KISS (Keep It Simple) in LLD - System Design Exercise

Design: Simple Task Management System

Design focuses on core task management features with simple architecture. Authentication and notifications are out of scope.

Functional Requirements

FR1: Users can create, update, and delete tasks

FR2: Tasks have a title, description, and status (todo, in-progress, done)

FR3: Users can view a list of their tasks

FR4: The system should be easy to understand and maintain

Non-Functional Requirements

NFR1: Handle up to 1000 users concurrently

NFR2: API response time p99 under 300ms

NFR3: Availability target 99.5% uptime

NFR4: Keep the design minimal and avoid unnecessary complexity

Think Before You Design

Questions to Ask

❓ Question 1

❓ Question 2

❓ Question 3

❓ Question 4

Key Components

API server for task operations

Simple database for storing tasks

Basic client interface or API consumer

Design Patterns

CRUD (Create, Read, Update, Delete) pattern

Single responsibility principle

Minimal layered architecture

Reference Architecture

Client
  |
  v
API Server
  |
  v
Simple Database

Components

Client

Any frontend or API consumer

Send requests to create, update, delete, and list tasks

API Server

Lightweight REST API (e.g., Flask, Express)

Handle task operations with simple endpoints

Simple Database

Relational DB like SQLite or PostgreSQL

Store tasks with minimal schema

Request Flow

1. Client sends HTTP request to API server to create or update a task

2. API server validates request and performs operation on database

3. Database stores or updates task record

4. API server sends response back to client confirming success or failure

5. Client requests list of tasks; API server queries database and returns results

Database Schema

Task(id PK, user_id FK, title VARCHAR, description TEXT, status ENUM('todo','in-progress','done'), created_at TIMESTAMP, updated_at TIMESTAMP)

Scaling Discussion

Bottlenecks

Single API server may become overloaded with many users

Database can slow down with large number of tasks

Simple design may lack advanced features needed later

Solutions

Add load balancer and multiple API servers for horizontal scaling

Use database indexing and optimize queries; consider read replicas

Keep design modular to add features without complexity

Interview Tips

Time: 10 min for requirements and clarifications, 20 min for design and data flow, 15 min for scaling and trade-offs

Emphasize simplicity and maintainability

Explain how minimal components meet requirements

Discuss trade-offs of simple design versus future needs

Show understanding of scaling basics without overcomplicating

Practice

1. What is the main goal of the KISS principle in system design?

easy

A. To add as many features as possible

B. To keep designs simple and easy to understand

C. To use complex algorithms for better performance

D. To focus only on the user interface

Solution

1. Step 1: Understand the KISS principle meaning

   KISS stands for "Keep It Simple, Stupid" and encourages simplicity in design.

2. Step 2: Identify the goal of simplicity

   The goal is to make systems easier to build, test, and maintain by avoiding unnecessary complexity.

3. Final Answer:

   To keep designs simple and easy to understand -> Option B

4. Quick Check:

   KISS = Keep It Simple [OK]

Hint: KISS means simple and clear design only [OK]

Common Mistakes:

• Thinking KISS means adding more features
• Confusing simplicity with lack of functionality
• Assuming KISS ignores performance

2. Which of the following is the best example of applying KISS in system design?

easy

A. Implementing a custom encryption algorithm instead of a standard one

B. Adding multiple layers of caching without clear need

C. Using a single, clear API endpoint instead of multiple complex ones

D. Designing a UI with many animations and effects

Solution

1. Step 1: Identify simple design choices

   A single clear API endpoint reduces complexity and is easier to maintain.

2. Step 2: Compare other options

   Adding unnecessary caching layers, custom encryption, or flashy UI adds complexity without clear benefit.

3. Final Answer:

   Using a single, clear API endpoint instead of multiple complex ones -> Option C

4. Quick Check:

   Simplicity means fewer, clearer components [OK]

Hint: Choose the simplest clear solution [OK]

Common Mistakes:

• Believing more features always mean better design
• Ignoring maintainability for flashy effects
• Over-engineering solutions

3. Consider this simple system design: a service receives requests and stores data in a database. Which design follows KISS best?

medium

A. Service directly writes to one database without extra layers

B. Service writes to multiple databases with complex syncing logic

C. Service uses multiple caches and queues before writing

D. Service encrypts data multiple times before storing

Solution

1. Step 1: Analyze the simplest design

   Directly writing to one database is straightforward and easy to maintain.

2. Step 2: Evaluate complexity of other options

   Multiple databases, caches, queues, or repeated encryption add complexity and potential failure points.

3. Final Answer:

   Service directly writes to one database without extra layers -> Option A

4. Quick Check:

   Simple direct design = KISS [OK]

Hint: Pick the design with least moving parts [OK]

Common Mistakes:

• Assuming more layers improve reliability always
• Ignoring maintenance overhead of complex syncing
• Overcomplicating encryption unnecessarily

4. You have a system with multiple unnecessary components causing delays. Which change best applies KISS to fix it?

medium

A. Add more logging to track delays

B. Add retries and timeouts to every component

C. Increase hardware resources without changing design

D. Remove extra components and keep only essential parts

Solution

1. Step 1: Identify unnecessary complexity

   Extra components cause delays and complexity, violating KISS.

2. Step 2: Simplify by removing non-essential parts

   Removing unnecessary parts reduces complexity and improves performance.

3. Final Answer:

   Remove extra components and keep only essential parts -> Option D

4. Quick Check:

   Simplify by cutting non-essential parts [OK]

Hint: Cut what is not needed to simplify [OK]

Common Mistakes:

• Adding more features instead of simplifying
• Ignoring root cause of delays
• Relying only on hardware upgrades

5. You must design a notification system that sends alerts to users. Which approach best follows KISS while ensuring scalability?

hard

A. Use a simple queue to process notifications one by one with retry logic

B. Build a complex microservices architecture with multiple databases and caches

C. Send notifications directly from the main application without queueing

D. Implement a custom protocol for notifications with encryption and compression

Solution

1. Step 1: Identify simple scalable design

   A queue with retry logic is simple and can handle load by processing messages sequentially.

2. Step 2: Compare complexity and scalability

   Complex microservices add overhead; direct sending risks overload; custom protocol adds unnecessary complexity.

3. Final Answer:

   Use a simple queue to process notifications one by one with retry logic -> Option A

4. Quick Check:

   Simple queue = scalable and clear [OK]

Hint: Simple queue with retry scales well [OK]

Common Mistakes:

• Over-engineering with microservices too early
• Ignoring retry and failure handling
• Skipping queueing causing overload