What is a CDN?

A Content Delivery Network (CDN) is a network of geographically distributed servers that stores and serves copies of content closer to users.

Instead of serving every request from your main server, the CDN delivers content from the nearest location.

Why Do We Need a CDN?

Imagine your server is in Mumbai.

User (Delhi)
        │
        ▼
     Mumbai Server

A user from Delhi experiences relatively low latency.

But a user from London has to travel a much longer network path.

User (London)
        │
        ▼
     Mumbai Server

The greater the distance, the higher the latency.

Now imagine millions of users from different countries requesting the same image.

Your server has to send that image repeatedly, consuming bandwidth and processing resources.

Solution

Place copies of the content around the world.

             CDN Network

 Delhi      London      Singapore

   ●            ●             ●

        \      |      /

         Origin Server
          (Mumbai)

Now:

The content travels a much shorter distance.

How Does a CDN Work?

Suppose a browser requests:

GET /images/logo.png

First Request

User

↓

Nearest CDN

↓

Content not found

↓

Origin Server

↓

Returns image

↓

CDN stores a copy

↓

Returns image to user

This is called a Cache Miss.

Later Requests

User

↓

Nearest CDN

↓

Image found

↓

Returned immediately

The origin server is not contacted.

This is called a Cache Hit.

Cache Hit vs Cache Miss

Cache Hit

The requested content already exists in the CDN cache.

User

↓

CDN

↓

Response

Fast.

Cache Miss

The content is not in the CDN cache.

User

↓

CDN

↓

Origin Server

↓

CDN stores response

↓

User

Slightly slower the first time.

What Can a CDN Cache?

Mostly static content, such as:

Some CDNs can also cache dynamic API responses when configured appropriately.

Benefits of a CDN

1. Faster Loading

Content comes from a nearby server.

Before

User (London)

↓

Mumbai

After

User (London)

↓

London CDN

Lower latency leads to faster page loads.

2. Reduced Server Load

Without a CDN:

1,000,000 image requests

↓

Origin Server

With a CDN:

999,000

↓

CDN

1,000

↓

Origin Server

The origin server handles far fewer requests.

3. Saves Bandwidth

The same file is not sent repeatedly from the origin server.

Bandwidth usage decreases.

4. Better Availability

If one CDN edge server has problems, traffic can often be routed to another nearby location.

This improves reliability.

5. DDoS Protection

Many CDN providers include protection against Distributed Denial of Service (DDoS) attacks.

Instead of malicious traffic reaching your server directly:

Attack

↓

CDN

↓

Filtered

↓

Origin Server

The CDN absorbs or filters much of the unwanted traffic.

Real-World Example

Suppose YouTube stores a video in one central data centre.

Every viewer worldwide would have to stream it from there.

Instead:

User (India)

↓

Indian CDN

User (Germany)

↓

German CDN

User (USA)

↓

US CDN

Each user streams from a nearby CDN edge server, improving performance.

CDN vs Reverse Proxy

Many people confuse these concepts.

CDN Reverse Proxy
Distributed across many geographic locations Usually sits in front of your backend servers
Mainly optimises content delivery Mainly routes and manages requests
Primarily caches content close to users May cache, but also handles TLS termination, routing, compression, etc.
Focuses on reducing latency globally Focuses on protecting and managing backend infrastructure

Think of it this way:

CDN + Reverse Proxy + Load Balancer

A common production architecture looks like this:

                Users
                   │
                   ▼
              CDN (Cloudflare)
                   │
                   ▼
       Reverse Proxy (Nginx)
                   │
                   ▼
          Load Balancer
          /      |      \
         ▼       ▼       ▼
   Backend1 Backend2 Backend3
                   │
                   ▼
               Database

Flow:

  1. User requests a page or asset.
  2. CDN serves cached content if available.
  3. If not cached, the request goes to the reverse proxy.
  4. The reverse proxy handles HTTPS, routing, compression, and security.
  5. The load balancer distributes the request to a healthy backend server.
  6. The backend processes the request and, if needed, accesses the database.

Popular CDN Providers

Some well-known CDN providers include: