Domain Name System and Content Delivery Network (CDN) PDF

09 Domain Name System, Content Delivery Network (GitHub System Design Primer) Domain name system *[ Source: DNS security presentation ]()* A Domain Name System (DNS) ** == == ** translates a ** domain name ** such as www.example.com...

09 Domain Name System, Content Delivery Network (GitHub System Design Primer) Domain name system *[ Source: DNS security presentation ]()* A Domain Name System (DNS) ** == == ** translates a ** domain name ** such as www.example.com [ ]() to an ** IP address. ** DNS is ** hierarchical, with a few ** ** authoritative servers at the ** ** top level. ** Your ** router or ** ** ISP ** provides information about which ** DNS server(s) ** to contact when doing a ** lookup. ** ** Lower level DNS servers ** ** cache mappings, ** which could become ** stale due to ** ** DNS propagation delays. ** ** DNS results ** can also be cached by your ** browser or ** ** OS ** for a certain period of time, determined by the [** time to live (TTL). **]() ** == NS record (name server) - Specifies the == ** ** DNS servers for your ** ** domain/subdomain. ** ** == MX record (mail exchange) - Specifies the == ** ** mail servers for accepting messages. ** ** == A record (address) - Points a name to an IP address. == ** ** == CNAME (canonical) - Points a name to another name or CNAME (example.com to == ** ` ` [ www.example.com) or to an A record. ]() ` ` ⠀ Services such as CloudFlare and Route 53 provide managed DNS services. Some DNS [ ]() [ ]() services can route traffic through various methods: [ Weighted round robin ]() Prevent traffic from going to servers under maintenance Balance between varying cluster sizes A/B testing [ Latency-based ]() [ Geolocation-based ]() ⠀ Disadvantage(s): DNS Accessing a DNS server introduces a slight delay, although mitigated by caching described above. DNS server management could be complex and is generally managed by governments, [ ISPs, and large companies. ]() DNS services have recently come under DDoS attack, preventing users from accessing [ ]() websites such as Twitter without knowing Twitter's IP address(es). ⠀ Source(s) and further reading [ DNS architecture ]() [ Wikipedia ]() [ DNS articles ]() ⠀ ** Content Delivery Network ** *[ Source: Why use a CDN ]()* A ** content delivery network (CDN) ** is a ** globally distributed network of proxy servers, serving content ** ** ** from ** locations closer to the user. ** Generally, ** static files such as ** ** HTML/CSS/JS, ** ** photos, and ** ** videos ** are served from CDN, although some CDNs such as ** Amazon's CloudFront ** support ** dynamic content. ** The site's ** DNS resolution ** will tell clients which ** (CDN Edge) server ** to contact. Serving content from CDNs can significantly improve performance in two ways: Users receive content from ** data centers ** ** close to them ** Your servers do not have to serve requests that the ~~ ~~ CDN fulfills ⠀ ** Push CDNs ** * Push CDNs receive * ** new content whenever ** changes occur on ** your server. ** You take ** full responsibility for providing content, ** ** uploading directly to the ** CDN and ** rewriting URLs ** to point to the CDN. You can ** configure when content expires and ** ** when it is updated. ** Content is uploaded only when it is ** new or ** ** changed, ** ** minimizing traffic, ** but ** maximizing storage. ** Sites with a ** small amount of traffic or sites with ** ** content that isn't often updated ** work well with push CDNs. Content is ** placed on the CDNs once, ** instead of being ~~ re-pulled at regular intervals. ~~ ** Pull CDNs ** Pull CDNs grab ** new content from your server when the ** ** first user requests the content. ** You ** leave the content on your server and ** ** rewrite URLs ** to point to the CDN. This results in a ** slower request until the content is ** * * ** cached on the CDN. ** Disadvantage(s): CDN CDN ** costs could be significant ** * depending on * ** traffic, ** * although this should be * ** weighed with additional costs ** you would incur ** not using a CDN. ** Content might be ** stale if it is updated before the TTL expires it. ** CDNs  ** require changing URLs ** for static content to point to the CDN. ⠀ Source(s) and further reading [ Globally distributed content delivery ]() [ The differences between push and pull CDNs ]() [ Wikipedia ]()