Andrzej Bialecki created LUCENE-4050:
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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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