Split strategies (equal, exact, percentage) in LLD - Scalability & System Analysis

Scalability Analysis - Split strategies (equal, exact, percentage)

Growth Table: Split Strategies at Different Scales

Users	Equal Split	Exact Split	Percentage Split
100 users	Simple division, low overhead	Manual allocation manageable	Basic percentage calculations
10,000 users	Automated equal partitioning needed	Complex manual tracking, error-prone	Dynamic percentage adjustments required
1,000,000 users	Load balancing with hashing or consistent hashing	Exact splits become impractical, high coordination cost	Requires scalable percentage distribution algorithms
100,000,000 users	Distributed hashing, multi-level partitioning	Exact splits infeasible, system bottlenecks	Automated, approximate percentage splits with monitoring

First Bottleneck

The first bottleneck is the coordination and state management for exact split strategy. As user count grows, tracking and enforcing exact allocations requires heavy synchronization and data consistency, which slows down the system.

Scaling Solutions

• Equal Split: Use consistent hashing or range partitioning to distribute load evenly without central coordination.
• Exact Split: Avoid at large scale; if needed, use distributed consensus systems and strong coordination, but prefer approximate methods.
• Percentage Split: Implement dynamic load balancing with feedback loops and approximate percentage calculations to reduce overhead.
• Cache split decisions to reduce repeated calculations.
• Use horizontal scaling with stateless services to handle increased traffic.

Back-of-Envelope Cost Analysis

At 1 million users, assuming each user generates 1 request per second:

• Total requests per second: ~1,000,000 QPS
• Single server handles ~5,000 QPS → Need ~200 servers
• Storage for split state: Exact split requires large, consistent state storage; equal and percentage splits need less.
• Network bandwidth: 1 Gbps server can handle ~125 MB/s; plan accordingly for data transfer.

Interview Tip

Start by explaining each split strategy simply. Then discuss how each scales with users and traffic. Identify the bottleneck clearly. Propose practical scaling solutions matching the bottleneck. Use real numbers to support your reasoning.

Self Check

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

Answer: Add read replicas and implement caching to reduce load on the primary database before scaling application servers.

Key Result

Exact split strategy breaks first due to coordination overhead; prefer equal or percentage splits with hashing and caching for scalability.

Practice

1. Which split strategy divides an amount so that everyone pays the same share regardless of individual preferences?

easy

A. Equal split

B. Exact split

C. Percentage split

D. Random split

Solution

1. Step 1: Understand the definition of equal split

   Equal split means dividing the total amount evenly among all participants.

2. Step 2: Compare with other splits

   Exact split assigns specific amounts, percentage split assigns based on percent, random split is not a standard method.

3. Final Answer:

   Equal split -> Option A

4. Quick Check:

   Equal split = same share for all [OK]

Hint: Equal split means everyone pays the same amount [OK]

Common Mistakes:

• Confusing exact split with equal split
• Thinking percentage split always equals equal split
• Assuming random split is a valid standard method

2. Which of the following is the correct syntax to represent a percentage split of 40% for user A and 60% for user B in a system design context?

easy

A. {'A': '40%', 'B': '60%'}

B. {'A': 40, 'B': 60}

C. {'A': 0.4, 'B': 0.6}

D. {'A': 4, 'B': 6}

Solution

1. Step 1: Understand percentage representation in decimals

   Percentages are often represented as decimals between 0 and 1 in code for calculations.

2. Step 2: Evaluate options

   {'A': 0.4, 'B': 0.6} uses decimals summing to 1, correct for percentage split. {'A': 40, 'B': 60} uses integers but not decimals. {'A': '40%', 'B': '60%'} uses strings which are not directly usable. {'A': 4, 'B': 6} uses incorrect smaller numbers.

3. Final Answer:

   {'A': 0.4, 'B': 0.6} -> Option C

4. Quick Check:

   Decimal form for percentages = {'A': 0.4, 'B': 0.6} [OK]

Hint: Use decimals (0.4) not integers (40) for percentage splits [OK]

Common Mistakes:

• Using integers instead of decimals for percentages
• Using strings with % symbol in code
• Not ensuring sum equals 1

3. Given a total amount of 100 and a split strategy: {'A': 30, 'B': 70} as exact amounts, what is the amount assigned to user B?

medium

A. 70

B. 100

C. 30

D. 0

Solution

1. Step 1: Identify the split type and amounts

   The split is exact, so amounts are assigned directly as given.

2. Step 2: Find user B's assigned amount

   User B is assigned 70 as per the exact split dictionary.

3. Final Answer:

   70 -> Option A

4. Quick Check:

   Exact split assigns given amounts = 70 [OK]

Hint: Exact split means use given amounts directly [OK]

Common Mistakes:

• Adding amounts instead of reading assigned value
• Confusing exact with percentage split
• Assuming equal split when exact is given

4. In a percentage split system, if the sum of percentages provided is 110%, what is the main issue and how should it be fixed?

medium

A. The sum is valid, no fix needed

B. The sum should be exactly 0%, reset all percentages

C. The sum is less than 100%, add missing percentage

D. The sum exceeds 100%, fix by normalizing percentages to sum to 100%

Solution

1. Step 1: Identify the problem with sum of percentages

   Percentages must sum to 100% (or 1 in decimal) to correctly split amounts.

2. Step 2: Determine the fix

   If sum is 110%, it exceeds total amount. Normalize by scaling percentages so they sum to 100%.

3. Final Answer:

   The sum exceeds 100%, fix by normalizing percentages to sum to 100% -> Option D

4. Quick Check:

   Sum > 100% requires normalization [OK]

Hint: Percentages must sum to 100%, else normalize [OK]

Common Mistakes:

• Ignoring sum validation
• Assuming sum can be more than 100%
• Trying to add missing percentage when sum is too high

5. You need to design a system that supports splitting a bill among users using equal, exact, or percentage strategies. Which approach best ensures scalability and correctness when handling thousands of users?

hard

A. Allow users to input any split without checks and calculate on demand

B. Use a unified split interface that validates input and applies the correct split logic per strategy

C. Store all splits as exact amounts without validation

D. Hardcode equal split only to simplify calculations

Solution

1. Step 1: Identify requirements for scalability and correctness

   System must handle many users and ensure splits are valid and consistent.

2. Step 2: Evaluate design approaches

   Unified interface with validation ensures correctness and flexibility. Hardcoding or no validation risks errors and poor scalability.

3. Final Answer:

   Use a unified split interface that validates input and applies the correct split logic per strategy -> Option B

4. Quick Check:

   Unified validated interface = scalable & correct [OK]

Hint: Validate splits and use unified logic for scalability [OK]

Common Mistakes:

• Ignoring validation leading to incorrect splits
• Hardcoding one strategy limits flexibility
• Allowing unchecked input causes errors at scale