Why insert static files ( css, images, js, ecc ) in a subdomain?

Question

Why so many big and little sites inserts static files ( css, images, js, ecc ) in a subdomain like media.example.com or s2.static.example.com ?

What are the advantages ?

Why not just a directory like example.com/media/ ?

Answer 1

I see at least three possible (good) reasons :

• Use another machine to serve the static content
  • Including some CDN
• Use another web-server to serve the static content
  • Something more lightweight
  • And faster
  • No need for a full PHP / .NET / JAVA server to serve static content !
• Using another domainname means that you'll be able to not have the cookies that are used on the main domain
  • Which means that HTTP requests will be smaller
  • Which can count, if you have a lot of traffic (see this post, for example : Performance Research, Part 3: When the Cookie Crumbles)
  • That's what is done on stackoverflow, if I remember correctly

Answer 2

The primary reason I think, is for cookies.

Cookies will be sent along with every request, let's say if you have 2kb of cookie data and load 20 images on a page.

That's an extra 40kb of data, multiply that by the number of page-views you have in a month, and you might be surprised about how much bandwidth you have lost in something useless, and bandwidth is not free...

Also, cookies set on the top-level domain, are sent across all requests made to any subdomain, in those cases, is even recommended to buy a new domain to host the static components there, in a cookie-free domain.

For example, StackOverflow uses sstatic.net, Yahoo uses yimg.com, YouTube uses ytimg.com, Amazon uses images-amazon.com, etc...

Give a look to this:

• Use Cookie-Free Domains for Components

Answer 3

To add to some of the answers above: Some web browsers can only download two files simultaneously from any domain.

Serving static content from a different host name (or names - e.g. a.domain, b.domain, c.domain) allows these older browsers to download more files in parallel.

Answer 4

You can read about why it was done on stackoverflow in this blog post. Here's an extract:

... use cookie-free domains for components.

When the browser makes a request for a static image and sends cookies together with the request, the server doesn’t have any use for those cookies. So they only create network traffic for no good reason. You should make sure static components are requested with cookie-free requests. Create a subdomain and host all your static components there.

This advise was taken from Yahoo's Best Practices for Speeding Up Your Web Site