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From Christian Kohlschütter (JIRA) <j...@apache.org>
Subject [jira] Created: (LUCENE-2133) [PATCH] IndexCache: Refactoring of FieldCache, FieldComparator, SortField
Date Tue, 08 Dec 2009 07:42:18 GMT
[PATCH] IndexCache: Refactoring of FieldCache, FieldComparator, SortField
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                 Key: LUCENE-2133
                 URL: https://issues.apache.org/jira/browse/LUCENE-2133
             Project: Lucene - Java
          Issue Type: Improvement
          Components: Search
    Affects Versions: 3.1
            Reporter: Christian Kohlschütter


Hi all,

up to the current version Lucene contains a conceptual flaw, that is the FieldCache. The FieldCache
is a singleton which is supposed to cache certain information for every IndexReader that is
currently open

The FieldCache is flawed because it is incorrect to assume that:
1. one IndexReader instance equals one index. In fact, there can be many clones (of SegmentReader)
or decorators (FilterIndexReader) which all access the very same data.
2. the cache information remains valid for the lifetime of an IndexReader. In fact, some IndexReaders
may be reopen()'ed and thus they may contain completely different information.
3. all IndexReaders need the same type of cache. In fact, because of the limitations imposed
by the singleton construct there was no implementation other than FieldCacheImpl.

Furthermore, FieldCacheImpl and FieldComparator are bloated by several static inner-classes
that could be moved to package level.

There have been a few attempts to improve FieldCache, namely LUCENE-831, LUCENE-1579 and LUCENE-1749,
but the overall situation remains the same: There is a central registry for assigning Caches
to IndexReader instances.

I now propose the following:
1. Obsolete FieldCache and FieldCacheKey and provide index-specific, extensible cache instances
("IndexCache"). IndexCaches provide common caching functionality for all IndexReaders and
may be extended (for example, SegmentReader would have a SegmentReaderIndexCache and store
different data than a regular IndexCache)
2. Add the index-specific field cache (IndexFieldCache) to the IndexCache. IndexFieldCache
is an interface just like FieldCache and may support different implementations.
3. The IndexCache instances may be flushed/closed by the associated IndexReaders whenever
necessary.
4. Obsolete FieldCacheSanityChecker because no more "insanities" are expected (or at least,
they do not impact the overall performance)
5. Refactor FieldCacheImpl and the related classes (FieldComparator, SortField) 

I have provided an patch which takes care of all these issues. It passes all JUnit tests.

The patch is quite large, admittedly, but the change required several modifications and some
more to preserve backwards-compatibility. Backwards-compatibility is preserved by moving some
of the updated functionality in the package org.apache.lucene.search.fields (field comparators
and parsers, SortField) while adding wrapper instances and keeping old code in org.apache.lucene.search.

In detail and besides the above mentioned improvements, the following is provided:
1. An IndexCache specific for SegmentReaders. The two ThreadLocals are moved from SegmentReader
to SegmentReaderIndexCache.
2. A housekeeping improvement to CloseableThreadLocal. Now delegates the close() method to
all registered instances by calling an onClose() method with the threads' instances.
3. Analyzer.close now may throw an IOException (this already is covered by java.io.Closeable).
4. A change to Collector: allow IndexCache instead of IndexReader being passed to setNextReader()
5. SortField's numeric types have been replaced by direct assignments of FieldComparatorSource.
This removes the "switch" statements and the possibility to throw IllegalArgumentExceptions
because of unsupported type values.

The following classes have been deprecated and replaced by new classes in org.apache.lucene.search.fields:
- FieldCacheRangeFilter (=> IndexFieldCacheRangeFilter)
- FieldCacheTermsFilter (=> IndexFieldCacheTermsFilter)
- FieldCache (=> IndexFieldCache)
- FieldCacheImpl (=> IndexFieldCacheImpl)
- all classes in FieldCacheImpl (=> several package-level classes)
- all subclasses of FieldComparator (=> several package-level classes)

Final notes:
- The patch would be simpler if no backwards compatibility was necessary. The Lucene community
has to decide which classes/methods can immediately be removed, which ones later, which not
at all. Whenever new classes depend on the old ones, an appropriate notice exists in the javadocs.
- The patch introduces a new, deprecated class IndexFieldCacheSanityChecker.java which is
just there for testing purposes, to show that no sanity checks are necessary any longer. This
class may be removed at any time.
- I expect that the patch does not impact performance. On the contrary, as the patch removes
a few unnecessary checks we might even see a slight speedup. No benchmarking has been done
so far, though.
- I have tried to preserve the existing functionality wherever possible and to focus on the
class/method structure only. We certainly may improve the caches' behavior, but this out of
scope for this patch.
- The refactoring finally makes the high duplication of code visible: For all supported atomic
types (byte, double, float, int, long, short) three classes each are required: *Cache, *Comparator
and *Parser. I think that further simplification might be possible (maybe using Java generics?),
but I guess the current patch is large enough for now.

Cheers,
Christian


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