Code review checklist for LLD - Scalability & System Analysis

Scalability Analysis - Code review checklist for LLD

Growth Table: Code Review Checklist for Low-Level Design (LLD)

Users / Scale	Checklist Focus	Common Changes	Review Complexity
100 users	Basic design correctness	Simple class and method structure, naming conventions	Low - manual review feasible
10K users	Performance and modularity	Check for efficient algorithms, modular components, avoid duplication	Medium - need automated tools support
1M users	Scalability and fault tolerance	Design patterns for concurrency, error handling, resource management	High - requires detailed analysis and peer reviews
100M users	Maintainability and extensibility	Code standards, documentation, interface contracts, versioning	Very High - formal processes and tooling mandatory

First Bottleneck in LLD Code Review

The first bottleneck is often the complexity and clarity of design. As user scale grows, unclear or tightly coupled components cause maintenance and scalability issues. Reviewers struggle to understand and verify design correctness, leading to delays and errors.

Scaling Solutions for LLD Code Review

• Standardized Checklists: Use detailed, evolving checklists tailored to scale and system complexity.
• Automated Tools: Employ static analysis and design validation tools to catch common issues early.
• Modular Design: Encourage modular, loosely coupled components to simplify reviews.
• Peer Reviews & Pair Programming: Distribute review workload and improve knowledge sharing.
• Documentation & Diagrams: Maintain clear design documents and UML diagrams for easier understanding.
• Training & Guidelines: Regularly train teams on best practices and evolving standards.

Back-of-Envelope Cost Analysis

• At 100 users: ~10-20 design reviews per week, manual effort manageable.
• At 10K users: ~1000+ reviews weekly, need automation to maintain speed.
• At 1M users: Review complexity grows, requiring specialized reviewers and tooling.
• At 100M users: Formal processes, dedicated review teams, and continuous integration with automated checks are essential.
• Storage: Design documents and diagrams require minimal storage but must be version controlled.
• Bandwidth: Sharing large diagrams or models may require optimized formats or cloud collaboration tools.

Interview Tip: Structuring Scalability Discussion for LLD Code Review

Start by identifying the scale and complexity of the system. Discuss common bottlenecks like design clarity and review capacity. Propose solutions such as checklists, automation, and modular design. Highlight trade-offs between manual and automated reviews. Conclude with how processes evolve as scale grows.

Self-Check Question

Your team can manually review 1000 LLD designs per week. The number of designs grows 10x. What is your first action and why?

Answer: Introduce automated static analysis and design validation tools to reduce manual workload and maintain review quality. This addresses the bottleneck of limited manual review capacity.

Key Result

As system scale grows, the main challenge in LLD code review shifts from basic correctness to managing complexity and review capacity. Early bottlenecks arise from unclear design and limited manual review bandwidth. Solutions include standardized checklists, automation, modular design, and formal processes to maintain quality and speed.

Practice

1. Which of the following is the MOST important focus when reviewing a Low-Level Design (LLD) document?

easy

A. Clarity and correctness of the design

B. The color scheme of the document

C. The number of pages in the document

D. The font style used

Solution

1. Step 1: Understand the purpose of LLD review

   The main goal is to ensure the design is clear and correct so developers can implement it properly.

2. Step 2: Evaluate options based on relevance

   Options A, B, and D relate to formatting, which is less critical than clarity and correctness.

3. Final Answer:

   Clarity and correctness of the design -> Option A

4. Quick Check:

   Focus on clarity and correctness = C [OK]

Hint: Focus on design clarity and correctness first [OK]

Common Mistakes:

• Focusing on document style over content
• Ignoring correctness for aesthetics
• Confusing LLD with high-level design

2. Which checklist item should be included to ensure modularity in an LLD review?

easy

A. Ensure the document has a table of contents

B. Check if the design uses consistent variable names

C. Count the total number of classes

D. Verify if components have clear, single responsibilities

Solution

1. Step 1: Understand modularity in design

   Modularity means breaking the system into parts that each do one thing well.

2. Step 2: Match options to modularity

   Verify if components have clear, single responsibilities directly relates to single responsibility, a key modularity principle. Others are unrelated.

3. Final Answer:

   Verify if components have clear, single responsibilities -> Option D

4. Quick Check:

   Modularity = single responsibility components [OK]

Hint: Look for single responsibility to ensure modularity [OK]

Common Mistakes:

• Confusing naming consistency with modularity
• Counting classes without checking responsibilities
• Focusing on document formatting

3. Given this checklist item: "Ensure all public methods have clear input and output definitions." What is the MOST likely outcome if this is NOT followed?

medium

A. Developers may implement methods incorrectly due to unclear contracts.

B. The system will automatically generate method documentation.

C. The code will run faster.

D. The design will have fewer classes.

Solution

1. Step 1: Understand the role of input/output definitions

   Clear definitions guide developers on how to use methods correctly.

2. Step 2: Analyze consequences of missing definitions

   Without clear contracts, developers may misunderstand method usage, causing errors.

3. Final Answer:

   Developers may implement methods incorrectly due to unclear contracts. -> Option A

4. Quick Check:

   Unclear method contracts = incorrect implementation [OK]

Hint: Clear method contracts prevent implementation errors [OK]

Common Mistakes:

• Assuming code runs faster without definitions
• Thinking documentation is auto-generated
• Confusing method count with clarity

4. You find a design where security considerations are missing in the LLD. What is the BEST immediate action during the code review?

medium

A. Approve the design since security is handled later.

B. Ignore security as it is not part of LLD.

C. Request adding security checks and data validation in the design.

D. Suggest removing modularity to simplify security.

Solution

1. Step 1: Recognize importance of security in LLD

   Security should be considered early to avoid costly fixes later.

2. Step 2: Choose action that improves design quality

   Requesting security checks and validation ensures design is robust and safe.

3. Final Answer:

   Request adding security checks and data validation in the design. -> Option C

4. Quick Check:

   Missing security = request additions [OK]

Hint: Always include security early in design reviews [OK]

Common Mistakes:

• Ignoring security until later stages
• Approving incomplete designs
• Removing modularity harms security

5. During a code review for a complex LLD, you notice the design lacks scalability considerations and testability is weak. Which combined checklist items should you prioritize to improve the design?

hard

A. Increase code comments and skip testing details.

B. Add modular components with clear interfaces and include unit test plans.

C. Focus only on UI design and ignore backend scalability.

D. Reduce the number of classes and remove error handling.

Solution

1. Step 1: Identify key improvements for scalability and testability

   Modular components with clear interfaces help scale and isolate parts for testing.

2. Step 2: Match checklist items to these improvements

   Including unit test plans ensures testability is addressed alongside modularity.

3. Final Answer:

   Add modular components with clear interfaces and include unit test plans. -> Option B

4. Quick Check:

   Modularity + testing plans = better scalability and testability [OK]

Hint: Combine modularity and testing for scalable, testable design [OK]

Common Mistakes:

• Removing error handling weakens design
• Ignoring backend scalability
• Focusing only on comments without tests