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From "Adam Fuchs (JIRA)" <>
Subject [jira] [Commented] (ACCUMULO-759) remove priority setting for scan-time iterators
Date Tue, 11 Sep 2012 21:54:07 GMT


Adam Fuchs commented on ACCUMULO-759:

What if we went with a more immutable form of Scanner, which was more representative of the
data set? This would allow things like:


Scanner allTheThings = connector.createScanner(...);
ScanIteratorSetting aggregateEverything = new ScanIteratorSetting(MyAggregatingIterator.class,properties);
Scanner aggregatedThings = allTheThings.transform(aggregateEverything);

// scan over the table with the iterator
for(Entry<Key,Value> aggregate: aggregatedThings)

// scan over the table without extra scan-time iterators
for(Entry<Key,Value> thing: allTheThings)


I think this might be an easier way for users to conceptualize iterators.
> remove priority setting for scan-time iterators
> -----------------------------------------------
>                 Key: ACCUMULO-759
>                 URL:
>             Project: Accumulo
>          Issue Type: Improvement
>            Reporter: Adam Fuchs
>              Labels: newbie
> 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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