Pricing strategy (discounts, coupons) in LLD - Scalability & System Analysis

Scalability Analysis - Pricing strategy (discounts, coupons)

Growth Table: Pricing Strategy (Discounts, Coupons)

Users	Coupon Requests/Second	Database Reads/Writes	Cache Usage	System Complexity
100 users	~10 req/s	Low (few DB queries)	Minimal or none	Simple validation logic
10,000 users	~1,000 req/s	Moderate (more reads, some writes)	Basic caching for coupons	Introduce cache, rate limiting
1,000,000 users	~100,000 req/s	High (heavy DB load)	Distributed cache (e.g., Redis cluster)	Sharding, load balancing, async processing
100,000,000 users	~10,000,000 req/s	Very high (DB bottleneck)	Multi-region caches, CDN for static coupon data	Microservices, event-driven, global distribution

First Bottleneck

At small to medium scale, the database is the first bottleneck. Coupon validation requires frequent reads and writes (e.g., usage count, expiration). As traffic grows, DB query volume and write contention increase, causing latency and failures.

Scaling Solutions

• Caching: Use in-memory caches (Redis) to store coupon data and validation rules to reduce DB reads.
• Read Replicas: Add DB replicas to distribute read load.
• Sharding: Partition coupon data by user or coupon ID to spread writes and reads across multiple DB instances.
• Async Processing: Use message queues to handle coupon usage updates asynchronously to reduce write contention.
• Rate Limiting: Prevent abuse by limiting coupon validation requests per user/IP.
• Microservices: Separate pricing and coupon logic into dedicated services for independent scaling.
• CDN: Cache static coupon metadata globally to reduce origin load.

Back-of-Envelope Cost Analysis

Assuming 1M users with 10% using coupons, each making 1 validation request per minute:

• Requests per second: (1,000,000 * 0.1) / 60 ≈ 1,667 req/s
• Database QPS: ~1,667 (mostly reads, some writes)
• Cache size: Coupon data ~100B per coupon * 10,000 active coupons = ~1MB RAM
• Bandwidth: Each request ~1KB, total ~1.6MB/s (13Mbps)

At 100M users, scale these numbers by 100x, requiring distributed caching, sharded DB, and load balancing.

Interview Tip

Start by clarifying traffic patterns and coupon usage frequency. Identify the main bottleneck (usually DB). Discuss caching and async updates to reduce load. Explain how sharding and microservices help at large scale. Mention rate limiting to prevent abuse. Always justify your choices with traffic and data estimates.

Self Check

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

Answer: Introduce caching to reduce DB reads and add read replicas to distribute load. Also consider async processing for writes to reduce contention.

Key Result

The database is the first bottleneck as coupon validation grows. Caching, read replicas, and sharding are key to scaling pricing strategies with discounts and coupons.

Practice

1. What is the primary purpose of implementing discounts and coupons in a pricing strategy?

easy

A. To reduce product quality

B. To increase the base price of products

C. To make the checkout process slower

D. To attract more customers and increase sales volume

Solution

1. Step 1: Understand the role of discounts and coupons

   Discounts and coupons are marketing tools used to lower prices temporarily.

2. Step 2: Identify the business goal

   Lower prices attract more customers, which can increase sales volume and customer loyalty.

3. Final Answer:

   To attract more customers and increase sales volume -> Option D

4. Quick Check:

   Discounts and coupons = attract customers [OK]

Hint: Discounts attract customers by lowering prices [OK]

Common Mistakes:

• Thinking discounts increase prices
• Confusing discounts with product quality
• Assuming coupons slow checkout

2. Which of the following is the correct way to represent a discount of 20% in a pricing system?

easy

A. discount = 20

B. discount = 2

C. discount = 0.2

D. discount = 200

Solution

1. Step 1: Understand percentage representation in code

   Percentages are usually represented as decimals for calculations, so 20% is 0.2.

2. Step 2: Check each option

   discount = 0.2 uses 0.2 which is correct; others are incorrect as they represent wrong values.

3. Final Answer:

   discount = 0.2 -> Option C

4. Quick Check:

   20% = 0.2 decimal [OK]

Hint: Use decimal for percentage (20% = 0.2) [OK]

Common Mistakes:

• Using whole number 20 instead of decimal
• Confusing 2 or 200 as percentage
• Not converting percentage to decimal

3. Consider this code snippet for applying a coupon discount:

price = 100
coupon_discount = 15  # fixed amount
final_price = price - coupon_discount
print(final_price)

What will be the output?

medium

A. 85

B. 15

C. 100

D. 115

Solution

1. Step 1: Understand the variables

   Price is 100, coupon_discount is 15 fixed amount.

2. Step 2: Calculate final price

   final_price = 100 - 15 = 85.

3. Final Answer:

   85 -> Option A

4. Quick Check:

   100 - 15 = 85 [OK]

Hint: Subtract fixed coupon value from price [OK]

Common Mistakes:

• Confusing discount as percentage
• Adding instead of subtracting discount
• Printing coupon_discount instead of final price

4. In the following code, what is the error that prevents correct discount application?

price = 200
discount = 20  # intended as 20%
final_price = price - discount
print(final_price)

medium

A. Price should be multiplied by discount directly

B. Discount should be converted to decimal before calculation

C. Discount should be added to price

D. No error, code is correct

Solution

1. Step 1: Identify discount representation

   Discount is 20 but intended as 20%, so it should be 0.2 in decimal.

2. Step 2: Correct calculation method

   final_price should be price - (price * discount_decimal), not price - discount integer.

3. Final Answer:

   Discount should be converted to decimal before calculation -> Option B

4. Quick Check:

   20% = 0.2 decimal needed [OK]

Hint: Convert percentage to decimal before subtracting [OK]

Common Mistakes:

• Subtracting integer discount directly
• Adding discount instead of subtracting
• Ignoring percentage to decimal conversion

5. You are designing a system that applies multiple discounts: a 10% seasonal discount and a $5 coupon. If the original price is $50, what is the correct final price after applying both discounts sequentially?

hard

A. $40.5

B. $45

C. $40

D. $42

Solution

1. Step 1: Apply the $5 coupon discount first

   $50 - $5 = $45.

2. Step 2: Apply the 10% seasonal discount

   $45 * 0.9 = $40.5.

3. Step 3: Sequential discounts

   Coupon before percentage discount yields $40.5.

4. Final Answer:

   $40.5 -> Option A

5. Quick Check:

   Coupon then percentage = 40.5 [OK]

Hint: Order of discounts affects final price [OK]

Common Mistakes:

• Applying discounts in wrong order
• Adding discounts instead of subtracting
• Ignoring percentage vs fixed discount difference