Caching Fundamentals

What You Will Learn

Understand what caching is, why it works, and where a cache fits in an architecture.

Core Idea

A cache stores data that can be reused quickly.

Without caching:

Request -> Application -> Database

With caching:

Request -> Application -> Cache
                         |
                       miss
                         v
                      Database

A cache is useful when the same data is requested repeatedly and the data does not need to be fetched from the source every time.

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

A product page may be requested thousands of times.

Instead of querying the database every time:

GET product:42

The application can return a cached representation and refresh it when necessary.

Key Points

['Caching reduces repeated expensive work.', 'Cache entries can become stale.', 'Every cache needs an invalidation or expiration strategy.', 'A cache should not be added without understanding the consistency requirement.']

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 Redis concepts commonly used for application caching.