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System Design and AI Engineering, by Roni Das. 984 interactive lessons.
Start LearningFoundation · Caching Strategies
A network of servers distributed globally that caches content close to users. Netflix uses CDNs to stream video from servers near you, not from one central location.
In short
A CDN (Content Delivery Network) is a globally distributed group of servers that store copies of your content close to users, so a request is answered by a nearby server instead of traveling all the way to your origin. By serving cached files from the nearest edge location, a CDN cuts latency, reduces load on your origin servers, and keeps sites fast and available even under heavy traffic.
In short
A CDN (Content Delivery Network) is a globally distributed group of servers that store copies of your content close to users, so a request is answered by a nearby server instead of traveling all the way to your origin. By serving cached files from the nearest edge location, a CDN cuts latency, reduces load on your origin servers, and keeps sites fast and available even under heavy traffic.
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The System Design course covers CDN across a run of lessons, not one page. These 4 alone are about 175 minutes of step-by-step reading, every one with a quiz.
A CDN is thousands of caches you do not operate, and the cache key decides everything about whether it helps you.
Design a CDN from scratch - edge caching, origin shield, cache purging, anycast routing, TLS termination, and global distribution.
What Edge Side Includes are, how esi:include works on Varnish and Fastly, and ESI vs SSI. On a real Varnish lab: 426 ms without ESI, 118 ms with it.
System design of a video platform like YouTube or Netflix. Covers upload, transcoding, adaptive bitrate (HLS vs DASH), CDN and recommendations.
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A CDN is a network of servers placed in data centers around the world, often called edge locations or points of presence (PoPs). Each edge server keeps cached copies of content that originally lives on your own server, which the CDN calls the origin.
When someone in Tokyo loads your website, they do not pull every image and script from your server in Virginia. The CDN routes them to a nearby edge server, maybe in Tokyo or Osaka, that already holds those files. The data travels a few hundred kilometers instead of crossing the Pacific, so the page loads faster.
Large CDNs are huge. Cloudflare advertises a presence in over 300 cities. Akamai runs hundreds of thousands of servers across thousands of locations. The point of all that hardware is simple: keep a copy of your content physically close to as many people as possible.
Two pieces do the heavy lifting: request routing and caching. When a user requests a file, the CDN uses DNS or anycast routing to send them to the closest healthy edge server, usually measured by network distance rather than raw geography.
On the first request for a file, the edge server does not have it yet. This is a cache miss. The edge fetches the file from your origin, sends it to the user, and stores a copy. Every later request for that file from that region is a cache hit, served directly from the edge with no trip back to origin.
How long a file stays cached is controlled by HTTP headers, mainly Cache-Control and the file's TTL (time to live). A logo with a one year TTL almost never hits your origin again. When you ship a new version, you either change the filename or send a purge command that tells edge servers to drop the old copy.
CDNs were built for static content like images, CSS, JavaScript, fonts, and video segments. Modern ones also do more: terminating TLS, compressing responses, blocking DDoS attacks, and running code at the edge with platforms like Cloudflare Workers or AWS Lambda@Edge.
Reach for a CDN any time you serve the same files to many users in different places: a marketing site, a video platform, an app's static assets, or software downloads. The bigger your geographic spread, the bigger the win. A site whose users are all in one city sees far less benefit.
The main trade-off is staleness. Aggressive caching means an edge server can keep serving an old file after you have updated the origin. You manage this with sensible TTLs, versioned filenames, and cache purges, but a misconfigured cache can show users yesterday's content or, worse, leak a private response to the wrong person.
CDNs are weaker for content that is unique per user or changes every second, like a personalized dashboard or live stock prices. You can still route those requests through the CDN for TLS and DDoS protection, but you usually mark them as uncacheable so they pass straight to origin.
Other costs to weigh: most CDNs bill per gigabyte of egress and per request, debugging gets harder because a cached bug can persist after a deploy, and you are trusting a third party with your traffic. For the vast majority of public web content, the latency and reliability gains are worth it.
Netflix is the textbook case. Instead of streaming every movie from a central data center, Netflix built its own CDN called Open Connect and places storage appliances, called OCAs, directly inside internet service provider networks and at internet exchange points.
Each appliance is preloaded overnight with the shows most likely to be watched in that region. When you press play, the video streams from an OCA that might be in the same building as your ISP, often just one network hop away. That is why a 4K stream starts in a second or two without buffering.
This design saves enormous bandwidth. Netflix can account for a large share of evening internet traffic in many countries, and Open Connect keeps that traffic off expensive long haul links by serving it locally. It is the same core idea as Cloudflare or Akamai, just owned and tuned end to end by one company for one workload.
Netflix Open Connect
Netflix's own CDN; storage appliances sit inside ISP networks and stream video from a hop or two away.
Cloudflare
Runs edge servers in 300+ cities, serving cached content plus TLS, DDoS protection, and edge compute via Workers.
Amazon CloudFront
AWS's CDN with hundreds of edge locations; commonly fronts S3 buckets and origin servers for static assets and video.
Akamai
One of the oldest and largest CDNs, with hundreds of thousands of servers delivering web, software, and media at global scale.
A web server (the origin) holds the original, authoritative copy of your content and runs your application. A CDN is a layer of caching servers in front of it that stores copies of that content close to users. The CDN answers most requests itself and only falls back to the origin on a cache miss.
By default no. CDNs shine with static files that are identical for everyone, like images, CSS, JavaScript, and video segments. Personalized or constantly changing responses are usually marked uncacheable so they pass through to the origin, though the CDN can still provide TLS termination and DDoS protection for them.
The origin sets HTTP headers, mainly Cache-Control with a max-age value, which defines the TTL (time to live). When the TTL expires the edge revalidates with the origin. You can also force an immediate update by purging the cache or by changing the filename, such as adding a content hash.
Often yes for compute and origin bandwidth, since the CDN absorbs most repeat traffic and your servers handle far fewer requests. But CDNs charge for their own egress and request volume, so for low traffic sites the net cost can be similar. The bigger and more global your traffic, the more you save.
The origin is your source of truth where content is created and stored. An edge server is a CDN node near users that caches copies of that content. Edge servers serve cache hits directly and only contact the origin when they lack a fresh copy.
Design a Content Delivery Network
Design a CDN from scratch - edge caching, origin shield, cache purging, anycast routing, TLS termination, and global distribution
Edge Side Includes (ESI): How a CDN Builds One Page From Cached Parts
What Edge Side Includes are, how esi:include works on Varnish and Fastly, and ESI vs SSI. On a real Varnish lab: 426 ms without ESI, 118 ms with it.
Design YouTube/Netflix
System design of a video platform like YouTube or Netflix. Covers upload, transcoding, adaptive bitrate (HLS vs DASH), CDN and recommendations.
Edge Caching
The same 200 URLs became 200, 501, 7,628 or 18,404 cache entries under four values of one response header, and the origin went from 200 requests to 18,706
Anycast Routing
One-to-nearest routing, how the same IP address serves users from hundreds of locations
Caching
Storing frequently accessed data in a faster storage layer so you don't have to fetch it from the original (slower) source every time.
Latency
The time delay between sending a request and getting a response. Amazon found every 100ms of extra latency costs 1% in sales.
DNS
The phonebook of the internet. Translates human-readable domain names (google.com) into IP addresses that computers understand.
Service Worker
A script the browser runs in the background, separate from the web page. Enables offline caching, push notifications, and background sync for progressive web apps.
Cache Eviction
The process of removing entries from a cache when it's full. Common policies: LRU (least recently used), LFU (least frequently used), FIFO (first in, first out).
Cache-Aside
A caching pattern where the application checks the cache first; on a miss, it fetches from the database and populates the cache. Also called lazy loading.
This page explains the idea. The "Content Delivery Network (CDN)" lesson builds it up step by step, with diagrams. It is part of the System Design Masterclass course, which has 770 lessons. 18 of them are free to read. You pay once and keep it for life, with no subscription.
Open the Content Delivery Network (CDN) lessoncourse 1
From absolute beginner to principal engineer, drawn step by step.
Asked about CDN in an interview?
Each question has why it is asked, a full answer, what the interviewer listens for, the follow-ups, the common mistakes and a diagram (268 diagrams in all). A PDF you keep, junior to staff level.
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