What System Design Is

A few years ago, a small team launched an online platform for students. The system was tested by the team itself. Everything looked fine.

• Login worked.
• Videos played.
• Certificates were generated.

On launch day, thousands of students logged in at the same time.

Within minutes:

• Login requests became slow
• Video buffering increased
• Certificate downloads failed

The code had no bugs. The features were correct. The problem was simple: the system was never designed for real usage. This is the exact reason companies care so much about system design.

What System Design Means

System design is not about writing code.

System design means:

Deciding how different parts of a system work together when real users, real data, and real failures exist.

When you design a system, you are answering questions like:

• What happens when users increase?
• What happens when something fails?
• Where will the system become slow?
• How can we fix problems without stopping everything?

Good system design is about thinking before building.

The Three Things Every System Design Is Built On

Every system design, no matter how big or small, comes down to three ideas.

Requirements

Requirements describe what the system must do. For example, in an online learning system:

• Users should be able to sign up, watch videos, and download certificates.
• These are functional requirements.

Then there are quality requirements:

• The system should be fast, reliable, and secure.
• These are non-functional requirements.

In interviews, if you don’t clarify requirements, your design will always be wrong.

Constraints

Constraints are real-world limits. You never design in a perfect world. You have limited money, limited time, and limited people. Maybe the system must launch in one month. Maybe the budget is small. Maybe data must stay inside a country. Constraints shape your design more than technology does.

Tradeoffs

This is the most important part of system design. You cannot optimize everything at once.

• If you make the system faster, it may cost more.
• If you make it very consistent, it may become less available.

System design is about choosing what matters most and clearly explaining why. Interviewers care more about your reasoning than your final answer.

Case Study: Certificate Generation System

Let’s design something small. Students complete a course and download a certificate.

One design choice is:

• Generate the certificate every time the user clicks “Download”.

Another choice is:

• Generate the certificate once and store it.

Generating every time saves storage but increases server load. Generating once uses storage but makes downloads fast and reliable. A good design explains why one option is chosen instead of pretending both are perfect.

High-Level Design vs Low-Level Design

This is where many beginners get confused.

High-Level Design (HLD)

High-level design shows the big picture.

You talk about:

• Client (web or mobile)
• Backend service
• Database
• Cache
• File storage

You explain how data flows between these parts.

You do not talk about classes, functions, or exact queries.

In interviews, always start with HLD.

Low-Level Design (LLD)

Low-level design goes deeper.

You explain:

• API endpoints
• Database tables
• Request and response formats
• Detailed logic

LLD is useful when building the system, but in interviews it usually comes later.

If you jump into LLD too early, interviewers may stop you.

How Interviewers Judge Your System Design

Interviewers are not looking for complex diagrams or big words. They want to see how you think. First, they look for clarity. Can you explain your ideas in a simple way?

Second, they look at assumptions. Do you ask questions? Do you state what you are assuming?

Third, they look for awareness of problems. Can you point out what might break? Do you think about scale and failure? You don’t need perfect answers. You need structured thinking.

A Common Mistakes

Many candidates design for millions of users when the problem needs thousands. They add too many services too early. Simple systems that scale later are better than complex systems that fail early. Good system design starts simple and grows when needed.

The One Output Template You Will Reuse Everywhere

This template keeps your thinking organized. Use it in interviews, projects, and real jobs. Start by clearly stating the problem. List the main features.
Mention performance and scale needs. State your assumptions. Then explain the high-level architecture. Describe how data is stored. Explain key tradeoffs. Finally, talk about possible bottlenecks.

If you follow this structure, you will never feel lost during a system design discussion.

System design shows whether you can build real systems, not just write code.

AI can help you generate ideas, but you must control the design.

This course will teach you how to think like an engineer who designs for the real world.