CDN caching for static content in HLD - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

How does a CDN improve performance for static content?

Imagine you have a website serving images and CSS files globally. How does using a CDN help improve the loading speed for users?

A By requiring users to download all content from the origin server directly.

B By compressing files on the origin server before sending to users.

C By dynamically generating static content on each user request.

D By storing copies of static files closer to users in multiple locations worldwide.

💡 Hint

Think about how distance affects loading times and where files are stored.

A CDN caches static files in servers near users, reducing latency and speeding up content delivery.

❓ Architecture

intermediate

Which component is responsible for cache invalidation in a CDN setup?

In a CDN architecture for static content, which part typically handles cache invalidation when content updates?

A CDN edge servers automatically detect changes without any signals.

B The origin server sends cache purge requests to CDN edge servers.

C Users manually clear their browser cache to update content.

D DNS servers update cache entries for static files.

💡 Hint

Consider who controls the original content and how updates propagate.

The origin server notifies CDN edge servers to remove or refresh cached content when updates happen.

❓ scaling

advanced

What is the main benefit of using a CDN for scaling static content delivery during traffic spikes?

Your website experiences sudden high traffic. How does a CDN help handle this increased load for static files?

A By increasing the origin server's CPU and memory automatically.

B By storing static content only on the origin server to avoid duplication.

C By distributing requests across many edge servers, reducing load on the origin server.

D By blocking excess user requests to limit traffic.

💡 Hint

Think about how spreading work helps handle more users.

CDNs spread user requests to many servers worldwide, preventing overload on the origin server during spikes.

❓ tradeoff

advanced

What is a tradeoff when setting a very long cache expiration time for static content in a CDN?

Setting a long cache expiration time improves performance but what is a potential downside?

A Users may see outdated content if the origin updates but CDN cache is not refreshed.

B The CDN will consume more bandwidth from the origin server.

C The origin server will have to handle more requests directly.

D Static content will load slower for users globally.

💡 Hint

Think about content freshness versus speed.

Long cache times mean cached content stays longer, so updates may not appear immediately to users.

❓ estimation

expert

Estimate the bandwidth savings when using a CDN caching 90% of static content requests for a website serving 1TB/month of static files.

Your website serves 1TB of static files monthly. If the CDN cache hit ratio is 90%, how much bandwidth is saved on the origin server?

A 900 GB saved, only 100 GB served by origin.

B 100 GB saved, 900 GB served by origin.

C 1 TB saved, origin serves 0 GB.

D No bandwidth saved, origin serves full 1 TB.

💡 Hint

Calculate based on percentage of requests served by CDN cache.

90% cache hit means 90% of data served by CDN edges, saving that bandwidth on origin.

Practice

1. What is the main purpose of using a CDN cache for static content?

easy

A. To delete static files after use

B. To store static files closer to users for faster access

C. To encrypt all static content

D. To increase the size of static files

Solution

1. Step 1: Understand CDN caching basics

   A CDN cache stores copies of static files like images and scripts near users to reduce loading time.

2. Step 2: Identify the main benefit

   By storing files closer, users get faster access and the main server has less load.

3. Final Answer:

   To store static files closer to users for faster access -> Option B

4. Quick Check:

   CDN caching = faster static content delivery [OK]

Hint: CDN caches static files near users to speed up loading [OK]

Common Mistakes:

• Thinking CDN increases file size
• Confusing caching with encryption
• Assuming CDN deletes files after use

2. Which HTTP header is commonly used to control CDN caching behavior for static content?

easy

A. Cache-Control

B. Content-Type

C. Authorization

D. User-Agent

Solution

1. Step 1: Identify headers related to caching

   Cache-Control header tells browsers and CDNs how long to keep cached content.

2. Step 2: Exclude unrelated headers

   Content-Type defines file type, Authorization is for access, User-Agent identifies client.

3. Final Answer:

   Cache-Control -> Option A

4. Quick Check:

   Cache-Control header manages caching [OK]

Hint: Cache-Control header sets caching rules for CDNs [OK]

Common Mistakes:

• Confusing Content-Type with caching
• Using Authorization header for caching
• Thinking User-Agent controls cache

3. Given the following Cache-Control header: Cache-Control: max-age=3600, public, what does it mean for CDN caching?

medium

A. Cache content publicly for 3600 seconds

B. Do not cache content at all

C. Cache content privately for 3600 seconds

D. Cache content indefinitely

Solution

1. Step 1: Interpret max-age directive

   max-age=3600 means cache content for 3600 seconds (1 hour).

2. Step 2: Understand public directive

   public means content can be cached by any cache, including CDNs and browsers.

3. Final Answer:

   Cache content publicly for 3600 seconds -> Option A

4. Quick Check:

   max-age=3600 + public = public cache for 1 hour [OK]

Hint: max-age sets cache time; public allows shared caching [OK]

Common Mistakes:

• Confusing public with private caching
• Assuming max-age disables caching
• Thinking max-age means indefinite caching

4. A CDN is serving outdated static content even after you updated the files on your origin server. What is the most likely cause?

medium

A. The origin server is down

B. The user's browser cache is disabled

C. The static files were deleted from the origin

D. The CDN cache was not invalidated or purged

Solution

1. Step 1: Understand CDN cache behavior

   CDNs keep cached copies until they expire or are explicitly purged.

2. Step 2: Identify cause of stale content

   If files updated on origin but CDN still serves old files, cache invalidation likely missing.

3. Final Answer:

   The CDN cache was not invalidated or purged -> Option D

4. Quick Check:

   Stale CDN content = missing cache purge [OK]

Hint: Stale CDN content? Check cache invalidation first [OK]

Common Mistakes:

• Assuming origin server down causes stale CDN cache
• Thinking deleted origin files cause stale CDN content
• Blaming user browser cache for CDN stale content

5. You want to ensure users always get the latest version of your static files after you update them, but still benefit from CDN caching. Which approach is best?

hard

A. Disable CDN caching and serve files directly from origin

B. Set Cache-Control to no-cache for all static files

C. Use versioned URLs for static files and set long cache expiration

D. Use short cache expiration without versioning

Solution

1. Step 1: Understand versioned URLs

   Versioned URLs (e.g., style.v2.css) change file names on update, forcing CDN to fetch new files.

2. Step 2: Combine with long cache expiration

   Long cache expiration lets CDN cache files efficiently without frequent revalidation.

3. Step 3: Avoid disabling caching or short expiration

   Disabling caching or short expiration hurts performance and increases load.

4. Final Answer:

   Use versioned URLs for static files and set long cache expiration -> Option C

5. Quick Check:

   Versioned URLs + long cache = fresh and fast [OK]

Hint: Version URLs + long cache = fresh content with speed [OK]

Common Mistakes:

• Disabling CDN caching unnecessarily
• Using no-cache header causing slow loads
• Relying on short expiration without versioning