Details
Major Description
A further step from LUCENE-5029.
The short story is, previous API change brings two problems:
- it somewhat breaks backward compatibility: although we can still read old format,
we can no longer reproduce it; - pulsing codec have problem with it.
And long story...
With the change, current PostingsBase API will be like this:
- term dict tells PBF we start a new term (via startTerm());
- PBF adds docs, positions and other postings data;
- term dict tells PBF all the data for current term is completed (via finishTerm()),
then PBF returns the metadata for current term (as long[] and byte[]); - term dict might buffer all the metadata in an ArrayList. when all the term is collected,
it then decides how those metadata will be located on disk.
So after the API change, PBF no longer have that annoying 'flushTermBlock', and instead
term dict maintains the <term, metadata> list.
However, for each term we'll now write long[] blob before byte[], so the index format is not consistent with pre-4.5.
like in Lucne41, the metadata can be written as longA,bytesA,longB, but now we have to write as longA,longB,bytesA.
Another problem is, pulsing codec cannot tell wrapped PBF how the metadata is delta-encoded, after all
PulsingPostingsWriter is only a PBF.
For example, we have terms=["a", "a1", "a2", "b", "b1" "b2"] and itemsInBlock=2, so theoretically
we'll finally have three blocks in BTTR: ["a" "b"] ["a1" "a2"] ["b1" "b2"], with this
approach, the metadata of term "b" is delta encoded base on metadata of "a". but when term dict tells
PBF to finishTerm("b"), it might silly do the delta encode base on term "a2".
So I think maybe we can introduce a method 'encodeTerm(long[], DataOutput out, FieldInfo, TermState, boolean absolute)',
so that during metadata flush, we can control how current term is written? And the term dict will buffer TermState, which
implicitly holds metadata like we do in PBReader side.
For example, if we want to reproduce old lucene41 format , we can simple set longsSize==0, then PBF
writes the old format (longA,bytesA,longB) to DataOutput, and the compatible issue is solved.
For pulsing codec, it will also be able to tell lower level how to encode metadata.