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From "Robert Muir (Created) (JIRA)" <j...@apache.org>
Subject [jira] [Created] (LUCENE-3490) Restructure codec hierarchy
Date Wed, 05 Oct 2011 16:16:34 GMT
Restructure codec hierarchy
---------------------------

                 Key: LUCENE-3490
                 URL: https://issues.apache.org/jira/browse/LUCENE-3490
             Project: Lucene - Java
          Issue Type: Improvement
            Reporter: Robert Muir
             Fix For: 4.0


Spinoff of LUCENE-2621. (Hoping we can do some of the renaming etc here in a rote way to make
progress).

Currently Codec.java only represents a portion of the index, but there are other parts of
the index 
(stored fields, term vectors, fieldinfos, ...) that we want under codec control. There is
also some 
inconsistency about what a Codec is currently, for example Memory and Pulsing are really just

PostingsFormats, you might just apply them to a specific field. On the other hand, PreFlex
actually
is a Codec: it represents the Lucene 3.x index format (just not all parts yet). I imagine
we would
like SimpleText to be the same way.

So, I propose restructuring the classes so that we have something like:
* CodecProvider <-- codec name to Class resolution only
* Codec <-- represents the index format (PostingsFormat + FieldsFormat + ...)
* PostingsFormat: this is what Codec controls today, and Codec will return one of these for
a field.
* FieldsFormat: Stored Fields + Term Vectors + FieldInfos?

I think for PreFlex, it doesnt make sense to expose its PostingsFormat as a 'public' class,
because preflex
can never be per-field so there is no use in allowing you to configure PreFlex for a specific
field.
Similarly, I think we should do the same thing for SimpleText. Nobody needs SimpleText for
production, it should
just be a Codec where we try to make as much of the index as plain text and simple as possible
for debugging/learning/etc.
So we don't need to expose its PostingsFormat. On the other hand, I don't think we need Pulsing
or Memory codecs,
because its pretty silly to make your entire index use one of their PostingsFormats. To parallel
with analysis:
PostingsFormat is like Tokenizer and Codec is like Analyzer, and we don't need Analyzers to
"show off" every Tokenizer.

Later, once we abstract FieldInfos reading/writing out of o.a.l.index into codec control,
we can also then
move the baked in PerFieldCodecWrapper out (it would basically be PerFieldPostingsFormat).
Privately it would
write the ids to the file like it does today. all 3.x hairy backwards code would move to PreflexCodec.
SimpleTextCodec
would get a plain text fieldinfos impl, etc.

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