How AdSense works: Each time a visitor visits a page with an AdSense tag, a piece of JavaScript writes an iframe tag, whose src attribute includes the URL of the page. Google's servers use a cache of the page for the URL or the keywords in the URL itself to determine a set of high-value keywords. (Some of the details are described in the AdSense patent.) If keywords have been cached already, ads are served for those keywords based on the AdWords bidding system.

The storage requirements of an AdSense system are stunningly modest. If each URL has just 8 "high-value" keywords, each represented by a single 32-bit number, then the keywords for each URL could be represented with just 32 bytes. The high value keywords of 4 billion URLs could be stored in 128GB, which would cost only $100 (circa 2006). 400 billion URLs or 100 drives (for a redundancy of 100) would require only $10,000 in storage costs.

AdSense serves a very large number of pages each day. If each day around 1B people saw 10 AdSense impressions (or 100M people saw 100 AdSense impressions), then AdSense would serve around 10B requests/day, or 115,741 requests/sec. If one machine can serve 20 reqs/second (seek times to read a random 4096-byte location on a drive allow for bursts of well over 100 reqs/second), then Google would require 5,787 servers to serve these 10B reqs/day. If each of these servers were hosted at a cost of $100/month, then it would cost $579K/month to run the adservers needed.

Suppose these 10B impressions/day generated clicks at a clickthrough rate of .3% and an average CPC of $.10. Then each day Google would receive 30M clicks/day (347 clicks/sec), generating $3M/day ($34.77/sec), or 900M clicks/month, generating $90M/month.

Abuse: Some webmasters create sites tailored to lure searchers from Google and other engines onto their AdSense to make money from clicks. These "zombie" sites often contain nothing but a large amount of interconnected, automated content (e.g. a directory with content from the Open Directory Project). Possibly the most popular form of such "AdSense farms" are splogs ("spam blogs"), which are centered around known high-paying keywords. Also many sites use free content from other web sites, such as Wikipedia, to attract visitors. These and related approaches are considered to be search engine spam and can be reported to Google.

Criticism: Due to concerns about click fraud, Google AdSense has been criticized by some SEO firms as a large source of what Google calls "invalid clicks". In response, Google says that it "removes publishers from their partner network on a daily basis". Some disabled publishers have complained that the process is not transparent or accountable.

To help prevent click fraud, publishers can choose from a number of click tracking programs. These programs will display detailed information about the visitors who click on the AdSense pages. Publishers can use that data to determine if they've been a victim of click fraud or not. There seems to be many such commercial scripts available. An open-source alternative is AdLogger.

Google has also come under fire for not doing enough to combat the misuse of trademarks. Since 2004, Google had stopped prohibiting advertisers from bidding on any keyword, including trademarked terms.

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