Local data access is when the data required for code to execute is physiacally close to it, in the sense of on the same proessor, or at least close network. This is an issue for big data processing especially on cloud computing platforms, where data may be stored in multiple computers often in physically remote data centres. The goal is to perfom as much procesisng locally without moving data around. Where the code is accessing ultiple data items, it is important that these are then stored physically close to one another. This may be because theer are interincis commonlaity about data so they can be permamnently stored close to one another, or brought together for the purposes of processing. An exmaple all the former is data about a single person, and maybe summary inorrmation about their friends, in a social network which can be stored as a single record or data item. An example of the latter is MapReduce, which uses a common hash value to bring items that need to be processed together by the reduce step on the same processor.
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