[jira] [Created] (LUCENE-9669) N-2 compatibility for file formats

["Simon Willnauer (Jira)" <ji...@apache.org>]

Simon Willnauer created LUCENE-9669:
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             Summary: N-2 compatibility for file formats
                 Key: LUCENE-9669
                 URL: https://issues.apache.org/jira/browse/LUCENE-9669
             Project: Lucene - Core
          Issue Type: Improvement
            Reporter: Simon Willnauer


Currently Lucene supports reading and writing indices that have been
created with the current or previous (N-1) version of Lucene. Lucene
refuses to open an index created by N-2 or earlier versions.
I would like to propose that Lucene adds support for opening indices
created by version N-2 in read-only mode. Here's what I have in mind:

- Read-only support. You can open a reader on an index created by
version N-2, but you cannot open an IndexWriter on it, meaning that
you can neither delete, update, add documents or force-merge N-2
indices.

- File-format compatibility only. File-format compatibility enables
reading the content of old indices, but not more. Everything that is
done on top of file formats like analysis or the encoding of length
normalization factors is not guaranteed and only supported on a
best-effort basis.

The reason I came up with these limitations is because I wanted to
make the scope minimal in order to retain Lucene's ability to move
forward. If there is consensus to move forward with this, I would like
to target Lucene 9.0 with this change.

This is a follow-up of the mailinglist thread [here|http://mail-archives.apache.org/mod_mbox/lucene-dev/202101.mbox/%3CCAAHmpkg1BD9RtmVFR4jweYgYAXwz5OsY1hV9ubZkS0fHSFnTCQ@mail.gmail.com%3E] 



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