Domain-Driven Design basics in LLD - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to name the central concept that represents the core business logic.

LLD

The [1] is the main focus in Domain-Driven Design where business rules are implemented.

Drag options to blanks, or click blank then click option'

ADomain Model

BAggregate

CEntity

DRepository

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing Repository with Domain Model

Choosing Entity instead of the overall model

The Domain Model represents the core business logic and rules in Domain-Driven Design.

2fill in blank

medium

Complete the code to identify the pattern that groups related objects and enforces consistency.

LLD

An [1] is a cluster of domain objects that can be treated as a single unit.

Drag options to blanks, or click blank then click option'

AService

BFactory

CAggregate

DEvent

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing Service with Aggregate

Choosing Factory which is for object creation

An Aggregate groups related domain objects and ensures consistency within its boundaries.

3fill in blank

hard

Fix the error in the statement about the role of a Repository.

LLD

A Repository is responsible for [1] domain objects to and from the data store.

Drag options to blanks, or click blank then click option'

Apersisting

Bcreating

Cdeleting

Dvalidating

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing creating instead of persisting

Confusing validation with repository responsibility

Repositories handle persisting and retrieving domain objects from storage.

4fill in blank

hard

Fill both blanks to complete the definition of a Value Object.

LLD

A Value Object is [1] and [2]; it has no identity and is immutable.

Drag options to blanks, or click blank then click option'

Aimmutable

Bmutable

Cstateless

Dstateful

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing mutable or stateful which contradict immutability

Confusing Value Objects with Entities

Value Objects are immutable and stateless, meaning they do not have identity and cannot be changed after creation.

5fill in blank

hard

Fill all three blanks to complete the description of a Domain Service.

LLD

A Domain Service encapsulates [1] that don't naturally belong to [2] or [3].

Drag options to blanks, or click blank then click option'

Abusiness logic

BEntities

CValue Objects

Dinfrastructure code

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing infrastructure code instead of business logic

Confusing Domain Services with Repositories

Domain Services hold business logic that does not fit naturally into Entities or Value Objects.

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