Media storage and CDN in HLD - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the primary purpose of a Content Delivery Network (CDN)?

A CDN is designed to deliver media and other content to users quickly by caching copies of the content on servers located close to the users, reducing latency and load on the origin server.

beginner

Why is media storage often separated from application servers in system design?

Separating media storage from application servers helps scale storage independently, improves performance by offloading heavy media data, and allows specialized storage solutions optimized for large files.

intermediate

What is 'cache invalidation' in the context of CDNs?

Cache invalidation is the process of removing or updating cached content on CDN servers when the original media changes, ensuring users receive the latest version.

intermediate

Name two common storage types used for media storage in large-scale systems.

Object storage (like Amazon S3) and block storage are commonly used. Object storage is preferred for its scalability and metadata support, ideal for media files.

intermediate

How does a CDN improve scalability for media-heavy applications?

By distributing media content across many edge servers worldwide, a CDN reduces the load on the origin server and handles many user requests simultaneously, allowing the system to scale efficiently.

What does a CDN primarily reduce for end users?

A Database queries

B Storage cost

C CPU usage

D Latency

A CDN reduces latency by serving content from servers closer to the user.

Which storage type is best suited for storing large media files?

A In-memory cache

B Object storage

C Relational database

D Block storage only

Object storage is optimized for large files and scalability, making it ideal for media.

What happens during cache invalidation in a CDN?

A Cached content is updated or removed

B New content is uploaded to origin server

C User requests are blocked

D Media files are compressed

Cache invalidation updates or removes outdated cached content to keep it fresh.

Why separate media storage from application servers?

A To scale storage independently

B To reduce network bandwidth

C To increase CPU power

D To simplify database queries

Separating storage allows independent scaling and better performance.

Which is NOT a benefit of using a CDN?

A Improved content delivery speed

B Reduced load on origin servers

C Automatic media file editing

D Better handling of traffic spikes

CDNs do not edit media files automatically; they focus on delivery.

Explain how a CDN works to improve media delivery performance.

Think about how content gets closer to the user.

Describe the key considerations when designing media storage for a large-scale system.

Focus on storage type, separation, and integration with CDN.

Practice

1. What is the main purpose of a Content Delivery Network (CDN) in media storage systems?

easy

A. To store original media files permanently

B. To cache media files closer to users for faster access

C. To compress media files before storage

D. To encrypt media files for security

Solution

1. Step 1: Understand CDN role

   A CDN stores copies of media files in servers near users to reduce latency.

2. Step 2: Differentiate storage vs delivery

   Media storage holds original files; CDN speeds up delivery by caching.

3. Final Answer:

   To cache media files closer to users for faster access -> Option B

4. Quick Check:

   CDN = cache near users [OK]

Hint: CDN = cache near users for speed [OK]

Common Mistakes:

• Confusing CDN with permanent storage
• Thinking CDN compresses files
• Assuming CDN encrypts files

2. Which of the following is the correct sequence for serving media using storage and CDN?

easy

A. User requests -> CDN cache -> Origin storage if cache miss

B. User requests -> Origin storage -> CDN cache

C. CDN cache -> User requests -> Origin storage

D. Origin storage -> CDN cache -> User requests

Solution

1. Step 1: Understand request flow

   User first hits CDN cache to get media quickly.

2. Step 2: Handle cache miss

   If CDN cache misses, it fetches from origin storage and caches it.

3. Final Answer:

   User requests -> CDN cache -> Origin storage if cache miss -> Option A

4. Quick Check:

   Request flow = User -> CDN -> Storage [OK]

Hint: Requests hit CDN first, then storage if needed [OK]

Common Mistakes:

• Thinking origin storage serves user directly every time
• Reversing CDN and storage order
• Ignoring cache miss step

3. Consider a system where media files are stored in cloud storage and served via CDN. If the CDN cache TTL (time-to-live) is set to 1 hour, what happens when a media file is updated in storage immediately after a user requests it?

medium

A. User gets the updated file immediately from CDN

B. CDN automatically invalidates cache and fetches new file

C. User gets the old cached file from CDN until TTL expires

D. User request fails until cache refresh

Solution

1. Step 1: Understand CDN TTL effect

   TTL controls how long CDN keeps cached copy before checking origin.

2. Step 2: Effect of update during TTL

   Until TTL expires, CDN serves cached old file despite origin update.

3. Final Answer:

   User gets the old cached file from CDN until TTL expires -> Option C

4. Quick Check:

   Cache TTL = old file served until expiry [OK]

Hint: Cache TTL controls update delay [OK]

Common Mistakes:

• Assuming CDN always fetches fresh file immediately
• Thinking cache invalidates automatically on update
• Believing user request fails on stale cache

4. A developer implemented a media delivery system using CDN and storage but users report slow media loading. Which of the following is the most likely cause?

medium

A. CDN is caching files for too long

B. Media files are too small to benefit from CDN

C. Storage is located in the same region as users

D. CDN cache miss due to incorrect cache headers

Solution

1. Step 1: Analyze slow loading cause

   Slow loading often happens if CDN cache misses and fetches from origin repeatedly.

2. Step 2: Identify cache header role

   Incorrect or missing cache headers prevent CDN from caching files properly.

3. Final Answer:

   CDN cache miss due to incorrect cache headers -> Option D

4. Quick Check:

   Cache headers control CDN caching [OK]

Hint: Check cache headers if CDN misses often [OK]

Common Mistakes:

• Assuming small files don't benefit from CDN
• Thinking storage location alone causes slowness
• Believing long cache time causes slow loading

5. You are designing a global media storage system with CDN for a video streaming service. Which combination best improves scalability and user experience?

hard

A. Store media in a single central region and use CDN with regional edge caches

B. Store media in multiple regions without CDN to reduce latency

C. Use CDN only without origin storage to reduce costs

D. Store media locally on user devices to avoid network delays

Solution

1. Step 1: Consider storage and CDN roles

   Central storage simplifies management; CDN edge caches bring content close to users.

2. Step 2: Evaluate options for scalability and latency

   Multiple regions without CDN adds complexity; CDN without origin lacks source; local storage is impractical.

3. Final Answer:

   Store media in a single central region and use CDN with regional edge caches -> Option A

4. Quick Check:

   Central storage + CDN edges = scalable & fast [OK]

Hint: Central storage + CDN edges = best scalability [OK]

Common Mistakes:

• Ignoring CDN benefits by using multi-region storage only
• Thinking CDN can replace origin storage
• Assuming local user storage is feasible