CDN concept and usage in HLD - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

How does a CDN improve website load times?

Imagine you have a website hosted in one country, but visitors come from all over the world. How does a CDN help make the website load faster for these visitors?

A By blocking visitors from faraway locations to reduce server load.

B By compressing the website content only at the origin server before sending it to visitors.

C By increasing the bandwidth of the origin server to handle more visitors at once.

D By storing copies of website content in servers closer to visitors, reducing the distance data travels.

💡 Hint

Think about how distance affects delivery speed of packages in real life.

A CDN stores copies of content in many locations worldwide. This means visitors get data from a nearby server, making loading faster. Other options do not reduce distance or improve delivery speed globally.

❓ Architecture

intermediate

Which component is NOT part of a typical CDN architecture?

Consider the main parts of a CDN system. Which of these is usually NOT included in the CDN architecture?

A User devices that store permanent copies of website data.

B Edge servers that cache content close to users.

C Load balancers to distribute traffic among edge servers.

D Origin servers where the original content is stored.

💡 Hint

Think about which components the CDN controls versus user devices.

CDNs include edge servers, origin servers, and load balancers to manage traffic and caching. User devices are outside the CDN architecture and do not store permanent website copies as part of the CDN.

❓ scaling

advanced

How does a CDN handle sudden traffic spikes to a website?

A popular website suddenly gets a huge number of visitors. How does the CDN help the website handle this spike without crashing?

A By sending all traffic directly to the origin server to keep edge servers free.

B By shutting down some edge servers to reduce the number of requests processed.

C By distributing the traffic load across many edge servers worldwide, preventing overload on the origin server.

D By limiting the number of visitors allowed to access the website at once.

💡 Hint

Think about how spreading work among many helpers can prevent one from getting overwhelmed.

CDNs spread incoming requests across many edge servers, so no single server or the origin is overwhelmed. This helps the website stay available during traffic spikes. Other options reduce availability or increase load on origin.

❓ tradeoff

advanced

What is a tradeoff when using a CDN for dynamic content?

CDNs work well for static content like images. What is a common tradeoff when using a CDN to deliver dynamic content that changes often?

A Dynamic content may be delayed or outdated because caching can cause stale data to be served.

B Using a CDN for dynamic content eliminates the need for an origin server.

C Dynamic content is always faster because CDNs do not cache it at all.

D Dynamic content cannot be encrypted when delivered through a CDN.

💡 Hint

Think about how caching works and what happens if content changes frequently.

CDNs cache content to speed delivery. For dynamic content that changes often, caching can cause users to see old data until the cache updates. Other options are incorrect because CDNs can cache dynamic content with care, origin servers are still needed, and encryption is supported.

❓ estimation

expert

Estimate bandwidth savings using a CDN for a global website

A website serves 1 TB of data monthly from its origin server without a CDN. After adding a CDN, 80% of requests are served from edge caches. How much bandwidth does the origin server save monthly?

A 1 TB saved, because the CDN handles all traffic now.

B 0.8 TB saved, because 80% of data is served from the CDN edges.

C 0.2 TB saved, because only 20% of data is served from the CDN edges.

D No bandwidth saved, because the origin still sends all data.

💡 Hint

Calculate the percentage of data served by the CDN edges versus the origin.

If 80% of requests are served by CDN edges, the origin only sends 20% of data. So the origin saves 80% of bandwidth, which is 0.8 TB out of 1 TB.

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