The Google Dance was the name given to the behaviour observed by the Google search engine during the monthly period when it updated its index. During that period, webmasters were able to view the effect of the update on the search engine rankings for their web pages by comparing the results across different data centres. In July 2003, the Google Dance was discontinued when the search engine began to update its index continuously instead.

Although the Google Dance itself has become a thing of the past, the individual data centres remain in operation, and this is likely to continue to be the case. Indeed, a new data centre, www-mc, appeared in October 2003. The Google Dance Viewer can therefore still be used to compare results. I have left the remainder of this article online, partly for historical interest, and partly because the description of Google's data centres remains relevant.

Mark Horrell, 3 November 2003

Because the traffic to Google is vast, numbering hundreds of millions of queries a day to some 3 billion documents, it spreads its database across several data centres, each comprising thousands of individual servers. Since it aims to assemble a complete map of the web once a month, updating all its data centres with the new index can take several days.

Each time you query the main Google domain at www.google.com, the domain name itself could direct you to the IP number for any one of its data centres, depending on your location in the world and the relative loads in terms of internet traffic using the data centres' servers.

Google runs two additional domains, www2.google.com, and www3.google.com, which are used as testing grounds for the new index after re-indexing has occurred and while the search engine's relevance ranking algorithms and PageRank iterations are being re-calculated. Once the new index has been tested and its results deemed satisfactory, it is then transferred in turn to each of Google's individual data centres.

Most of the time when you query Google, you will not observe this behaviour taking place. During the period of the transfer however, you may notice the results on Google 'dancing' up and down depending on which data centre the domain www.google.com is directing you to and whether or not that data centre has been updated with the new index. It is for this reason that the updating period is known as The Google Dance.

The Google Dance Viewer is simply a tool which allows you to query each of Google's data centres from one convenient interface, and compare the results against those of the main Google site. If you are a webmaster, you can thus see whether the Dance is taking place and how the latest update may be affecting the ranking of your web sites.

Mark Horrell, 4 December 2002

Click here to try the Google Dance Viewer.