CDN concept and usage in HLD - Scalability & System Analysis

Scalability Analysis - CDN concept and usage

Growth Table: CDN Usage at Different Scales

Users	Traffic Pattern	Latency	Origin Server Load	Cache Hit Rate	Bandwidth Usage
100 users	Low, local requests	Low, direct origin access	Low	Minimal caching needed	Low
10,000 users	Moderate, regional spread	Improved with CDN edge nodes	Moderate	Cache hit ~30-50%	Moderate
1,000,000 users	High, global distribution	Significantly reduced latency	High without CDN, reduced with CDN	Cache hit ~70-90%	High, but origin bandwidth offloaded
100,000,000 users	Very high, global and peak loads	Low latency worldwide	Origin server offloaded heavily	Cache hit >90%	Very high, mostly edge bandwidth

First Bottleneck: Origin Server Bandwidth and Latency

As user count grows beyond 10,000, the origin server bandwidth and response time become the first bottleneck. Without a CDN, the origin must serve all requests, causing high latency and potential overload. Network congestion and geographic distance increase delays for users far from the origin.

Scaling Solutions Using CDN

• Edge Caching: Store static content (images, scripts, videos) on CDN edge servers close to users to reduce origin load and latency.
• Geographic Distribution: Use multiple CDN PoPs (Points of Presence) worldwide to serve users locally.
• Cache Invalidation: Implement smart cache refresh strategies to keep content fresh without overloading origin.
• Load Balancing: Distribute requests among CDN nodes and origin servers to avoid hotspots.
• Compression and Optimization: Use gzip, Brotli, and image optimization at CDN edges to reduce bandwidth.
• SSL Termination: Offload SSL/TLS processing to CDN to reduce origin CPU usage.

Back-of-Envelope Cost Analysis

Assuming 1 million users each making 1 request per second:

• Requests per second (RPS): 1,000,000
• Origin server without CDN: must handle 1M RPS, likely impossible on a single server.
• With CDN cache hit rate 80%, origin handles 200,000 RPS.
• Bandwidth saved at origin: 80% reduction.
• CDN edge bandwidth: high, but distributed globally.
• Storage at CDN edges: depends on cached content size, typically tens of TBs.
• Network bandwidth: 1 Gbps server handles ~125 MB/s; origin needs multiple servers or CDN to scale.

Interview Tip: Structuring CDN Scalability Discussion

Start by explaining the problem of origin server overload and latency. Then describe how CDN edge caching reduces load and improves user experience. Discuss cache hit rates and geographic distribution. Finally, mention cache invalidation and SSL offloading as optimizations. Use numbers to show impact on requests and bandwidth.

Self Check Question

Your origin server handles 1,000 requests per second. Traffic grows 10x to 10,000 RPS. What do you do first and why?

Answer: Deploy a CDN to cache static content at edge servers. This reduces origin load by serving most requests from cache, lowering bandwidth and latency before scaling origin servers.

Key Result

A CDN reduces origin server load and latency by caching content at edge locations, enabling systems to scale from thousands to hundreds of millions of users efficiently.

Practice

1. What is the primary purpose of a Content Delivery Network (CDN) in system design?

easy

A. To store copies of content closer to users for faster delivery

B. To increase the size of the main server's database

C. To replace the main server completely

D. To encrypt all user data end-to-end

Solution

1. Step 1: Understand CDN's role in content delivery

   A CDN caches copies of content at locations near users to reduce latency and speed up access.

2. Step 2: Compare options with CDN purpose

   Only To store copies of content closer to users for faster delivery describes storing content near users for faster delivery, which is the main CDN function.

3. Final Answer:

   To store copies of content closer to users for faster delivery -> Option A

4. Quick Check:

   CDN = content near users for speed [OK]

Hint: CDN = cache content near users to speed delivery [OK]

Common Mistakes:

• Thinking CDN replaces main server
• Confusing CDN with encryption service
• Assuming CDN increases database size

2. Which of the following is the correct way to describe how a CDN handles user requests?

easy

A. User requests always go directly to the origin server

B. CDN stores user credentials for authentication

C. CDN modifies user requests to change content

D. User requests are routed to the nearest CDN edge server if content is cached

Solution

1. Step 1: Understand CDN request routing

   CDNs route user requests to the closest edge server that has cached content to reduce latency.

2. Step 2: Evaluate options for request handling

   User requests are routed to the nearest CDN edge server if content is cached correctly states requests go to nearest CDN edge if cached; others are incorrect or unrelated.

3. Final Answer:

   User requests are routed to the nearest CDN edge server if content is cached -> Option D

4. Quick Check:

   CDN routes requests to nearest cache [OK]

Hint: Requests go to nearest cached CDN server, not origin [OK]

Common Mistakes:

• Assuming all requests hit origin server
• Believing CDN changes content on the fly
• Confusing CDN with authentication service

3. Consider a system using a CDN with edge servers in multiple regions. If a user in Europe requests a static image, which server will most likely serve the image?

medium

A. A CDN edge server located in Europe

B. The origin server located in the US

C. A random CDN edge server in Asia

D. The user's local device cache only

Solution

1. Step 1: Identify CDN edge server selection logic

   CDNs serve content from the closest edge server to the user to reduce latency.

2. Step 2: Match user location with server location

   A user in Europe will get content from a European CDN edge server if available, not from origin or distant servers.

3. Final Answer:

   A CDN edge server located in Europe -> Option A

4. Quick Check:

   User location matches nearest CDN edge server [OK]

Hint: User served by nearest CDN edge server geographically [OK]

Common Mistakes:

• Assuming origin server always serves content
• Choosing random or distant CDN servers
• Ignoring CDN edge server caching

4. A developer notices that users are still experiencing slow load times despite using a CDN. Which of the following is the most likely cause?

medium

A. The CDN is encrypting content which slows delivery

B. The CDN is not caching the content properly, causing requests to go to the origin server

C. The CDN is blocking user requests randomly

D. The CDN is storing user passwords insecurely

Solution

1. Step 1: Analyze slow load cause with CDN

   If CDN cache misses occur, requests go to origin server, increasing latency and slowing load times.

2. Step 2: Evaluate options for likely cause

   The CDN is not caching the content properly, causing requests to go to the origin server explains cache miss causing slow loads; others are unrelated or incorrect.

3. Final Answer:

   The CDN is not caching the content properly, causing requests to go to the origin server -> Option B

4. Quick Check:

   Cache miss = slow load [OK]

Hint: Slow CDN? Check if cache is missing content [OK]

Common Mistakes:

• Blaming encryption for speed issues
• Assuming CDN blocks requests randomly
• Confusing CDN with security storage

5. You are designing a global video streaming service. Which CDN usage strategy best improves performance and reduces origin server load?

hard

A. Disable CDN caching to ensure fresh content always

B. Cache only small static files like logos, serve videos directly from origin

C. Cache all static content including videos on CDN edge servers worldwide

D. Use CDN only for user authentication requests

Solution

1. Step 1: Identify content types for CDN caching

   Videos are large static files that benefit greatly from CDN caching near users to reduce latency and origin load.

2. Step 2: Evaluate CDN strategies for global streaming

   Caching all static content including videos on CDN edges worldwide improves performance and reduces origin server load.

3. Final Answer:

   Cache all static content including videos on CDN edge servers worldwide -> Option C

4. Quick Check:

   Cache large static files globally for best performance [OK]

Hint: Cache all static content globally on CDN edges [OK]

Common Mistakes:

• Caching only small files and ignoring videos
• Disabling CDN caching unnecessarily
• Using CDN for unrelated tasks like authentication