Above figure shows average response time for www.pageflakes.com from Washington, DC, where the servers are in Dallas, Texas. The average response time is around 0.4 seconds. This response includes server side execution time as well. Generally it takes around 0.3 to 0.35 seconds to execute the page on the server. So, time spent on network is around 0.05 seconds or 50ms. This is a really fast connectivity as there are only 4 to 6 hops to reach Dallas from Washington DC.
This figure shows average response time from Sydney, Australia. The average response time is 1.5 seconds which is significantly higher than Washington DC. It2019s almost 4 times compared to USA. There's almost 1.2 seconds overhead on network only. Moreover, there are around 17 to 23 hops from Sydney to Dallas. So, the site downloads at least 4 times slower in Australia than it is from anywhere in USA.
A content delivery network (CDN) is a system of computers networked together across the Internet. The computers cooperate transparently to deliver content (especially large media content) to end users. CDN nodes (cluster of servers in a specific location) are deployed in multiple locations, often over multiple backbones. These nodes cooperate with each other to serve requests for content by end users. They also transparently move content behind the scenes to optimize the delivery process. CDN serves request by intelligently choosing the nearest server. It looks for the fastest connectivity between your computer to a nearest node that has the content you are looking for.A0 The number of nodes in different countries and the number of redundant backbone connectivity a CDN has measures its strength. Some of the most popular CDNs are Akamai, Limelight, EdgeCast. Akamai is used by large companies like Microsoft, Yahoo, AOL. It2019s comparatively expensive solution. However, Akamai has the best performance throughout the world because they have servers in almost every prominent city in the world. However, Akamai is very expensive and they only accept a customer who can spend minimum 5K on CDN per month. For smaller companies, Edgecast is a more affordable solution.A0
This figure shows CDN Nodes that is closest to the browser intercepts traffic and serves response. If it does not have the response in cache, it fetches it from origin server using a faster route and much more optimized connectivity that the browser2019s ISP can provide. If the content is already cached, then it2019s served directly from the node
There are generally two types of CDN. One is where you upload content to CDN's servers via FTP and you get a subdomain in their domain like dropthings.somecdn.net. You change all the URL of static content throughout your site to download content from the CDN domain instead of relative URL to your own domain. So, a URL like /logo.gif will be renamed to http://dropthings.somecdn.net/logo.gif. This is easy to configure but has maintenance problems. You will have to keep CDN2019s store synchronized with the files all the time. Deployment becomes complicated because you need to update both your website and the CDN store at the same time. Example of such a CDN (which is very cheap) is Cachefly.
A more convenient approach is to store static content on your own site but use domain aliasing. You can store your content in a subdomain that points to your own domain like static.dropthings.com. Then you use CNAME to map that subdomain to a CDN2019s nameserver like cache.somecdn.net. When a browser tries to resolve static.dropthigns.com, the DNS lookup request goes to the CDN nameserver. The nameserver then returns IP of a CDN node which is closest to you and can give you the best download performance. Browser then sends requests for files to that CDN node. When CDN node sees the request, it checks whether it has the content already cached. If it is cached, it delivers the content directly from its local store. If not, it makes a request to your server and then looks at the cache header generated in response. Based on the cache header it decides how long to cache the response in its own cache. In the meantime, browser does not wait for CDN node to get content and return to it. CDN does interesting trick on the internet backbone to actually route request to the origin server so that the browser gets the response directly served from origin server while CDN is updating its cache. Sometimes CDN act as a proxy, intercepting each request and then fetching uncached content from origin using a faster route and optimized connectivity to the origin server.Note: I will be posting some stuffs from my old blog to new blog. Please ignore if you have read them before.