Affects Version/s: 2.9.1, 3.0
Fix Version/s: None
Lucene Fields:New, Patch Available
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)
- 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.