Media storage and CDN in HLD - Scalability & System Analysis

Scalability Analysis - Media storage and CDN

Growth Table: Media Storage and CDN Scaling

Users	Media Storage	CDN Usage	Network Traffic	Latency
100 users	Single storage server, local disk	Minimal or no CDN	Low, direct fetch from storage	Low latency, direct access
10,000 users	Distributed storage cluster, object storage	Basic CDN with few edge nodes	Moderate, some caching	Improved latency via CDN edges
1,000,000 users	Highly scalable object storage (S3-like), multi-region	Global CDN with many edge locations	High, CDN offloads origin	Low latency globally
100,000,000 users	Multi-cloud, geo-redundant storage, sharded data	Advanced CDN with dynamic content optimization	Very high, optimized delivery	Consistent low latency worldwide

First Bottleneck

At small scale, the media storage server disk I/O and network bandwidth limit throughput.

At medium scale (~10K users), the origin storage bandwidth and read latency become bottlenecks.

At large scale (1M+ users), the CDN edge cache capacity and cache miss rate impact performance.

Without CDN, origin servers get overwhelmed by traffic spikes.

Scaling Solutions

• Horizontal scaling: Add more storage nodes and CDN edge servers to distribute load.
• Caching: Use CDN to cache media close to users, reducing origin load.
• Sharding: Partition media storage by region or content type to improve access speed.
• Multi-region replication: Store copies of media in multiple geographic locations.
• Compression and optimization: Reduce media size for faster delivery.
• Load balancing: Distribute requests evenly across storage and CDN nodes.

Back-of-Envelope Cost Analysis

Assuming 1M users, each streaming 1 video per day of 5 MB:

• Requests per second (QPS): ~12 (1M users * 1 request / 86400 seconds)
• Daily data transfer: 5 TB (1M * 5 MB)
• Bandwidth needed at origin: Reduced by CDN cache hit ratio (e.g., 90% cache hit reduces origin bandwidth to 0.5 TB)
• Storage needed: Depends on retention, e.g., 30 days = 150 TB
• Network bandwidth: CDN edges handle most traffic, origin bandwidth is bottleneck without caching

Interview Tip

Start by defining user scale and media size.

Identify origin storage limits and CDN role early.

Discuss caching strategies and geographic distribution.

Explain how to handle cache misses and data consistency.

Always mention cost and latency trade-offs.

Self Check Question

Your media storage origin handles 1000 QPS. Traffic grows 10x. What do you do first?

Answer: Deploy or increase CDN edge caching to offload origin servers and reduce direct requests, preventing origin overload.

Key Result

Media storage scales by adding distributed storage and using CDN caching to offload origin servers; the first bottleneck is origin bandwidth and disk I/O, solved by CDN and horizontal scaling.

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