Why ww2 sub domains?

I have seen 开发者_运维知识库on the web some domain names having prefix of ww2 or ww3 or so (ww2.somedomain.example, ww3.yourdomain.example). And these happen mostly when traveling from a page to page. What would be the reason of having such subdomains? Is there anything special about them or are they just another sub domain? I mean, are they useful in any particular context?

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People running large(-ish) sites used to do this when they needed to break up the load between more than one server. One machine would be called www then the next one would be called www2, etc.

Today, much better load balancing solutions are available that don't require you to expose your internal machine naming conventions to the browser clients.

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Technically, the initials before the primary domain name (e.g. the "mail" in mail.yahoo.com) can be best though of as a machine name, identifying the web server/mail server, whatever. They can also identify a group of machines (a web farm).

So the person building up that machine can call it anything they want. The initials www are a (somewhat arbitrary) convention.

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Oftentimes, ww{x} is used to indicate a particular server of a set of mirrored servers. If properly configured, I could have www.mydomain.example point to my web site on a load balancer, while I could use ww1, ww2, ww3, etc to access the site guaranteed from a specific LBed server.

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I can see 3 possibilities

• make the browser load resources more faster. the browser would open a fixed number of connection to same domain not to load the server
• they are using more then one server so they can share the load between servers
• separate some content to a separate virtual host or server. some kind of organization ...

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As various answers have pointed out, modern day load-balancers can balance load without having to resort to using different sub-domains for each machine. However, there is still one benefit of dividing your site into various sub-domains: maximize browser connections.

All browsers limit the number of concurrent connections to a particular host (6 for most modern browsers). If a page contains lots of assets, page-load would be slow as the browser queue those requests because of connection limit. By loading different assets from different subdomain, you get around the connection limit, speeding up page-load.

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Typically it's a partitioning strategy. When sites get sufficiently large that they can't run (or run well) on a single server you then have to look at solutions for scaling the application out horizontally (ie more servers) rather than vertically (ie bigger servers).

Some example partitioning strategies are:

1. Certain users always use certain servers. This can be arbitrary or based on some criteria (user type, geographic location, etc);
2. When a user gets a session that session is assigned to a particular server (sometimes called "sticky sessions" although this can also be used where such different machines are transparent); and
3. Certain activities are always on certain machines.

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Another common case is organizational reasons. In an extremely large company, www might be for their main marketing website. And, ww2 might be, say, for product documentation pages.

In an ideal world, all departments would share perfectly. In practise, a big company might have their (www) marketing pages managed by an external agency. Their internal (ww2) pages done by their internal team. Often, the marketing agency just doesn't update pages quickly or refuses to run certain stacks, may be too limiting in terms of bureaucratic needs.

The marketing agency may insist on controlling the www and not sharing due to past situations where a company website went down due to internal reasons and yet the agency got blamed, or vice versa.

So, theoretically, there's no need to do this with modern load balancing and such. But, in practise, it can be a lot cheaper, straightforward and allow better business productivity.