CDN caching for static content in HLD - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the main purpose of a CDN in caching static content?

A CDN (Content Delivery Network) caches static content close to users to reduce latency and server load, improving load times and user experience.

intermediate

Explain the concept of cache invalidation in CDN caching.

Cache invalidation is the process of updating or removing cached content in the CDN when the original content changes, ensuring users get fresh data.

beginner

What role do TTL (Time To Live) values play in CDN caching?

TTL defines how long cached content stays in the CDN before it expires and must be refreshed from the origin server.

intermediate

Why is it important to separate static and dynamic content in CDN caching?

Static content is cacheable and served quickly from CDN, while dynamic content changes often and usually bypasses CDN to ensure accuracy.

intermediate

How does CDN caching improve scalability of a web application?

By offloading static content delivery to distributed CDN servers, the origin server handles fewer requests, allowing the system to serve more users efficiently.

What type of content is best suited for CDN caching?

A Database queries

B Static content like images and CSS files

C Real-time chat messages

D User-specific dynamic pages

Static content does not change often and can be cached to improve load times.

What happens when cached content's TTL expires in a CDN?

A Content is refreshed from the origin server

B Content is deleted permanently

C Content is served from cache indefinitely

D CDN shuts down

After TTL expires, CDN fetches fresh content from the origin to keep data up to date.

Which method helps ensure users get updated content after a change?

A Cache invalidation

B Increasing TTL

C Ignoring cache

D Using only dynamic content

Cache invalidation updates or removes stale cached content.

How does CDN caching reduce load on the origin server?

A By running database queries

B By blocking user requests

C By storing user data

D By serving cached static content from edge servers

CDN edge servers serve cached content, reducing requests to the origin.

Which of these is NOT a benefit of CDN caching?

A Lower latency for users

B Improved server scalability

C Automatic content creation

D Reduced bandwidth costs

CDNs cache existing content; they do not create content.

Describe how CDN caching works for static content and why it improves user experience.

Think about how content travels from server to user.

Explain the importance of cache invalidation and TTL in maintaining content freshness in CDN caching.

Consider what happens when content changes.

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