Domain Services When business logic doesn't belong to a single entity, it belongs in a **domain service** — a stateless function in the domain layer that operates across multiple entities or aggregates. ```typescript // ❌ WRONG — cramming cross-entity logic into one entity const addContribution = (occasion: Occasion, contribution: Contribution): Occasion => { // This needs to check the contributor's wallet balance — wrong aggregate! }; // ✅ CORRECT — domain service operates across aggregates const pledgeContribution = ( occasion: Occasion, contributor: Contributor, amount: Money, ): PledgeResult => { if (amount.amount > contributor.walletBalance.amount) { return { success: false, reason: 'insufficient-balance' }; } return { success: true, occasion: addContribution(occasion, { contributorId: contributor.id, amount }), contributor: deductBalance(contributor, amount), }; }; ``` **Domain service vs use case (application service):** | | Domain Service | Use Case | |--|----------------|----------| | Contains business logic? | Yes | No — orchestration only | | Lives in | `domain/` | `domain/` — identifiable by taking ports as params | | Depends on | Domain types only | Repositories, ports, domain services | | Example | `pledgeContribution(occasion, contributor, amount)` | `handlePledge(repo, dto)` — loads, calls domain service, saves | For detailed guidance, see `resources/domain-services.md`. --- ## Error Modeling Use discriminated union result types for expected business outcomes. Reserve exceptions for programmer mistakes and infrastructure failures. ```typescript type PledgeResult = | { readonly success: true; readonly occasion: Occasion; readonly contributor: Contributor } | { readonly success: false; readonly reason: 'insufficient-balance' | 'funding-closed' | 'not-found' }; ``` **The test:** Could a user's action legitimately cause this outcome? If yes → result type. If no (it would mean a bug) → exception. For detailed error modeling patterns and how errors propagate through layers, see `resources/error-modeling.md`. --- ## Repository Pattern Repositories provide collection-like access to aggregates. **Interfaces** in the domain layer, **implementations** in the adapter layer. Repositories use `interface` (not `type`) because they define behavior contracts — this aligns with the TypeScript strict rule "reserve `interface` for behavior contracts." Name methods using domain language. ```typescript // Port (domain layer) — interface because it's a behavior contract interface OccasionRepository { readonly findById: (id: OccasionId) => Promise<Occasion | undefined>; readonly save: (occasion: Occasion) => Promise<void>; } // Adapter (infrastructure layer) — see hexagonal-architecture skill ```