Hadoop is an implementation of the Map/Reduce algorithm developed by Google, running atop the Hadoop distributed file system (HDFS). Hadoop sees to it that distributed instances of your routines are executed, that input data is partitioned and sent to your routines, that the results are gathered and passed on to the next stage, and even that crashed instances are restarted and the situation is "healed." And Hadoop's scalability is practically linear. If your data gets twice as big, then double the number of systems in your Hadoop network. You need write no new code to accommodate the increase in the number of processors or expanded disk space; Hadoop sees to all that.
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