Hotel, Room, Booking classes in LLD - Scalability & System Analysis

Scalability Analysis - Hotel, Room, Booking classes

Growth Table: Hotel Booking System

Scale	Users	Rooms	Bookings per day	System Changes
Small	100	500	200	Single server, simple database, no caching
Medium	10,000	50,000	10,000	Load balancer, read replicas, caching for availability checks
Large	1,000,000	500,000	100,000	Sharded database, distributed cache, asynchronous booking processing
Very Large	100,000,000	10,000,000	10,000,000	Multi-region deployment, event-driven architecture, CDN for static content

First Bottleneck

At small to medium scale, the database is the first bottleneck. It handles all booking transactions and availability checks. As users and bookings grow, the database CPU and I/O get overwhelmed, causing slow responses and possible booking conflicts.

Scaling Solutions

• Database Read Replicas: Offload read queries like room availability checks to replicas to reduce load on the primary database.
• Caching: Use in-memory caches (e.g., Redis) for frequently accessed data like room availability to reduce database hits.
• Horizontal Scaling: Add more application servers behind a load balancer to handle increased user requests.
• Database Sharding: Split booking data by hotel or region to distribute load across multiple database instances.
• Asynchronous Processing: Use message queues to handle booking confirmation and payment processing asynchronously to improve responsiveness.
• Multi-region Deployment: Deploy services closer to users to reduce latency and improve availability.

Back-of-Envelope Cost Analysis

• At 10,000 bookings/day (~0.12 QPS), a single database can handle the load easily.
• At 100,000 bookings/day (~1.2 QPS), database load increases; read replicas and caching become necessary.
• Storage: Each booking record ~1 KB, so 10 million bookings need ~10 GB storage, manageable with modern databases.
• Network bandwidth: Assuming 1 KB per booking request/response, 100,000 bookings/day require ~100 MB data transfer, well within 1 Gbps network capacity.

Interview Tip

Start by describing the system components and their roles (Hotel, Room, Booking). Then discuss expected traffic and data growth. Identify the first bottleneck (usually the database). Propose scaling solutions step-by-step, explaining why each is needed. Use real numbers to justify your choices. Finally, mention trade-offs and monitoring strategies.

Self Check

Your database handles 1000 QPS. Traffic grows 10x to 10,000 QPS. What do you do first?

Answer: Add read replicas to distribute read queries and reduce load on the primary database. Also, implement caching for frequent read operations like room availability checks. This reduces database CPU and I/O usage, preventing overload.

Key Result

The database is the first bottleneck as bookings grow; scaling requires read replicas, caching, and sharding to maintain performance.

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