Code review checklist for LLD - System Design Exercise

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

Focus on reviewing low-level design diagrams and documents. Out of scope: code syntax, high-level architecture, deployment details.

Functional Requirements

FR1: Ensure design correctness and completeness

FR2: Verify adherence to design principles and patterns

FR3: Check for scalability and performance considerations

FR4: Validate modularity and separation of concerns

FR5: Confirm clarity and maintainability of design

FR6: Identify potential security and error handling gaps

FR7: Ensure consistency with overall system architecture

Non-Functional Requirements

NFR1: Checklist must be applicable to designs of varying complexity

NFR2: Must be easy to use by both junior and senior engineers

NFR3: Should cover both functional and non-functional aspects

NFR4: Checklist items must be clear and actionable

Think Before You Design

Questions to Ask

❓ Question 1

❓ Question 2

❓ Question 3

❓ Question 4

❓ Question 5

❓ Question 6

❓ Question 7

Key Components

Class and object diagrams

Sequence and interaction diagrams

Data models and schemas

Interfaces and APIs

Error handling mechanisms

Security controls

Performance optimization techniques

Design Patterns

Single Responsibility Principle

Open/Closed Principle

Factory and Builder patterns

Observer and Strategy patterns

Layered architecture

Dependency Injection

Fail-safe and retry mechanisms

Reference Architecture

 +---------------------+       +---------------------+       +---------------------+
 |  Design Document     | ----> |  Review Checklist    | ----> |  Feedback & Updates  |
 +---------------------+       +---------------------+       +---------------------+

Components

Design Document

LLD diagrams and documents

Contains detailed low-level design including classes, methods, and interactions

Review Checklist

Structured checklist template

Guides reviewers through key design aspects to verify correctness and quality

Feedback & Updates

Collaboration tools (e.g., comments, issue trackers)

Captures review feedback and tracks design improvements

Request Flow

1. Designer creates or updates the low-level design document.

2. Reviewer uses the checklist to systematically evaluate the design.

3. Reviewer notes issues or suggestions based on checklist items.

4. Feedback is communicated back to the designer.

5. Designer revises the design accordingly.

6. Cycle repeats until design meets quality standards.

Database Schema

Not applicable as this is a process checklist rather than a data-driven system.

Scaling Discussion

Bottlenecks

Checklist becomes too long or complex, reducing usability.

Inconsistent application of checklist by different reviewers.

Difficulty in tracking feedback and design iterations at scale.

Solutions

Prioritize checklist items to focus on critical design aspects.

Provide training and examples to standardize review approach.

Use integrated collaboration tools to manage feedback and version control.

Interview Tips

Time: Spend 10 minutes explaining the checklist purpose and key items, 15 minutes walking through example review scenarios, and 5 minutes discussing scaling and tooling.

Importance of systematic design review to catch issues early.

Balancing thoroughness with usability in checklist design.

How checklist supports maintainability, scalability, and security.

Role of collaboration and feedback in improving designs.

Adapting checklist for different project sizes and teams.

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