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What is CDN (content delivery network)?

A system that keeps copies of content such as images, video and scripts on cache servers (edge servers) placed in many locations and delivers it from a server close to the user. The shorter RTT makes pages load faster, the load on the original server (the origin) drops, and spreading traffic across servers also helps absorb DDoS attacks. Users are directed to a nearby server using DNS or anycast.

Japanese name: CDN(コンテンツデリバリネットワーク)

CDN (content delivery network) appears in 6 practice questions across 4 topics, including Web Technology Basics, System Architecture and Reliability, Cloud and Container Basics.

Field: Web Technology Basics

Try it: questions about CDN (content delivery network)

Which describes a CDN (content delivery network)?

  1. AA mechanism that gathers all users' requests to a single server to keep the delivered content consistent
  2. BA mechanism that encrypts communication to create a virtual private line between distant sites
  3. CA mechanism that places copies (caches) of content on many servers located near users and directs users to a nearby server for delivery
Show answer and explanation

Answer: C. A mechanism that places copies (caches) of content on many servers located near users and directs users to a nearby server for delivery

A CDN keeps copies of content such as images, videos and software update files on delivery servers placed in many locations, and directs users to a nearby server for delivery. Because the distance to users is shorter, responses are faster, and the load and traffic on the original server (origin server) are reduced. When access temporarily surges, the load can also be spread across the servers in many locations. Encrypting communication to create a virtual private line between sites is a VPN.

A CDN receives 2,000 requests per second, and its cache hit ratio rises from 90% to 98%. The response time is 20 ms on a hit, and 200 ms on a miss because the content is fetched from the origin server and returned; only requests that miss reach the origin server. Which of the following is the combination of the number of requests reaching the origin server and the average response time after the ratio reaches 98%?

  1. A40 per second, about 24 ms
  2. B40 per second, about 110 ms
  3. C160 per second, about 24 ms
Show answer and explanation

Answer: A. 40 per second, about 24 ms

The requests that miss are 100 − 98 = 2% of the total, so 2,000 × 0.02 = 40 per second reach the origin server. At 90% it was 200 per second, so a rise of just 8 points in the hit ratio cuts the origin server's load to one fifth. The average response time is 0.98 × 20 + 0.02 × 200 = 19.6 + 4 = 23.6 ms, about 24 ms. This is also much shorter than the 0.9 × 20 + 0.1 × 200 = 38 ms at 90%.

CDN (content delivery network) in the explanations

Questions that mention CDN (content delivery network) (6)

  1. Which describes a CDN (content delivery network)?
  2. A CDN receives 2,000 requests per second, and its cache hit ratio rises from 90% to 98%. The response time is 20 ms on a hit, and 200 ms on a miss because the content is fetched from the origin server and returned; only requests that miss reach the origin server. Which of the following is the combination of the number of requests reaching the origin server and the average response time after the ratio reaches 98%?
  3. Which of the following generally applies to how many public clouds charge for data transfer?
  4. The response for a "my page" whose content differs for each user is given Cache-Control: private, max-age=60. Which of the following correctly describes the effect of this setting?
  5. Among the ways of producing the pages of a website, which of the following generates the HTML files of all pages in advance at the build stage and returns those files as they are in response to requests?
  6. Which statement correctly describes a DDoS attack?

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Related terms

VPN (virtual private network) Availability zone Server-side rendering (SSR) Router IP address Cookie (HTTP cookie) CORS (cross-origin resource sharing) DOM (Document Object Model) Event loop HTTP (Hypertext Transfer Protocol)

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