Google's Mapper API provide full MapReduce capabilities on App Engine. "One area of particular interest is report generation. Most apps create and maintain large numbers of detail records: entities in data models, transaction history or event logs. In order to glean useful information out of these vast data sets, your app has to iterate over your entities, summarize and breakdown the results. That's where the Mapper API comes in.
Christopher O'Donnell, from Cambridge, Massachusetts, in a guest article, Modern Funnel Analytics Using Mapper, makes extensive use of the Mapper API to illustrate the process of generating rollup reports. With his approach, he's able to provide both summarized results and drill down capabilities."
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