Relationships (association, aggregation, composition) in LLD - Architecture Diagram

System Overview - Relationships (association, aggregation, composition)

This system models different types of relationships between objects in software design: association, aggregation, and composition. It helps visualize how objects connect and depend on each other, which is key for designing clear and maintainable software.

Architecture Diagram

  +----------------+       +----------------+       +----------------+
  |   Object A     |-------|   Object B     |       |   Object C     |
  | (Association)  |       | (Aggregation)  |-------| (Composition)  |
  +----------------+       +----------------+       +----------------+
         |                        |                        |
         |                        |                        |
         +------------------------+------------------------+
                              Relationship Types

Components

Object A

class_object

Represents an object with an association relationship to Object B

Object B

class_object

Represents an object with aggregation relationship to Object C

Object C

class_object

Represents an object composed by Object B

Request Flow - 3 Hops

Object A → Object B

Object B → Object C

Object B → Object C

Failure Scenario

Component Fails:Object B

Impact:If Object B is destroyed, composed Object C is also destroyed; aggregated Object C remains

Mitigation:Ensure proper lifecycle management; use aggregation when independent lifecycle is needed

Architecture Quiz - 3 Questions

Which relationship means Object C cannot exist without Object B?

A Composition

B Association

C Aggregation

D Inheritance

Design Principle

This architecture demonstrates how different object relationships affect ownership and lifecycle. Association is a loose connection, aggregation implies shared ownership but independent lifecycle, and composition means strong ownership with dependent lifecycle. Understanding these helps design clear and maintainable object models.

Practice

1. Which relationship describes two objects connected without ownership, where both can exist independently?

easy

A. Inheritance

B. Association

C. Composition

D. Aggregation

Solution

1. Step 1: Understand relationship types

   Association means objects are linked but do not own each other and can exist separately.

2. Step 2: Compare with other relationships

   Aggregation and composition imply ownership; inheritance is a different concept.

3. Final Answer:

   Association -> Option B

4. Quick Check:

   Association = connected without ownership [OK]

Hint: No ownership means association, not aggregation or composition [OK]

Common Mistakes:

• Confusing aggregation with association
• Thinking composition allows independent existence
• Mixing inheritance with relationships

2. Which of the following correctly shows a composition relationship in UML notation?

easy

A. A solid diamond at the owner end connected to the part

B. A hollow diamond at the owner end connected to the part

C. A plain line with no diamonds between objects

D. A dashed arrow pointing from part to owner

Solution

1. Step 1: Recall UML notation for composition

   Composition uses a solid diamond at the owner end to show strong ownership.

2. Step 2: Differentiate from aggregation and association

   Aggregation uses hollow diamond; association uses plain line; dashed arrow is for dependency.

3. Final Answer:

   A solid diamond at the owner end connected to the part -> Option A

4. Quick Check:

   Composition = solid diamond [OK]

Hint: Solid diamond means composition, hollow means aggregation [OK]

Common Mistakes:

• Mixing hollow and solid diamonds
• Confusing association line with composition
• Using dashed arrows for composition

3. Consider this code snippet representing relationships:

class Engine:
    pass

class Car:
    def __init__(self):
        self.engine = Engine()

What type of relationship does Car have with Engine?

medium

A. Association

B. Aggregation

C. Inheritance

D. Composition

Solution

1. Step 1: Analyze ownership in code

   Car creates and owns Engine instance inside its constructor, implying strong ownership.

2. Step 2: Match with relationship definitions

   Since Engine cannot exist meaningfully without Car here, this is composition.

3. Final Answer:

   Composition -> Option D

4. Quick Check:

   Car owns Engine fully = Composition [OK]

Hint: Creating owned object inside means composition [OK]

Common Mistakes:

• Thinking it's aggregation because of ownership
• Confusing association with composition
• Assuming inheritance from code

4. Identify the error in this aggregation example:

class Wheel:
    pass

class Car:
    def __init__(self, wheels):
        self.wheels = wheels

car = Car([Wheel(), Wheel()])

What is wrong with this aggregation?

medium

A. Wheels cannot exist independently in aggregation

B. Car should not hold wheels as a list

C. No error; this is correct aggregation

D. Wheels should be created inside Car for composition

Solution

1. Step 1: Understand aggregation ownership

   Aggregation means Car owns wheels but wheels can exist independently.

2. Step 2: Check code for ownership and independence

   Wheels are created outside Car and passed in, so they can exist alone, matching aggregation.

3. Final Answer:

   No error; this is correct aggregation -> Option C

4. Quick Check:

   Aggregation allows independent parts [OK]

Hint: Aggregation parts created outside owner, passed in [OK]

Common Mistakes:

• Thinking wheels must be created inside Car
• Confusing aggregation with composition
• Assuming list usage is wrong

5. You are designing a library system where a Library contains many Books. Books can exist outside the library and be moved between libraries. Which relationship best fits this scenario and why?

hard

A. Aggregation, because library owns books but books can exist alone

B. Composition, because books depend fully on the library

C. Association, because library and books are loosely connected

D. Inheritance, because books inherit properties from library

Solution

1. Step 1: Analyze ownership and existence

   Books can exist independently and move between libraries, so they are not fully owned.

2. Step 2: Match relationship type

   Aggregation fits because library owns books but books can exist alone and be shared.

3. Final Answer:

   Aggregation, because library owns books but books can exist alone -> Option A

4. Quick Check:

   Aggregation = ownership with independent parts [OK]

Hint: If parts can exist alone, use aggregation [OK]

Common Mistakes:

• Choosing composition thinking books depend fully
• Selecting association ignoring ownership
• Confusing inheritance with ownership