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184 lines
5.1 KiB
Markdown
184 lines
5.1 KiB
Markdown
# DDD Rules Checklist
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Use this checklist to validate your domain model against DDD principles.
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## Aggregate Rules
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### Boundaries
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- [ ] Aggregate has a clearly defined boundary
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- [ ] Aggregate Root is identified and documented
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- [ ] Only Aggregate Root is accessible from outside
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- [ ] Child entities are encapsulated (not exported or accessed directly)
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- [ ] Aggregate is the smallest consistent unit
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### Invariants
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- [ ] All invariants are documented with `// Invariant:` comments
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- [ ] Constructor validates all invariants
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- [ ] Every mutation method maintains invariants
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- [ ] Invalid state is impossible to create
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- [ ] Aggregate rejects invalid operations (returns error)
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### References
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- [ ] No direct object references to other Aggregates
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- [ ] References to other Aggregates use ID only
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- [ ] ID references are typed (not raw strings/ints)
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- [ ] Cross-aggregate consistency is eventual (not immediate)
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### Transactions
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- [ ] One Aggregate = one transaction boundary
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- [ ] No multi-aggregate transactions in domain layer
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- [ ] Application layer coordinates multiple aggregates if needed
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### Sizing
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- [ ] Aggregate is not too large (performance issues)
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- [ ] Aggregate is not too small (consistency issues)
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- [ ] "Just right" - protects business invariants, nothing more
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---
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## Entity Rules
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### Identity
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- [ ] Has a unique identifier
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- [ ] ID is assigned at creation time
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- [ ] ID is immutable after creation
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- [ ] ID type is specific (e.g., `AccountID`, not `string`)
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### Equality
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- [ ] Equals compares by ID only
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- [ ] Two entities with same ID are considered equal
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- [ ] Attribute changes don't affect equality
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### Mutability
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- [ ] State changes through explicit methods
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- [ ] No public setters
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- [ ] Methods express domain operations (not CRUD)
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- [ ] Methods return errors for invalid operations
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---
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## Value Object Rules
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### Immutability
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- [ ] All fields are private/unexported
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- [ ] No setter methods
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- [ ] No methods that modify internal state
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- [ ] "Modification" creates new instance
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### Equality
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- [ ] Equals compares ALL fields
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- [ ] Two VOs with same values are interchangeable
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- [ ] No identity concept
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### Validation
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- [ ] Constructor validates input
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- [ ] Invalid VOs cannot be created
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- [ ] Returns error for invalid input
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- [ ] Provides `MustXxx()` variant for tests (panics on error)
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### Self-Containment
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- [ ] VO contains all related logic
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- [ ] Domain logic lives in VO methods
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- [ ] Example: `Money.Add(other Money) Money`
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---
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## Repository Rules
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### Interface Location
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- [ ] Repository interface defined in domain layer
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- [ ] Implementation in infrastructure layer
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- [ ] Domain doesn't know about infrastructure
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### Methods
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- [ ] Methods operate on Aggregates (not entities)
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- [ ] `Save(aggregate)` - persists aggregate
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- [ ] `FindByID(id)` - retrieves aggregate
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- [ ] No methods that bypass Aggregate Root
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### Transactions
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- [ ] Repository doesn't manage transactions
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- [ ] Application layer manages transaction scope
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- [ ] One repository call = one aggregate
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---
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## Domain Service Rules
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### When to Use
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- [ ] Operation spans multiple aggregates
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- [ ] Operation doesn't naturally belong to any entity/VO
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- [ ] Significant domain logic that's not entity behavior
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### Implementation
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- [ ] Stateless (no instance variables)
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- [ ] Named after domain operation (e.g., `TransferService`)
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- [ ] Injected dependencies via constructor
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- [ ] Returns domain objects, not DTOs
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---
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## Domain Event Rules
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### Naming
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- [ ] Past tense (e.g., `AccountCreated`, `TransferCompleted`)
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- [ ] Describes what happened, not what to do
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- [ ] Domain language, not technical terms
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### Content
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- [ ] Contains all data needed by handlers
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- [ ] Immutable after creation
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- [ ] Includes aggregate ID and timestamp
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### Publishing
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- [ ] Events raised by Aggregates
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- [ ] Published after aggregate is saved
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- [ ] Handlers in application or infrastructure layer
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---
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## Clean Architecture Rules
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### Dependency Direction
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- [ ] Domain layer has no external dependencies
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- [ ] Application layer depends only on Domain
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- [ ] Infrastructure depends on Domain and Application
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- [ ] No circular dependencies
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### Layer Contents
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- [ ] Domain: Entities, VOs, Aggregates, Repository interfaces, Domain Services
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- [ ] Application: Use Cases, Application Services, DTOs
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- [ ] Infrastructure: Repository implementations, External services, Framework code
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### Interface Segregation
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- [ ] Small, focused interfaces
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- [ ] Defined by consumer (domain layer)
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- [ ] Implemented by provider (infrastructure)
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---
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## Common Anti-Patterns to Avoid
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### Anemic Domain Model
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- ❌ Entities with only getters/setters
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- ❌ All logic in services
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- ✅ Rich domain model with behavior
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### Large Aggregates
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- ❌ Loading entire object graph
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- ❌ Too many entities in one aggregate
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- ✅ Small, focused aggregates
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### Aggregate References
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- ❌ `order.Customer` (direct reference)
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- ✅ `order.CustomerID` (ID reference)
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### Business Logic Leakage
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- ❌ Validation in controllers
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- ❌ Business rules in repositories
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- ✅ All business logic in domain layer
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### Technical Concepts in Domain
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- ❌ `@Entity`, `@Column` annotations
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- ❌ JSON serialization tags
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- ✅ Pure domain objects, mapping in infrastructure
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