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From "Andrzej Bialecki (JIRA)" <j...@apache.org>
Subject [jira] [Created] (LUCENE-4050) Change SegmentInfos format to plain text
Date Sat, 12 May 2012 03:01:56 GMT
Andrzej Bialecki  created LUCENE-4050:
-----------------------------------------

             Summary: Change SegmentInfos format to plain text
                 Key: LUCENE-4050
                 URL: https://issues.apache.org/jira/browse/LUCENE-4050
             Project: Lucene - Java
          Issue Type: Improvement
          Components: core/codecs
            Reporter: Andrzej Bialecki 
             Fix For: 4.0


I propose to change the format of SegmentInfos file (segments_NN) to use plain text instead
of the current binary format.

SegmentInfos file represents a commit point, and it also declares what codecs were used for
writing each of the segments that the commit point consists of. However, this is a chicken
and egg situation - in theory the format of this file is customizable via Codec.getSegmentInfosFormat,
but in practice we have to first discover what is the codec implementation that wrote this
file - so the SegmentCoreReaders assumes a certain fixed binary layout of a preamble of this
file that contains the codec name... and then the file is read again, only this time using
the right Codec.

This is ugly. Instead I propose to use a simple plain text format, either line oriented properties
or JSON, in such a way that newer versions could easily extend it, and which wouldn't require
any special Codec to read and parse. Consequently we could remove SegmentInfosFormat altogether,
and instead add SegmentInfoFormat (notice the singular) to Codec to read single per-segment
SegmentInfo-s in a codec-specific way. E.g. for Lucene40 codec we could either add another
file or we could extend the .fnm file (FieldInfos) to contain also this information. 

Then the plain text SegmentInfos would contain just the following information:

* list of global files for this commit point (if any)
* list of segments for this commit point, and their corresponding codec class names
* user data map


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