Domain-Driven Design basics in LLD - Architecture Diagram

System Overview - Domain-Driven Design basics

Domain-Driven Design (DDD) helps build software by focusing on the core business ideas. It breaks down complex systems into smaller parts called domains and subdomains. The goal is to create clear boundaries and models that match real-world business needs.

Architecture Diagram

User
  |
  v
Application Layer
  |
  v
Domain Layer <--> Infrastructure Layer
  |
  v
Database

Legend:
- Application Layer: Handles user requests and coordinates tasks
- Domain Layer: Contains business logic and rules
- Infrastructure Layer: Manages technical details like database access
- Database: Stores persistent data

Components

User

actor

Interacts with the system by sending requests

Application Layer

service

Coordinates tasks and handles user requests

Domain Layer

service

Contains business logic and domain models

Infrastructure Layer

service

Handles technical details like database and external services

Database

database

Stores persistent data for the domain

Request Flow - 8 Hops

User → Application Layer

Application Layer → Domain Layer

Domain Layer → Infrastructure Layer

Infrastructure Layer → Database

Database → Infrastructure Layer

Infrastructure Layer → Domain Layer

Domain Layer → Application Layer

Application Layer → User

Failure Scenario

Component Fails:Database

Impact:Data cannot be read or written, causing business operations to fail

Mitigation:Use database replication and backups; cache recent data in Infrastructure Layer to serve reads temporarily

Architecture Quiz - 3 Questions

Which layer contains the core business rules in Domain-Driven Design?

A Application Layer

B Infrastructure Layer

C Domain Layer

D Database

Design Principle

Domain-Driven Design separates concerns by organizing software into layers that reflect business concepts and technical details. This clear separation helps teams focus on the domain logic while managing infrastructure independently, improving maintainability and scalability.

Practice

1. What is the main purpose of Domain-Driven Design (DDD)?

easy

A. To model software closely around real business concepts

B. To optimize database queries for performance

C. To create user interfaces quickly

D. To write code without any documentation

Solution

1. Step 1: Understand the goal of DDD

   DDD focuses on aligning software design with the core business domain and its logic.

2. Step 2: Compare options with DDD purpose

   Only To model software closely around real business concepts describes modeling software around business concepts, which is the essence of DDD.

3. Final Answer:

   To model software closely around real business concepts -> Option A

4. Quick Check:

   DDD = model software on business concepts [OK]

Hint: DDD = software models business ideas clearly [OK]

Common Mistakes:

• Confusing DDD with UI or database optimization
• Thinking DDD is about coding speed only
• Ignoring the business domain focus

2. Which of the following is the correct definition of an Entity in DDD?

easy

A. An object defined only by its attributes and no identity

B. A database table storing raw data

C. An object with a unique identity that persists over time

D. A service that performs calculations without state

Solution

1. Step 1: Recall Entity characteristics in DDD

   Entities have a unique identity that remains constant even if attributes change.

2. Step 2: Match definitions with Entity concept

   An object with a unique identity that persists over time correctly states that Entities have unique identity and persistence over time.

3. Final Answer:

   An object with a unique identity that persists over time -> Option C

4. Quick Check:

   Entity = unique identity object [OK]

Hint: Entity always has unique identity, not just attributes [OK]

Common Mistakes:

• Confusing Entities with Value Objects
• Thinking Entities have no identity
• Mixing Entities with Services

3. Consider this simplified DDD code snippet in Python:

class Order:
    def __init__(self, order_id, items):
        self.order_id = order_id
        self.items = items

order1 = Order(1, ['apple', 'banana'])
order2 = Order(1, ['apple', 'banana'])

print(order1 == order2)

What will be the output?

medium

A. True

B. False

C. SyntaxError

D. None

Solution

1. Step 1: Understand default equality in Python classes

   By default, Python compares object references, so two different instances with same data are not equal.

2. Step 2: Analyze the code output

   order1 and order2 are different objects with same data, so order1 == order2 returns False.

3. Final Answer:

   False -> Option B

4. Quick Check:

   Default object equality compares references = False [OK]

Hint: Default == compares object identity, not data [OK]

Common Mistakes:

• Assuming == compares data automatically
• Expecting True because attributes match
• Confusing syntax error with logic error

4. In a DDD model, a Value Object should be immutable. Which of the following code snippets violates this principle?

medium

A. class Money:
def __init__(self, amount, currency):
self.amount = amount
self.currency = currency

B. class Money:
def __init__(self, amount, currency):
self._amount = amount
self._currency = currency

C. class Money:
def __init__(self, amount, currency):
self._amount = amount
self._currency = currency
@property
def amount(self):
return self._amount

D. class Money:
def __init__(self, amount, currency):
self.amount = amount
self.currency = currency
def change_amount(self, new_amount):
self.amount = new_amount

Solution

1. Step 1: Recall immutability in Value Objects

   Value Objects should not allow changes after creation to keep consistency.

2. Step 2: Identify mutable code

   class Money: def __init__(self, amount, currency): self.amount = amount self.currency = currency def change_amount(self, new_amount): self.amount = new_amount has a method that changes the amount, violating immutability.

3. Final Answer:

   Code with method changing amount violates immutability -> Option D

4. Quick Check:

   Value Object must be immutable = no setters [OK]

Hint: Value Objects cannot change state after creation [OK]

Common Mistakes:

• Allowing setters or methods that modify attributes
• Confusing immutability with read-only properties only
• Ignoring methods that change internal state

5. You are designing a DDD model for an online store. Which of the following best represents an Aggregate?

hard

A. An Order object that contains multiple OrderItems and enforces business rules

B. A single Product object with price and description

C. A database table storing customer addresses

D. A utility service that calculates discounts

Solution

1. Step 1: Understand Aggregate in DDD

   An Aggregate is a cluster of related objects treated as a single unit with a root entity controlling consistency.

2. Step 2: Match options with Aggregate concept

   An Order object that contains multiple OrderItems and enforces business rules describes an Order with multiple OrderItems and business rules, fitting Aggregate definition.

3. Final Answer:

   An Order object containing multiple OrderItems and enforcing business rules -> Option A

4. Quick Check:

   Aggregate = root entity + related objects [OK]

Hint: Aggregate = root entity plus related objects as one unit [OK]

Common Mistakes:

• Confusing single entities with aggregates
• Thinking utility services are aggregates
• Mixing database tables with domain aggregates