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From "Andrzej Bialecki (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (LUCENE-4050) Change SegmentInfos format to plain text
Date Sat, 12 May 2012 20:14:50 GMT

    [ https://issues.apache.org/jira/browse/LUCENE-4050?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13274064#comment-13274064
] 

Andrzej Bialecki  commented on LUCENE-4050:
-------------------------------------------

bq. Plain text encoding of these files would be really nice but isn't as important, I think...

Yeah, it could be sufficient if we would agree on necessarily separate the "plain list of
segments:codec" from the segmentInfo/fieldInfo parts and push those parts down to the codec-specific
formats.

Then we could just use a version number as the first element of this file to allow for extensions
in the future, like e.g. switching to JSON or to some other format du jour.

bq. Surely this is just some problem only on windows 3.1 and java 1.2 or something and now
fixed, since this is how every other linux/cygwin program (e.g. vi) works.

I'm not so sure. I know for a fact that Windows doesn't allow renames or deletes of open files,
no matter if it's open by you or by some other process (e.g. user examining the file in Notepad.exe),
and IIRC the issue was that JVM doesn't release OS file handles quickly enough.
                
> 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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