Domain-Driven Design basics in LLD - System Design Exercise

Design: Domain-Driven Design Basics

Covers foundational concepts of Domain-Driven Design including modeling, bounded contexts, and key building blocks. Does not cover advanced tactical patterns or implementation details.

Functional Requirements

FR1: Understand how to model complex business domains using software

FR2: Identify core domain and subdomains

FR3: Define bounded contexts to separate different parts of the system

FR4: Use ubiquitous language shared by developers and domain experts

FR5: Design entities, value objects, aggregates, and repositories

FR6: Ensure clear separation between domain logic and infrastructure

Non-Functional Requirements

NFR1: Focus on clarity and maintainability over premature optimization

NFR2: Support evolving business requirements with flexible design

NFR3: Keep latency and performance reasonable for typical business applications

NFR4: Design for team collaboration between technical and non-technical members

Think Before You Design

Questions to Ask

❓ Question 1

❓ Question 2

❓ Question 3

❓ Question 4

❓ Question 5

Key Components

Entities with identity and lifecycle

Value objects representing immutable concepts

Aggregates as consistency boundaries

Repositories for data access abstraction

Bounded contexts to define clear model boundaries

Domain events to capture important changes

Design Patterns

Layered architecture separating domain, application, infrastructure

Context mapping to show relationships between bounded contexts

Ubiquitous language to unify communication

Event-driven design for decoupling

Anti-corruption layer to integrate with legacy systems

Reference Architecture

  +---------------------+       +---------------------+
  |  Bounded Context A   |       |  Bounded Context B   |
  |  +---------------+  |       |  +---------------+  |
  |  |   Entities    |  |       |  |   Entities    |  |
  |  | Value Objects |  |       |  | Value Objects |  |
  |  | Aggregates   |  |       |  | Aggregates   |  |
  |  +---------------+  |       |  +---------------+  |
  |  +---------------+  |       |  +---------------+  |
  |  | Repositories  |  |       |  | Repositories  |  |
  |  +---------------+  |       |  +---------------+  |
  +---------------------+       +---------------------+
            |                              |
            |          Context Map         |
            +------------------------------+
                       Integration

Components

Entity

Conceptual

Represents an object with a unique identity and lifecycle in the domain.

Value Object

Conceptual

Represents an immutable descriptive aspect of the domain without identity.

Aggregate

Conceptual

A cluster of domain objects treated as a single unit for data changes.

Repository

Interface/Pattern

Provides abstraction for retrieving and storing aggregates.

Bounded Context

Architectural Boundary

Defines a clear boundary within which a particular domain model applies.

Ubiquitous Language

Communication Practice

Shared language between developers and domain experts to avoid confusion.

Request Flow

1. 1. Domain experts and developers collaborate to define the ubiquitous language.

2. 2. Identify core domain and subdomains to focus modeling efforts.

3. 3. Define bounded contexts to separate different models and teams.

4. 4. Within each bounded context, design entities, value objects, and aggregates.

5. 5. Use repositories to abstract data access for aggregates.

6. 6. Implement domain logic inside aggregates to enforce business rules.

7. 7. Use context maps to manage integration between bounded contexts.

Database Schema

Entities have unique identifiers and attributes. Value objects are embedded within entities or aggregates without separate identity. Aggregates enforce consistency boundaries. Repositories map aggregates to database tables or collections. Bounded contexts may have separate schemas or databases to isolate models.

Scaling Discussion

Bottlenecks

Complex domain logic tightly coupled causing difficulty in changes

Overlapping bounded contexts leading to confusion and integration issues

Large aggregates causing performance bottlenecks during updates

Poor communication causing mismatch between domain model and business needs

Solutions

Refactor domain logic into smaller, focused aggregates and services

Clearly define and enforce bounded context boundaries with context maps

Design aggregates to be as small as possible to reduce locking and contention

Maintain continuous collaboration and update ubiquitous language regularly

Interview Tips

Time: Spend 10 minutes explaining core concepts and terminology, 15 minutes on how to identify bounded contexts and model aggregates, 10 minutes on integration and communication strategies, and 10 minutes on scaling and real-world challenges.

Importance of ubiquitous language for clear communication

How bounded contexts help manage complexity

Role of aggregates in maintaining consistency

Separation of domain logic from infrastructure

Strategies for evolving the domain model with business changes