Content delivery with CDN in HLD - Scalability & System Analysis

Scalability Analysis - Content delivery with CDN

Growth Table: Content Delivery with CDN

Scale	Users	Traffic	Origin Server Load	CDN Cache Hit Rate	Network Bandwidth	Latency
Small	100 users	Low (few requests/sec)	Low	Moderate	Low	Low
Medium	10,000 users	Moderate (hundreds req/sec)	Moderate	High (70-80%)	Moderate	Low
Large	1,000,000 users	High (tens of thousands req/sec)	High without CDN	Very High (90%+)	High	Very Low (due to edge caching)
Very Large	100,000,000 users	Very High (millions req/sec)	Origin overwhelmed without CDN	Critical to be >90%	Very High (multi Tbps)	Very Low (edge nodes worldwide)

First Bottleneck

The origin server becomes the first bottleneck as user traffic grows beyond thousands of requests per second. Without a CDN, the origin must serve all content, causing high CPU, memory, and bandwidth usage. This leads to slow responses and potential downtime.

Scaling Solutions

• Use CDN Edge Caching: Cache static content at edge servers close to users to reduce origin load and latency.
• Cache Invalidation: Implement smart cache invalidation to keep content fresh without overloading origin.
• Load Balancing: Distribute origin requests across multiple servers to handle higher load.
• Origin Scaling: Vertically scale origin servers (better CPU, RAM) or horizontally scale with multiple origins.
• Compression & Optimization: Compress content and optimize delivery to reduce bandwidth.
• Multi-CDN Strategy: Use multiple CDN providers for redundancy and global coverage at very large scale.

Back-of-Envelope Cost Analysis

• At 1M users, assuming 1 request per user per minute = ~16,667 requests/sec.
• Origin server can handle ~5,000 req/sec; CDN must serve ~11,667 req/sec to reduce origin load.
• Storage: If average content size is 500 KB, bandwidth = 500 KB * 16,667 req/sec = ~8.33 GB/sec (~66.6 Gbps).
• Network bandwidth at origin must support peak misses; CDN edges handle majority of traffic.
• Cost savings come from reduced origin bandwidth and server load due to CDN caching.

Interview Tip

Start by explaining the role of the CDN in reducing origin load and latency. Discuss how traffic grows and which components become bottlenecks. Then, describe caching strategies and scaling methods. Use numbers to justify your choices and show understanding of trade-offs.

Self Check Question

Your origin server handles 1,000 requests per second. Traffic grows 10x to 10,000 requests per second. What is your first action and why?

Key Result

A CDN reduces origin server load by caching content at edge locations, enabling the system to scale from thousands to millions of users by offloading traffic and lowering latency.

Practice

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

easy

A. To store user data permanently

B. To cache content closer to users for faster delivery

C. To replace the origin server completely

D. To encrypt all website content

Solution

1. Step 1: Understand CDN function

   A CDN stores copies of content in multiple locations worldwide to reduce latency.

2. Step 2: Identify main benefit

   This caching near users speeds up content delivery and reduces load on the origin server.

3. Final Answer:

   To cache content closer to users for faster delivery -> Option B

4. Quick Check:

   CDN = caching near users [OK]

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

Common Mistakes:

• Thinking CDN replaces origin server
• Confusing CDN with data storage
• Assuming CDN encrypts all content

2. Which of the following is the correct sequence when a user requests content served via a CDN?

easy

A. User -> Origin Server -> CDN -> User

B. Origin Server -> User -> CDN

C. User -> CDN -> Origin Server (if needed) -> CDN -> User

D. User -> CDN -> User -> Origin Server

Solution

1. Step 1: Trace request flow in CDN

   User first contacts the CDN edge server; if content is cached, it responds immediately.

2. Step 2: Understand cache miss handling

   If content is not cached, CDN fetches it from the origin server, caches it, then serves user.

3. Final Answer:

   User -> CDN -> Origin Server (if needed) -> CDN -> User -> Option C

4. Quick Check:

   Request flow = CDN first, origin fallback [OK]

Hint: User hits CDN first, origin only if cache miss [OK]

Common Mistakes:

• Assuming user contacts origin server first
• Ignoring CDN cache miss flow
• Confusing response direction

3. Consider this simplified code snippet representing a CDN cache check:

cache = {"index.html": "content"}
request = "style.css"

if request in cache:
    response = cache[request]
else:
    response = "fetch from origin"
print(response)

What will be printed?

medium

A. fetch from origin

B. KeyError

C. content

D. None

Solution

1. Step 1: Check if request key exists in cache

   The request is "style.css" which is not in cache keys (only "index.html" exists).

2. Step 2: Determine else branch execution

   Since request not found, else branch runs, setting response to "fetch from origin".

3. Final Answer:

   fetch from origin -> Option A

4. Quick Check:

   Cache miss prints fetch from origin [OK]

Hint: If key missing in cache, else branch runs [OK]

Common Mistakes:

• Assuming cache returns default value automatically
• Expecting KeyError without else
• Confusing cache keys with values

4. A CDN is configured but users report slow content loading. Which of these is the most likely cause?

medium

A. The CDN is caching all content correctly

B. Users have slow internet unrelated to CDN

C. The origin server is down but CDN cache is fresh

D. The CDN edge servers are geographically far from users

Solution

1. Step 1: Analyze CDN slow delivery causes

   If CDN edge servers are far from users, latency increases, causing slow loading.

2. Step 2: Evaluate other options

   The CDN is caching all content correctly means caching works, so unlikely cause. The origin server is down but CDN cache is fresh means cache serves content fast. Users have slow internet unrelated to CDN is user-side, not CDN issue.

3. Final Answer:

   The CDN edge servers are geographically far from users -> Option D

4. Quick Check:

   Distance to CDN edge affects speed [OK]

Hint: Far CDN edges cause slow delivery [OK]

Common Mistakes:

• Blaming origin server when cache is fresh
• Ignoring CDN server location
• Assuming caching always fixes speed

5. You want to design a CDN system that handles sudden traffic spikes without overloading the origin server. Which combination of strategies is best?

hard

A. Use aggressive caching with long TTL and multiple CDN edge locations

B. Disable caching and let all requests go to origin server

C. Use a single CDN edge server with short TTL caching

D. Serve content only from origin server with load balancer

Solution

1. Step 1: Understand traffic spike handling

   Long TTL caching reduces origin hits; multiple edges distribute load geographically.

2. Step 2: Evaluate options for scalability

   Disable caching and let all requests go to origin server and D overload origin server. Use a single CDN edge server with short TTL caching limits caching benefits and single edge risks bottleneck.

3. Final Answer:

   Use aggressive caching with long TTL and multiple CDN edge locations -> Option A

4. Quick Check:

   Long TTL + many edges = scalable CDN [OK]

Hint: Long cache + many edges handle spikes best [OK]

Common Mistakes:

• Disabling caching under traffic spikes
• Relying on single edge server
• Ignoring TTL impact on origin load