Hotel, Room, Booking classes in LLD - System Design Exercise

Design: Hotel Booking System

Design focuses on core classes and their relationships for hotel, room, and booking management. Payment processing, user authentication, and UI are out of scope.

Functional Requirements

FR1: Allow creation and management of hotels with multiple rooms

FR2: Support room types and availability tracking

FR3: Enable customers to book rooms for specific dates

FR4: Prevent double booking of the same room for overlapping dates

FR5: Allow cancellation and modification of bookings

Non-Functional Requirements

NFR1: Support up to 10,000 hotels and 100,000 rooms

NFR2: Booking operations should respond within 200ms

NFR3: System availability target is 99.9%

NFR4: Data consistency is important for booking operations

Think Before You Design

Questions to Ask

❓ Question 1

❓ Question 2

❓ Question 3

❓ Question 4

❓ Question 5

Key Components

Hotel class to represent hotel details

Room class to represent individual rooms and their types

Booking class to represent reservations with date ranges

Availability checking logic

Booking management service

Design Patterns

Aggregation between Hotel and Room classes

Composition of Booking with Room and Customer

Use of date range overlap checks

Factory pattern for creating bookings

Observer pattern for availability updates

Reference Architecture

HotelBookingSystem
  |
  +-- Hotel
  |     +-- Room
  |
  +-- Booking
  
Classes:
Hotel -- owns --> Rooms
Booking -- references --> Room
Booking -- has --> date range

Components

Hotel

Class

Represents a hotel with attributes like name, location, and a list of rooms

Room

Class

Represents a room with attributes like room number, type, and status

Booking

Class

Represents a reservation with customer info, room reference, and booking dates

Request Flow

1. User requests to book a room for specific dates

2. System checks the requested hotel's rooms for availability

3. For each room, system checks existing bookings for date overlaps

4. If an available room is found, system creates a Booking instance

5. Booking is saved and room availability is updated

6. Confirmation is sent to the user

Database Schema

Entities: - Hotel(id, name, location) - Room(id, hotel_id, room_number, type, status) - Booking(id, room_id, customer_id, start_date, end_date, status) Relationships: - Hotel 1:N Room - Room 1:N Booking Constraints: - Booking dates must not overlap for the same room

Scaling Discussion

Bottlenecks

Checking room availability for many rooms and bookings can be slow

Booking conflicts due to concurrent requests

Database write contention on booking records

Solutions

Use caching for room availability data with short TTL

Implement optimistic locking or transactions to prevent double booking

Partition booking data by hotel or room to reduce contention

Use asynchronous processing for non-critical updates

Interview Tips

Time: Spend 10 minutes understanding requirements and clarifying assumptions, 20 minutes designing classes and relationships, 10 minutes discussing scaling and trade-offs, 5 minutes summarizing

Explain class responsibilities and relationships clearly

Discuss how to handle booking conflicts and availability checks

Mention data consistency and concurrency control

Highlight scalability considerations and possible optimizations

Show awareness of real-world constraints and trade-offs

Practice

1. Which class is primarily responsible for storing information about individual rooms in a hotel system?

easy

A. Room

B. Hotel

C. Booking

D. Guest

Solution

1. Step 1: Understand the role of each class

   The Hotel class manages the overall hotel, Booking handles reservations, and Room stores details about each room.

2. Step 2: Identify which class holds room details

   Since Room is designed to represent individual rooms, it stores room number, type, and availability.

3. Final Answer:

   Room -> Option A

4. Quick Check:

   Room class = stores room info [OK]

Hint: Room class holds room details, not Hotel or Booking [OK]

Common Mistakes:

• Confusing Hotel with Room class
• Thinking Booking stores room details
• Assuming Guest class stores room info

2. Which of the following is the correct way to represent a Booking class constructor in Python that takes room, guest, and date as parameters?

easy

A. def __init__(self, room, guest, date):

B. def Booking(room, guest, date):

C. def __booking__(self, room, guest, date):

D. def init(self, room, guest, date):

Solution

1. Step 1: Recall Python constructor syntax

   Python constructors use the special method __init__ with self as the first parameter.

2. Step 2: Match the correct method signature

   def __init__(self, room, guest, date): correctly uses def __init__(self, room, guest, date): which is the standard constructor format.

3. Final Answer:

   def __init__(self, room, guest, date): -> Option A

4. Quick Check:

   Constructor = __init__ method [OK]

Hint: Python constructors always use __init__(self, ...) [OK]

Common Mistakes:

• Using method name other than __init__
• Omitting self parameter
• Using class name as method name

3. Given the following code snippet, what will be the output?

class Room:
    def __init__(self, number):
        self.number = number
        self.is_available = True

class Booking:
    def __init__(self, room):
        self.room = room
        self.room.is_available = False

room101 = Room(101)
print(room101.is_available)
booking1 = Booking(room101)
print(room101.is_available)

medium

A. True\nTrue

B. False\nTrue

C. False\nFalse

D. True\nFalse

Solution

1. Step 1: Check initial availability of room101

   When room101 is created, is_available is set to True, so first print outputs True.

2. Step 2: Booking changes room availability

   Booking constructor sets room101.is_available to False, so second print outputs False.

3. Final Answer:

   True\nFalse -> Option D

4. Quick Check:

   Initial True, then set False by Booking [OK]

Hint: Booking sets room availability to False immediately [OK]

Common Mistakes:

• Assuming availability stays True after booking
• Confusing order of prints
• Ignoring side effect on room object

4. Identify the error in the following Booking class code snippet:

class Room:
    def __init__(self, number):
        self.number = number
        self.is_available = True

class Booking:
    def __init__(self, room, guest):
        self.room = room
        self.guest = guest
    def book(self):
        if self.room.is_available:
            self.room.is_available = False
            print("Booking successful")
        else:
            print("Room not available")

room = Room(201)
booking = Booking(room)
booking.book()

medium

A. is_available should be a method, not attribute

B. Missing guest argument when creating Booking instance

C. book method should return a value

D. Room class is not defined

Solution

1. Step 1: Check Booking constructor parameters

   Booking __init__ requires room and guest, but only room is passed when creating booking instance.

2. Step 2: Identify missing argument error

   Omitting guest argument causes a TypeError at runtime.

3. Final Answer:

   Missing guest argument when creating Booking instance -> Option B

4. Quick Check:

   Constructor args mismatch = missing guest [OK]

Hint: Match all constructor parameters when creating objects [OK]

Common Mistakes:

• Ignoring missing guest argument
• Assuming book method must return value
• Thinking is_available must be a method

5. You want to design a system where a Hotel manages multiple Rooms and allows Bookings only if rooms are available. Which design approach best supports scalability and maintainability?

hard

A. Make Booking class manage all Rooms and Guests directly, without Hotel involvement.

B. Store all booking data inside Room class only, without separate Booking class.

C. Have Hotel class contain a list of Room objects, and Booking class references Room and Guest; Hotel checks availability before booking.

D. Use a single class combining Hotel, Room, and Booking functionalities.

Solution

1. Step 1: Analyze class responsibilities

   Hotel should manage Rooms, Booking should link Rooms and Guests, keeping clear separation.

2. Step 2: Evaluate design for scalability

   Have Hotel class contain a list of Room objects, and Booking class references Room and Guest; Hotel checks availability before booking. cleanly separates concerns, allowing Hotel to check availability and Booking to handle reservations, supporting easy maintenance and scaling.

3. Final Answer:

   Hotel manages Rooms; Booking references Room and Guest; Hotel checks availability -> Option C

4. Quick Check:

   Separation of concerns = Have Hotel class contain a list of Room objects, and Booking class references Room and Guest; Hotel checks availability before booking. [OK]

Hint: Separate Hotel, Room, Booking roles for clean design [OK]

Common Mistakes:

• Combining all logic in one class
• Booking managing Rooms directly
• Ignoring availability checks in Hotel