MapReduce

MapReduce is a distributed computing framework for dealing with big data. It is inspired by the map and reduce functions found in Lisp and other functional programming languages: the map stage initially processes each item of data separately and then the reduce stage collects and aggregates related intermediate results. However, MapReduce has two crucial additional features : (i) the use of a hash as means to distribute intermediate results over different computers, so reducing the likelihood of Byzantine conditions; and (ii) means to monitor for failure of individual computers and recover from this, so making it overall more robust to failure.

each data item	→	map	→	hash + processed data
all data for a hash	→	reduce	→	one or more agggregated calculations for the hashed data (possibly including fresh hash for further reduce steps)

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MapReduce was initially developed by engineers at Google, but has since become part of the open source Apache Apache Hadoop project.