What high level design encompasses in HLD - Scalability & System Analysis

Scalability Analysis - What high level design encompasses

Growth Table: What Changes at Each Scale

Users	System Components	Design Focus	Challenges
100 users	Single server, simple database	Basic architecture, clear modules	Basic reliability and usability
10,000 users	Multiple servers, load balancer, database replicas	Scalability, fault tolerance	Handling concurrent requests, data consistency
1 million users	Distributed services, caching layers, sharded databases	High availability, performance optimization	Data partitioning, network latency, failover
100 million users	Global data centers, CDN, microservices, event-driven systems	Global scalability, disaster recovery	Data synchronization, cost management, complex monitoring

First Bottleneck

At the start, the database is usually the first bottleneck because it handles all data storage and queries. As users grow, the single database server struggles with read/write operations and connection limits.

Scaling Solutions

• Vertical Scaling: Upgrade server hardware to handle more load temporarily.
• Horizontal Scaling: Add more servers behind load balancers to distribute traffic.
• Caching: Use caches like Redis or Memcached to reduce database load.
• Database Replication: Use read replicas to spread read queries.
• Sharding: Split database into parts to handle more data and queries.
• Content Delivery Network (CDN): Cache static content closer to users to reduce latency.

Back-of-Envelope Cost Analysis

Assuming 10,000 users with 1 request per second each:

• Requests per second: 10,000 QPS
• Database: Needs to handle ~10,000 QPS (likely requires replicas and caching)
• Storage: Depends on data size per user; e.g., 1MB/user -> 10GB total
• Bandwidth: 10,000 QPS * 100KB response ≈ 1GB/s (8Gbps network needed)

Interview Tip

Start by explaining the main components of high level design: modules, data flow, and interactions. Then discuss how these components change as the system scales. Always mention bottlenecks and your approach to solve them step-by-step.

Self Check Question

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 main database before considering more complex solutions.

Key Result

High level design starts simple but must evolve with user growth by adding scalability, fault tolerance, and performance optimizations to handle bottlenecks like databases and servers.

Practice

1. What does a high level design (HLD) primarily show in a system?

easy

A. The user interface colors and fonts

B. The main components and how they connect

C. The detailed database schema

D. The exact code for each module

Solution

1. Step 1: Understand the purpose of HLD

   HLD focuses on the big picture, showing main parts and their relationships.

2. Step 2: Differentiate from detailed design

   Detailed code or UI specifics are not part of HLD but come later in design phases.

3. Final Answer:

   The main components and how they connect -> Option B

4. Quick Check:

   HLD = Big parts and connections [OK]

Hint: HLD = big parts + connections, not code or UI [OK]

Common Mistakes:

• Confusing HLD with detailed design
• Thinking HLD includes UI styling
• Assuming HLD shows exact code

2. Which of the following is a correct element to include in a high level design document?

easy

A. Component diagrams showing system parts

B. Detailed API code snippets

C. Exact SQL queries for database

D. Pixel-perfect UI mockups

Solution

1. Step 1: Identify typical HLD contents

   HLD includes diagrams of components and their interactions, not code or UI details.

2. Step 2: Eliminate incorrect options

   Code snippets, SQL queries, and UI mockups belong to detailed design or implementation.

3. Final Answer:

   Component diagrams showing system parts -> Option A

4. Quick Check:

   HLD = component diagrams [OK]

Hint: HLD shows diagrams, not code or UI details [OK]

Common Mistakes:

• Including code snippets in HLD
• Confusing HLD with low-level design
• Adding UI mockups in HLD

3. Given a high level design showing components A, B, and C connected in sequence, what is the most likely data flow?

medium

A. Data flows from A to B to C

B. Data flows from C to B to A

C. Data flows randomly between components

D. No data flows between components

Solution

1. Step 1: Analyze component connections

   Sequence connection means data moves in order from first to last component.

2. Step 2: Determine flow direction

   Data flows from A to B to C as per the sequence shown.

3. Final Answer:

   Data flows from A to B to C -> Option A

4. Quick Check:

   Sequence = A -> B -> C data flow [OK]

Hint: Follow arrows in sequence for data flow direction [OK]

Common Mistakes:

• Reversing data flow direction
• Assuming random data flow
• Ignoring component connections

4. A high level design diagram shows a database connected directly to the user interface without any backend service. What is the main issue here?

medium

A. Database should connect only to frontend

B. Too many components included

C. Missing backend layer to handle business logic

D. User interface should connect directly to database

Solution

1. Step 1: Identify missing architectural layer

   Backend services handle logic and security between UI and database.

2. Step 2: Understand why direct UI to database is bad

   Direct connection risks security and breaks separation of concerns.

3. Final Answer:

   Missing backend layer to handle business logic -> Option C

4. Quick Check:

   Backend needed between UI and database [OK]

Hint: UI never connects directly to database [OK]

Common Mistakes:

• Thinking UI can safely connect to database
• Ignoring backend role
• Confusing component count with design quality

5. When creating a high level design for a scalable web application, which of the following should be included?

hard

A. Detailed SQL queries for data retrieval

B. Exact CSS styles for responsive design

C. User passwords and encryption keys

D. Components like load balancers, web servers, and databases with their interactions

Solution

1. Step 1: Identify scalable system components

   Load balancers, web servers, and databases are key parts for scalability.

2. Step 2: Exclude irrelevant details

   CSS styles, SQL queries, and sensitive data are not part of HLD.

3. Final Answer:

   Components like load balancers, web servers, and databases with their interactions -> Option D

4. Quick Check:

   HLD = scalable components + interactions [OK]

Hint: Include scalable components, exclude code and secrets [OK]

Common Mistakes:

• Adding UI styles in HLD
• Including sensitive data in design
• Confusing HLD with implementation details