CDN and Media or Static Content Systems

Let’s say you are building an application that serves images, videos, CSS, JavaScript files, and downloadable documents.

When your application has a small number of users, all requests for these files go directly to your backend servers. Everything works fine at first.

As the application grows, users from different cities and countries start accessing your content. Images load slowly. Videos buffer. Your backend servers spend most of their time serving static files instead of handling business logic.

If you don’t handle static and media content properly, your system becomes slow, expensive, and fragile.

In this lesson, we’ll walk through how CDNs work, how media systems are designed, and how real systems deliver content efficiently and securely.

1. CDN Basics

A CDN, or Content Delivery Network, is a network of servers distributed across many geographic locations. Instead of serving static files from one central server, the CDN stores copies of your content closer to users.

When a user requests an image or file, the request is served from the nearest CDN location instead of your backend. This reduces latency, improves load time, and protects your origin servers from excessive traffic.

The backend server that holds the original content is called the origin. The CDN acts as a shield between users and the origin.

2. Edge Caching and Origin Protection

CDNs cache content at the edge, meaning close to users.

When a file is requested for the first time, the CDN fetches it from the origin and stores it locally. Subsequent requests are served directly from the edge cache. This dramatically reduces load on your origin servers.

If traffic spikes suddenly, the CDN absorbs most of it. Your backend remains stable. This is called origin protection, and it is one of the biggest benefits of using a CDN.

3. Image Delivery Patterns

Images are one of the most common types of static content. Modern systems do not store multiple versions of the same image.

Instead, they store one original image and let the CDN generate variations on demand. The CDN can resize, compress, and convert images based on request parameters.

This allows:

• faster page loads,
• lower bandwidth usage,
• better user experience on mobile devices.

Image optimization is almost always handled at the CDN layer in large systems.

4. Video Delivery Patterns

Videos are heavier and more complex than images.

Instead of sending a full video file at once, systems use streaming.

Videos are split into small chunks and delivered gradually as the user watches.

The CDN caches these chunks at the edge, allowing smooth playback even for millions of users.

Adaptive bitrate streaming is commonly used.
The video quality adjusts based on the user’s internet speed.

This prevents buffering and reduces wasted bandwidth.

5. Signed URLs and Token-Based Access

Not all content should be public. Some files should only be accessible to authorized users.

CDNs support signed URLs and token-based access to control this.

A signed URL contains a cryptographic signature and an expiration time.
Only users with a valid URL can access the content.

Token-based access works similarly but uses headers or cookies instead of URLs.

These methods allow secure delivery without exposing backend servers.

6. Preventing Hotlinking and Abuse

Without protection, anyone can link directly to your images or videos. This is called hotlinking and can increase your costs dramatically.

CDNs can block requests based on:

• domain,
• IP address,
• headers,
• or tokens.

This ensures that only your application can use your content.

7. Cost Optimization Tips

CDNs improve performance, but they also cost money. Smart systems optimize costs by:

• setting proper cache TTLs,
• compressing images and videos,
• using correct file formats,
• avoiding unnecessary cache invalidation.

Serving content from the edge is cheaper than hitting the origin repeatedly. Cost awareness is part of good system design.

8. When You Might Not Need a CDN

Very small applications with low traffic may not need a CDN initially. However, once users are spread across regions or traffic increases, a CDN becomes essential. Most production systems adopt a CDN sooner than later.

Final Thoughts

CDNs are not just about speed. They protect your backend, improve user experience, and reduce costs when used correctly.

In system design interviews, interviewers look for:

• an understanding of edge caching,
• origin protection,
• secure content delivery,
• and cost awareness.

If you can explain why a CDN is needed and how it fits into the architecture, you are thinking like a real system designer.