Media storage and CDN in HLD - System Design Guide

Problem Statement

When a website or app serves media files like images or videos directly from a single storage server, users far from that server experience slow loading times. High traffic spikes can overload the storage server, causing delays or failures in delivering media content.

Solution

Media files are stored in a scalable storage system optimized for large files. A Content Delivery Network (CDN) caches copies of these files in servers distributed worldwide. When a user requests media, the CDN serves it from the nearest cache, reducing latency and load on the main storage.

Architecture

User 1

(Location A)

→CDN Edge Node

↓

User 2

(Location B)

→CDN Edge Node

This diagram shows users from different locations requesting media files. The CDN edge nodes cache media close to users, reducing direct requests to the central media storage.

Trade-offs

✓ Pros

→

Reduces latency by serving media from geographically closer servers.

→

Decreases load on the origin media storage, improving scalability.

→

Improves availability by serving cached content even if origin is temporarily unreachable.

✗ Cons

→

Cache invalidation can be complex, leading to stale content if not managed properly.

→

Initial cache misses cause requests to hit the origin, which may cause latency spikes.

→

Costs increase due to CDN usage and data transfer fees.

When your system serves large media files to a global user base with high read traffic exceeding thousands of requests per second.

When your user base is small and localized, or media files are rarely accessed, making CDN overhead unjustified.

Real World Examples

Netflix

Uses CDN to cache and deliver video streams close to users worldwide, reducing buffering and improving playback quality.

Amazon

Delivers product images and videos via CDN to ensure fast page loads globally during high traffic events like Prime Day.

Spotify

Caches album art and audio files on CDN edge servers to reduce latency and server load during peak usage.

Alternatives

Direct Origin Serving

All media requests go directly to the central storage without caching or distribution.

Use when: When traffic is low and user locations are concentrated near the storage server.

Peer-to-Peer (P2P) Distribution

Users share media files directly with each other, reducing load on central servers.

Use when: When building decentralized systems or when bandwidth costs must be minimized.

Summary

Serving media directly from a single server causes slow load times and overloads under high traffic.

CDNs cache media files on servers near users to reduce latency and origin load.

Proper cache management and origin scalability remain essential for reliable media delivery.

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