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From "Adam Fuchs (JIRA)" <j...@apache.org>
Subject [jira] [Created] (ACCUMULO-759) remove priority setting for scan-time iterators
Date Sat, 08 Sep 2012 20:32:07 GMT
Adam Fuchs created ACCUMULO-759:
-----------------------------------

             Summary: remove priority setting for scan-time iterators
                 Key: ACCUMULO-759
                 URL: https://issues.apache.org/jira/browse/ACCUMULO-759
             Project: Accumulo
          Issue Type: Improvement
            Reporter: Adam Fuchs


Iterators have a priority setting that allows a user to order iterators arbitrarily. However
that priority is an integer that doesn't directly convey the iterator's relationship to other
iterators. I would postulate that nobody has ever needed to sneak in a scan-time iterator
underneath a configured table iterator (please let me know if I'm wrong about this), and the
effect of doing so is not easy to calculate. Many people have chosen a bad iterator priority
and seen commutativity problems with previously configured iterators.

I propose that we use more of an agglomerative approach to configuring scan-time iterators,
in which the order of the iterator tree is the same order in which the addScanIterator method
is called, and all scan-time iterators apply after the configured iterators apply. The change
to the API should just be to remove the priority number, and the existing IteratorSetting
constructor and accessors should be deprecated.

With this change, we can think of an iterator as more of a functional modification to a data
set, as in T' = f(T) or T'' = g(f(T)). This should make it easier for developers to use iterators
correctly.

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