Google reveals scope of Web-crawling task

Multiple times each day, Google recomputes the relative ranking of the world's Web sites. And it must index several billion new Web pages added daily.

It's a pity the National Security Agency can't talk about its computational challenges, because it's leaving a lot of the boasting rights to Google.

Paul Ford

In a blog posting on Friday the company shared some detail about the challenges of one aspect of its search operation, the Web indexing and processing that must take place before the results are delivered to users. The short version: Google has no choice but to think big.

First comes surfing. "We start at a set of well-connected initial pages and follow each of their links to new pages. Then we follow the links on those new pages to even more pages and so on, until we have a huge list of links," said software engineers Jesse Alpert and Nissan Hajaj. "Even after removing...exact duplicates, we saw a trillion unique URLs, and the number of individual web pages out there is growing by several billion pages per day."

Next comes analyzing the "link graph"--the mathematical representation of what links to what. That's a key foundation of Google's PageRank algorithm, which brought the company's search engine to prominence by assigning importance to those pages that other important pages point toward.

In the early days of Google, computing PageRank for the company's collection of a mere 26 million pages took a workstation "a couple hours," and the results would be used for some unspecified period of time. Today, Google surfs the Web continuously and recalculates the link graph "several times per day."

"This graph of one trillion URLs is similar to a map made up of one trillion intersections. So multiple times every day, we do the computational equivalent of fully exploring every intersection of every road in the United States. Except it'd be a map about 50,000 times as big as the U.S., with 50,000 times as many roads and intersections," the engineers said.

Google likes to talk about how users have choice and competition just one click away, and that's a fair point. But the blog post also makes it even clearer just how high barriers to entry are in the search market. That's one of the reasons Yahoo's BOSS (build your own search service) program is intriguing: it lets search start-ups take advantage of Yahoo's crawling, indexing, and search technology in exchange for advertising or revenue-sharing partnerships.

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