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Round-robin means everyone taking a turn, in order, such as children arranged in a circle and the turn passing around the circle, with no set beginning or end.
The domain name system (DNS) allows use of something called called round-robin answering to distribute requests between servers (web servers, mail servers, or what-have-you). For example, suppose you have three web servers, all able to serve up the site www.example.com. With round-robin DNS, you give that name all three servers' IP addresses, as a set of three DNS records. Then when web browsers try to load the website, the address they get from DNS changes with each request (each DNS request, not each web request). In this way, the load is shared roughly equally between the servers.
Looking a little deeper, the way this is accomplished is not by returning one answer to one request and a different answer to the next. That would be against the rules of DNS, although some services such as Akamai do actually do this. With round-robin, though, the whole set of answers is returned to every requester, but in a different order each time. Web browsers, as an example, only use the first address returned, so changing the order allows different browsers to get a different first answer. They're all still, technically, getting the same set of answers.
With true round-robin, the order of records in successive responses is simply permuted, or rotated, so that the addresses are returned in order for the first request, and then the first one is moved to the end for the second request, and so on - each time, the previously-first record is moved to the bottom of the list. Modern BIND servers (BIND 9) don't do this by default. Instead, they randomize the set of records each time. This turns out to be less work for the DNS server than permuting, at least for smaller sets of records, while the effect is usually indistinguishable from round-robin.
Round-robin DNS answering does not provide any sort of fault tolerance or true load balancing. If one of those web servers should fail, using the example above, one third of new web visitors will be unable to connect.
Chris Buxton
Men & Mice
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