[LUCENE-3490] Restructure codec hierarchy

• Type: Improvement
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Fix Version/s: 4.0-ALPHA
• Lucene Fields: New
• Assignee: Unassigned
• Reporter: Robert Muir
• Created: 05/Oct/11 16:14
• Updated: 28/Aug/22 12:59
• Resolved: 04/Nov/11 16:02

Description

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 <-- dead, replaced by java ServiceProvider mechanism. All indexes are 'readable' if codecs are in classpath.
• 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 in the future 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.

we can also move the baked in PerFieldCodecWrapper out (it would basically be PerFieldPostingsFormat). Privately it would
write the ids to the file like it does today. in the future, all 3.x hairy backwards code would move to PreflexCodec.
SimpleTextCodec would get a plain text fieldinfos impl, etc.

Issue Links

• is part of: LUCENE-2621 Extend Codec to handle also stored fields and term vectors