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From "Michael McCandless (JIRA)" <>
Subject [jira] [Commented] (LUCENE-4050) Change SegmentInfos format to plain text
Date Sat, 12 May 2012 10:39:48 GMT


Michael McCandless commented on LUCENE-4050:

+1 to fully separate (separate files maybe?) the codec-neutral "list
of committed segments" from "the codec-specific details/metadata for
each segment".

Then, a codec can easily store its own stuff in the segment metadata.

And I agree the FieldInfo/SegmentInfo duality is confusing...

Plain text encoding of these files would be really nice but isn't as
important, I think... and will be a fair amount of work (I suspect we
need a JSON or YAML or something that represents lists, maps,
different native types, etc.).  I think this is separate / can come

We do a lot of funky stuff like writing fake bogus
data, seeking backwards, etc. Why not just a normal atomic rename like
any other computer program on the planet????

In fact Lucene used to use rename to commit the segments file but this
proved problematic on Windows (sometimes the rename would hit "access
denied" error).

> Change SegmentInfos format to plain text
> ----------------------------------------
>                 Key: LUCENE-4050
>                 URL:
>             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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