Requirements and use cases in LLD - Scalability & System Analysis

Scalability Analysis - Requirements and use cases

Growth Table: Requirements and Use Cases

Users	Requirements Complexity	Use Case Diversity	System Impact
100 users	Basic functional requirements, simple workflows	Few use cases, mostly core features	Minimal load, easy to manage
10,000 users	More detailed requirements, edge cases appear	Multiple use cases, some variations in user behavior	Moderate load, need for performance optimization
1,000,000 users	Complex requirements, scalability and reliability critical	Wide range of use cases, including rare and concurrent scenarios	High load, requires distributed systems and fault tolerance
100,000,000 users	Extensive requirements, global compliance, multi-tenancy	Very diverse use cases, internationalization, personalization	Massive load, advanced scaling, monitoring, and automation

First Bottleneck

At small scale, unclear or incomplete requirements cause delays and rework.

As users grow, the first technical bottleneck is often the system's inability to handle diverse use cases efficiently.

Without clear use cases, the system design may not support necessary workflows, causing performance and reliability issues.

Scaling Solutions

• Clarify and prioritize requirements: Regularly update requirements to reflect user needs and system goals.
• Use modular design: Break system into components to handle different use cases independently.
• Implement flexible workflows: Support variations in use cases without major redesign.
• Automate testing: Ensure all use cases are covered and system behaves as expected.
• Plan for scalability: Design with growth in mind, including load balancing and caching.

Back-of-Envelope Cost Analysis

At 100 users, requirements gathering and use case definition cost dominate.

At 10,000 users, development and testing costs increase due to more use cases.

At 1,000,000 users, infrastructure costs rise sharply to support complex workflows and high availability.

At 100,000,000 users, operational costs for monitoring, compliance, and continuous improvement become significant.

Interview Tip

Start by clearly defining requirements and use cases.

Explain how these influence system design and scalability.

Discuss how evolving requirements impact bottlenecks and solutions.

Use examples to show understanding of real-world complexity.

Self Check

Your system requirements are simple and support 1000 QPS. Traffic grows 10x. What do you do first?

Answer: Review and update requirements and use cases to ensure the system can handle increased load and complexity. Then plan scaling strategies accordingly.

Key Result

Clear, evolving requirements and well-defined use cases are essential to scale systems effectively from small to massive user bases.