Domain-Driven Design (DDD)
Domain-Driven Design is an approach to software development that centers the design around the business domain and its logic. Instead of letting technical concerns drive your architecture, DDD puts business concepts and rules at the heart of your system.
The Core Idea
DDD recognizes that complex business logic is the real challenge in most software. The goal is to create a model of the business domain that captures its rules, behaviors, and language in code.
Key insight: When your code reflects how the business actually works, it becomes easier to understand, maintain, and evolve alongside the business.
Ubiquitous Language
Use the same terminology in code as in business discussions. If the business talks about "deposits" and "withdrawals," your code should too—not "add value" and "remove value."
This shared language:
- Reduces translation errors between business and code
- Makes code review easier (business experts can read it)
- Keeps the model aligned with business needs
Bounded Contexts
Different parts of the business have different models. A "Customer" in the billing context might have different attributes and behaviors than a "Customer" in the support context.
Each bounded context has its own:
- Model (aggregates, entities, value objects)
- Language (terminology specific to that context)
- Events (business facts relevant to that context)
This prevents the mess of trying to create one unified model that serves all purposes.
DDD Building Blocks
DDD provides tactical patterns for modeling your domain:
Entities
Objects with identity that persists over time. A bank account with ID acc-123 is the same account even as its balance changes.
Value Objects
Objects defined by their attributes, not identity. Two Money objects with the same amount and currency are considered equal. Value objects are immutable.
Aggregates
Clusters of entities and value objects treated as a single unit for data changes. The aggregate root enforces business rules and maintains consistency within the aggregate boundary.
Repositories
Provide access to aggregates, hiding persistence details. You ask for an aggregate by ID, and the repository handles loading events and reconstructing state.
Domain Events
Represent something that happened in the domain. Named in past tense (MoneyDeposited, AccountClosed) because they're facts that already occurred.
Domain Services
Operations that don't naturally belong to a single entity, like transferring money between accounts.
How EventFlows Uses DDD
EventFlows provides building blocks that make DDD natural:
AggregateRoot: Base class for your aggregates that handles event application and history tracking
ValueObject: Base class for immutable value objects with equality based on attributes
Domain Events: First-class concept—aggregates emit events that capture business facts
Repositories: Pattern for loading and saving aggregates through event streams
Event Handlers: Subscribe to domain events to coordinate between bounded contexts
DDD + Event Sourcing
DDD and Event Sourcing work together naturally:
Traditional DDD: Aggregates update their state directly when business rules are satisfied
DDD + Event Sourcing: Aggregates emit events when business rules are satisfied, and those events become the state
This combination gives you:
- Complete audit trail of all business decisions
- Ability to replay history to understand how state was reached
- Natural way to publish domain events to other contexts
Why DDD Matters
Handles Complexity: DDD gives you patterns for taming complex business logic instead of letting it become a tangled mess.
Business Alignment: Code organized around business concepts stays aligned with how the business actually works.
Maintainability: Clear boundaries (aggregates, bounded contexts) make it obvious where code belongs and how to change it safely.
Shared Understanding: Ubiquitous language means developers and business experts can discuss the model together.
DDD in Practice with EventFlows
When you build with EventFlows:
- Model your domain using aggregates that enforce business rules
- Use ubiquitous language in your event names and aggregate methods
- Define bounded contexts for different areas of your system
- Emit domain events that represent business facts
- Coordinate between contexts using event subscriptions
EventFlows handles the technical mechanics while you focus on modeling the business domain correctly.
Next Steps
Learn how DDD works with other patterns:
- CQRS - Separating reads and writes around your domain model
- Event Sourcing - Storing domain events as the source of truth
- Clean Architecture - Keeping domain logic independent of infrastructure
For implementation details, explore:
- Value Objects - Building immutable domain values
- Aggregates - Enforcing business rules and consistency
- Commands & Handlers - Executing domain operations