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https://issues.apache.org/jira/browse/LUCENE-9669?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17268193#comment-17268193
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ASF subversion and git services commented on LUCENE-9669:
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Commit 1aeb1dcb86904f2cd3799bb467a56986d97f0d6e in lucene-solr's branch
refs/heads/master from Simon Willnauer
[ https://gitbox.apache.org/repos/asf?p=lucene-solr.git;h=1aeb1dc ]
LUCENE-9669: Restore Lucene70Codec (#2221)
This change restores the Lucene70Codec for file format compatibility of
indices that are created within the Lucene 7 major version. These indices
can be opened via an expert API on DirectoryReader in read-only mode. Changes
to these indices are prohibited and will be rejected by the IndexWriter.
In fact, IndexWriter will not open an index that is created with a major version
less than N-1 to the current major version.
> 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
> Assignee: Simon Willnauer
> Priority: Major
> Fix For: master (9.0)
>
> Time Spent: 3.5h
> Remaining Estimate: 0h
>
> 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/%3ccaahmpkg1bd9rtmvfr4jweygyaxwz5osy1hv9ubzks0fhsfn...@mail.gmail.com%3E]
>
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