Vertical vs Horizontal Scaling

What You Will Learn

Understand the two basic ways to increase system capacity.

Core Idea

Vertical scaling means making one machine stronger.

Small Server -> Bigger Server

Horizontal scaling means adding more machines.

        Load Balancer
        /    |        Server Server Server

Horizontal scaling is often useful for stateless application servers because traffic can be distributed across multiple instances.

A Simple Mental Model

Client
  |
  v
Application
  |
  +--> Database
  |
  +--> Cache
  |
  +--> Background Worker

Do not worry about building every component at once. System design is mainly about understanding why a component exists, what problem it solves, and how components communicate.

Practical Example

Suppose one API server handles 500 requests per second and traffic grows.

Instead of immediately buying a much larger machine, you might run several application instances:

Load Balancer
  |
  +-- API 1
  +-- API 2
  +-- API 3

The database and other shared resources may then become the next bottleneck.

Key Points

['Vertical scaling is simple but has hardware limits.', 'Horizontal scaling increases the number of instances.', 'Stateless applications are easier to scale horizontally.', 'Scaling one component can expose another bottleneck.']

Common Beginner Mistakes

Adding technologies before understanding the problem.
Designing for millions of users when the requirements do not justify it.
Ignoring failure cases.
Treating every system as if it needs the same architecture.
Focusing on diagrams instead of explaining the decisions behind them.

Quick Summary

The goal of this lesson is to build a simple mental model that you can reuse in future designs.

Next, you will learn how a load balancer distributes traffic.