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IBM WebSphere eXtreme Scale 6 by Anthony Chaves

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Aggregate results

One thing to be aware of with the MapGridAgent interface is its potential for a partition to send huge result maps to a client. This is the nature of the map function. Its output size can be proportional to its input size if we don't use a query to select specific objects to work with or specify a key set. In this case, we need a specific result for every key, with the key set as narrow as we can make it. We then just need to deal with large maps once in a while.

What if we need an aggregate result for a key set? Instead of an operation and result for each element, we need an operation over all elements with just one result. Simple examples include the highest or lowest number in a set, and the earliest or total payroll expenses ...

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