Refactor DocumentsWriter
------------------------
Key: LUCENE-1301
URL: https://issues.apache.org/jira/browse/LUCENE-1301
Project: Lucene - Java
Issue Type: Improvement
Components: Index
Affects Versions: 2.3.1, 2.3, 2.3.2, 2.4
Reporter: Michael McCandless
Assignee: Michael McCandless
Priority: Minor
Fix For: 2.4
I've been working on refactoring DocumentsWriter to make it more
modular, so that adding new indexing functionality (like column-stride
stored fields, LUCENE-1231) is just a matter of adding a plugin into
the indexing chain.
This is an initial step towards flexible indexing (but there is still
alot more to do!).
And it's very much still a work in progress -- there are intemittant
thread safety issues, I need to add tests cases and test/iterate on
performance, many "nocommits", etc. This is a snapshot of my current
state...
The approach introduces "consumers" (abstract classes defining the
interface) at different levels during indexing. EG DocConsumer
consumes the whole document. DocFieldConsumer consumes separate
fields, one at a time. InvertedDocConsumer consumes tokens produced
by running each field through the analyzer. TermsHashConsumer writes
its own bytes into in-memory posting lists stored in byte slices,
indexed by term, etc.
DocumentsWriter*.java is then much simpler: it only interacts with a
DocConsumer and has no idea what that consumer is doing. Under that
DocConsumer there is a whole "indexing chain" that does the real work:
* NormsWriter holds norms in memory and then flushes them to _X.nrm.
* FreqProxTermsWriter holds postings data in memory and then flushes
to _X.frq/prx.
* StoredFieldsWriter flushes immediately to _X.fdx/fdt
* TermVectorsTermsWriter flushes immediately to _X.tvx/tvf/tvd
DocumentsWriter still manages things like flushing a segment, closing
doc stores, buffering & applying deletes, freeing memory, aborting
when necesary, etc.
In this first step, everything is package-private, and, the indexing
chain is hardwired (instantiated in DocumentsWriter) to the chain
currently matching Lucene trunk. Over time we can open this up.
There are no changes to the index file format.
For the most part this is just a [large] refactoring, except for these
two small actual changes:
* Improved concurrency with mixed large/small docs: previously the
thread state would be tied up when docs finished indexing
out-of-order. Now, it's not: instead I use a separate class to
hold any pending state to flush to the doc stores, and immediately
free up the thread state to index other docs.
* Buffered norms in memory now remain sparse, until flushed to the
_X.nrm file. Previously we would "fill holes" in norms in memory,
as we go, which could easily use way too much memory. Really this
isn't a solution to the problem of sparse norms (LUCENE-830); it
just delays that issue from causing memory blowup during indexing;
memory use will still blowup during searching.
I expect performance (indexing throughput) will be worse with this
change. I'll profile & iterate to minimize this, but I think we can
accept some loss. I also plan to measure benefit of manually
re-cycling RawPostingList instances from our own pool, vs letting GC
recycle them.
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