When creating custom Operations (
Buffer) do not store operation state in class
For example, if implementing a custom "counter"
Aggregator, do not create a field named "count"
and increment it on every
Aggregator.aggregate() call. There is no
guarantee that your Operation will be called from a single thread in a
JVM - and future versions of Hadoop or Cascading local mode might
execute the same operation from multiple threads.
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