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https://issues.apache.org/jira/browse/LUCENE-584?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12547901
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Mark Harwood commented on LUCENE-584:
-------------------------------------
To go back to post #1 on this topic:
_"Sparsely populated =java.util.BitSet=s are not efficient and waste lots of
memory. It would be desirable to have an alternative BitSet implementation with
smaller memory footprint."_
Given the motivation to move to more memory efficient structures why is the
only attempt at caching dedicated exclusively to caching the very structures we
were trying to move away from?.....
_"I deprecated also CachingWrapperFilter and RemoteCachingWrapperFilter
and added corresponding CachingBitSetFilter and RemoteCachingBitSetFilter"_
Does this suggest we are to have type-specific CachingXxxxxFilters and
RemoteCachingXxxxxFilters created for every new filter type? Why not provide a
single caching mechanism that works for all those other, new, more
memory-efficient structures? I beleive the reason this hasn't been done is due
to the issue I highlighted earlier - the cachable artefacts (what I chose to
call "DocIdSet" here: [#action_12518642] ) are not modelled in a way which
promotes re-use. That's why we would end up needing a specialised caching
implementations for each type.
If we are to move forward from the existing Lucene implementation it's
important to note the change:
* Filters currently produce, at great cost, BitSets. Bitsets provide both a
cachable data structure and a thread-safe, reusable means of iterating across
the contents.
* By replacing BitSets with Matchers this proposal has removed an important
aspect of the existing design - the visibility (and therefore cachability) of
these expensive-to-recreate data structures. Matchers are single-use,
non-threadsafe objects and hide the data structure over which they iterate.
With this change if I want to implement a caching mechanism in my application I
need to know the Filter type and what sort of data structure it returns and get
it from it directly:
if(myFilter instanceof BitSetFilter) wrap specific data structure using
CachingBitSetFilter
else
if(myFilter instanceof OpenBitSetFilter) wrap specific data structure using
CachingXxxxxFilter
else...
...looks like an Anti-pattern to me. Worse, this ties the choice of
datastructure to the type of Filter that produces it. Why can't my RangeFilter
be free to create a SortedVIntList or a BitSet depending on the sparseness of
matches for a particular set of criteria?
I'm not saying "lets just stick with Bitsets", just consider caching more in
the design. Post [#action_12518642] lays out how this could be modelled with
the introduction of DocIdSet and DocIdSetIterator as separate responsibilities
(whereas Matcher currently combines them both).
Cheers
Mark
> Decouple Filter from BitSet
> ---------------------------
>
> Key: LUCENE-584
> URL: https://issues.apache.org/jira/browse/LUCENE-584
> Project: Lucene - Java
> Issue Type: Improvement
> Components: Search
> Affects Versions: 2.0.1
> Reporter: Peter Schäfer
> Assignee: Michael Busch
> Priority: Minor
> Attachments: bench-diff.txt, bench-diff.txt, lucene-584-take2.patch,
> lucene-584.patch, Matcher-20070905-2default.patch,
> Matcher-20070905-3core.patch, Matcher-20071122-1ground.patch, Some
> Matchers.zip
>
>
> {code}
> package org.apache.lucene.search;
> public abstract class Filter implements java.io.Serializable
> {
> public abstract AbstractBitSet bits(IndexReader reader) throws IOException;
> }
> public interface AbstractBitSet
> {
> public boolean get(int index);
> }
> {code}
> It would be useful if the method =Filter.bits()= returned an abstract
> interface, instead of =java.util.BitSet=.
> Use case: there is a very large index, and, depending on the user's
> privileges, only a small portion of the index is actually visible.
> Sparsely populated =java.util.BitSet=s are not efficient and waste lots of
> memory. It would be desirable to have an alternative BitSet implementation
> with smaller memory footprint.
> Though it _is_ possibly to derive classes from =java.util.BitSet=, it was
> obviously not designed for that purpose.
> That's why I propose to use an interface instead. The default implementation
> could still delegate to =java.util.BitSet=.
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