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As I understand it, when I open a website such as Google, the hostname is looked up and my browser uses the resulting IP address to connect to the server and retrieve the page.

However, how do high availability websites make sure that this single IP address can always be reached? isn't that a single point of failure?

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closed as not a real question by Jason Berg, ThatGraemeGuy, John Gardeniers, Warner, Massimo Sep 14 '10 at 14:13

It's difficult to tell what is being asked here. This question is ambiguous, vague, incomplete, overly broad, or rhetorical and cannot be reasonably answered in its current form. For help clarifying this question so that it can be reopened, visit the help center.If this question can be reworded to fit the rules in the help center, please edit the question.

Your question is ambiguous and difficult to reasonably answer in its current form. High availability is an expansive topic with numerous approaches. – Warner Sep 13 '10 at 21:14
Warner is exactly right. As it stands now your question is far too vague for any sort of meaningful answer. Any answer is likely to spawn multiple additional questions. I'd suggest you google around, check out some wikipedia pages, even search existing serverfault questions and then come back with more specific questions covering topics you don't understand. – ThatGraemeGuy Sep 13 '10 at 21:20
How do they do it? With a lot of money. – joeqwerty Sep 14 '10 at 3:11
up vote 8 down vote accepted

There are two common solutions to high availability for web sites: DNS round robin and IP load balancing.

DNS round robin means you get different IP addresses each time you query a DNS server for the site's name; this helps distributing requests across multiple servers, and it also avoids the single point of failure you pointed out. This is the DNS answer for (when asked to one of the authoritative name servers for the "" domain):

Address:  canonical name =        internet address =        internet address =        internet address =

Another common solution, which could also be used at the same time (and very likely is in this case), is IP load balancing; i.e. those IP addresses aren't actually assigned to servers, but instead to load balancing devices (or reverse proxies, or any other similar solution), which then forward the requests to one of several back-end servers; should one of those servers fail, another one would be used.

More info here:

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Actually given the size of google it is more than that. – TomTom Sep 13 '10 at 22:25
I'm sure you can add in DNS Anycasting as well – Mark Henderson Sep 14 '10 at 1:05
-1 for one reason - DNS round robin is NOT A high availability solution. It distributes load to all IP addresses, whether they are available or not. – TomTom Sep 14 '10 at 7:10
BGP is key as well. Really, too general a question. We could nitpick details all day long. – Warner Sep 14 '10 at 14:11
This is the answer I was looking for, I did not know that there is such a thing as DNS round robin! Thanks! – Chris Sep 15 '10 at 12:46

Google most likely uses THREE Approaches at the same time:

  • At the backend you have a number of servers to serve requests. They haveall their own IP addresses.
  • In front of them are Hardware Load balancers that distribute reuqests to servers behind them. They have one public IP each, but may cover 30, 60 or even more physical servers. They are themselves likely redundant from a large manufacturer.
  • In front DNS Round Robin is LIKELY used. Allows load sitribution to even more load balanders.

Actually all that is nicely described.

Note that we talk of HUNDREDS OF THOUSANDS OF SERVERS. MANY data centers full of stuff.

Google is very special in that the servers pretty much are read only. They get a copy of the index, and serve that until they are reimaged with a new updated copy. No updates are ever done to an answering cluster. This is unusual for an applicaiton - but not because google is smart or so, just because their requirements are unusual.

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Well, it's probably also because Google is smart. – mfinni Sep 14 '10 at 3:17
In this case not only - their simple scale pretty much makes ths the only viable approach. The number of servers is ridiculously high, but then... they are all similar and they simply need them ;) – TomTom Sep 14 '10 at 7:09

An IP address isn't necessarily a SPOF as it certainly can be re-affected dynamically (a.k.a. fail-over) to a healthy server should the previous one holding it goes wrong.

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High availability sites uses many technologies as like as DNS roots servers in the way to be reached at any time.

In the fact, to be safe of attacks and failure, we can deploy many solutions as :

  • Anycast solutions
  • DNS load balacing
  • Load balancing and reverse proxy.
  • Fail-over solutions
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