Why CDN concept and usage in HLD? - Purpose & Use Cases

The Big Idea

What if your website could load instantly for anyone, anywhere, without crashing?

The Scenario

Imagine you run a popular website that millions of people visit from all over the world. Every time someone clicks your site, their computer asks your main server for all the images, videos, and files. If everyone asks the same server, it gets very slow and sometimes crashes.

The Problem

Relying on a single server means slow loading times for visitors far away, high chances of server overload, and unhappy users who might leave your site. Fixing this by adding more servers manually is complex and expensive.

The Solution

A Content Delivery Network (CDN) solves this by placing copies of your files on many servers worldwide. When someone visits your site, the CDN sends files from the closest server, making loading fast and reliable without overloading your main server.

Before vs After

✗ Before

User -> Main Server -> Content

✓ After

User -> Nearest CDN Server -> Content

What It Enables

CDNs enable fast, reliable, and scalable delivery of content to users anywhere in the world, improving user experience and reducing server load.

Real Life Example

When you watch a video on a streaming site, the video loads quickly because a CDN delivers it from a server near you, not from a faraway central server.

Key Takeaways

Manual single-server delivery causes slow and unreliable access.

CDNs distribute content globally to speed up delivery.

Using CDNs improves user experience and scales easily.

Practice

1. What is the primary purpose of a Content Delivery Network (CDN) in system design?

easy

A. To store copies of content closer to users for faster delivery

B. To increase the size of the main server's database

C. To replace the main server completely

D. To encrypt all user data end-to-end

Solution

1. Step 1: Understand CDN's role in content delivery

   A CDN caches copies of content at locations near users to reduce latency and speed up access.

2. Step 2: Compare options with CDN purpose

   Only To store copies of content closer to users for faster delivery describes storing content near users for faster delivery, which is the main CDN function.

3. Final Answer:

   To store copies of content closer to users for faster delivery -> Option A

4. Quick Check:

   CDN = content near users for speed [OK]

Hint: CDN = cache content near users to speed delivery [OK]

Common Mistakes:

• Thinking CDN replaces main server
• Confusing CDN with encryption service
• Assuming CDN increases database size

2. Which of the following is the correct way to describe how a CDN handles user requests?

easy

A. User requests always go directly to the origin server

B. CDN stores user credentials for authentication

C. CDN modifies user requests to change content

D. User requests are routed to the nearest CDN edge server if content is cached

Solution

1. Step 1: Understand CDN request routing

   CDNs route user requests to the closest edge server that has cached content to reduce latency.

2. Step 2: Evaluate options for request handling

   User requests are routed to the nearest CDN edge server if content is cached correctly states requests go to nearest CDN edge if cached; others are incorrect or unrelated.

3. Final Answer:

   User requests are routed to the nearest CDN edge server if content is cached -> Option D

4. Quick Check:

   CDN routes requests to nearest cache [OK]

Hint: Requests go to nearest cached CDN server, not origin [OK]

Common Mistakes:

• Assuming all requests hit origin server
• Believing CDN changes content on the fly
• Confusing CDN with authentication service

3. Consider a system using a CDN with edge servers in multiple regions. If a user in Europe requests a static image, which server will most likely serve the image?

medium

A. A CDN edge server located in Europe

B. The origin server located in the US

C. A random CDN edge server in Asia

D. The user's local device cache only

Solution

1. Step 1: Identify CDN edge server selection logic

   CDNs serve content from the closest edge server to the user to reduce latency.

2. Step 2: Match user location with server location

   A user in Europe will get content from a European CDN edge server if available, not from origin or distant servers.

3. Final Answer:

   A CDN edge server located in Europe -> Option A

4. Quick Check:

   User location matches nearest CDN edge server [OK]

Hint: User served by nearest CDN edge server geographically [OK]

Common Mistakes:

• Assuming origin server always serves content
• Choosing random or distant CDN servers
• Ignoring CDN edge server caching

4. A developer notices that users are still experiencing slow load times despite using a CDN. Which of the following is the most likely cause?

medium

A. The CDN is encrypting content which slows delivery

B. The CDN is not caching the content properly, causing requests to go to the origin server

C. The CDN is blocking user requests randomly

D. The CDN is storing user passwords insecurely

Solution

1. Step 1: Analyze slow load cause with CDN

   If CDN cache misses occur, requests go to origin server, increasing latency and slowing load times.

2. Step 2: Evaluate options for likely cause

   The CDN is not caching the content properly, causing requests to go to the origin server explains cache miss causing slow loads; others are unrelated or incorrect.

3. Final Answer:

   The CDN is not caching the content properly, causing requests to go to the origin server -> Option B

4. Quick Check:

   Cache miss = slow load [OK]

Hint: Slow CDN? Check if cache is missing content [OK]

Common Mistakes:

• Blaming encryption for speed issues
• Assuming CDN blocks requests randomly
• Confusing CDN with security storage

5. You are designing a global video streaming service. Which CDN usage strategy best improves performance and reduces origin server load?

hard

A. Disable CDN caching to ensure fresh content always

B. Cache only small static files like logos, serve videos directly from origin

C. Cache all static content including videos on CDN edge servers worldwide

D. Use CDN only for user authentication requests

Solution

1. Step 1: Identify content types for CDN caching

   Videos are large static files that benefit greatly from CDN caching near users to reduce latency and origin load.

2. Step 2: Evaluate CDN strategies for global streaming

   Caching all static content including videos on CDN edges worldwide improves performance and reduces origin server load.

3. Final Answer:

   Cache all static content including videos on CDN edge servers worldwide -> Option C

4. Quick Check:

   Cache large static files globally for best performance [OK]

Hint: Cache all static content globally on CDN edges [OK]

Common Mistakes:

• Caching only small files and ignoring videos
• Disabling CDN caching unnecessarily
• Using CDN for unrelated tasks like authentication