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From "Michael McCandless (JIRA)" <>
Subject [jira] Updated: (LUCENE-710) Implement "point in time" searching without relying on filesystem semantics
Date Fri, 09 Mar 2007 09:44:24 GMT


Michael McCandless updated LUCENE-710:

    Attachment: LUCENE-710.take3.patch

Woops, looks like the commit for LUCENE-825 messed up the patch.  OK I updated and re-diff'd
and attached take3.

It's too bad we don't have a patch that's better integrated with svn such that if even you
have a more recent svn revision checked out, applying the patch would do so back against the
revision it was based on, and then svn would merge the changes committed to the trunk since
then.  In this case an svn update on the checkout with the diffs produced no conflicts, so
if we had such a combined patch tool, it would have worked find here.  I suppose the person
applying the patch could first "svn update" to its base revision, apply the patch, then svn
up, but that's kind of a hassle

> Implement "point in time" searching without relying on filesystem semantics
> ---------------------------------------------------------------------------
>                 Key: LUCENE-710
>                 URL:
>             Project: Lucene - Java
>          Issue Type: Improvement
>          Components: Index
>    Affects Versions: 2.1
>            Reporter: Michael McCandless
>         Assigned To: Michael McCandless
>            Priority: Minor
>         Attachments: LUCENE-710.patch, LUCENE-710.take2.patch, LUCENE-710.take3.patch
> This was touched on in recent discussion on dev list:
> and then more recently on the user list:
> Lucene's "point in time" searching currently relies on how the
> underlying storage handles deletion files that are held open for
> reading.
> This is highly variable across filesystems.  For example, UNIX-like
> filesystems usually do "close on last delete", and Windows filesystem
> typically refuses to delete a file open for reading (so Lucene retries
> later).  But NFS just removes the file out from under the reader, and
> for that reason "point in time" searching doesn't work on NFS
> (see LUCENE-673 ).
> With the lockless commits changes (LUCENE-701 ), it's quite simple to
> re-implement "point in time searching" so as to not rely on filesystem
> semantics: we can just keep more than the last segments_N file (as
> well as all files they reference).
> This is also in keeping with the design goal of "rely on as little as
> possible from the filesystem".  EG with lockless we no longer re-use
> filenames (don't rely on filesystem cache being coherent) and we no
> longer use file renaming (because on Windows it can fails).  This
> would be another step of not relying on semantics of "deleting open
> files".  The less we require from filesystem the more portable Lucene
> will be!
> Where it gets interesting is what "policy" we would then use for
> removing segments_N files.  The policy now is "remove all but the last
> one".  I think we would keep this policy as the default.  Then you
> could imagine other policies:
>   * Keep past N day's worth
>   * Keep the last N
>   * Keep only those in active use by a reader somewhere (note: tricky
>     how to reliably figure this out when readers have crashed, etc.)
>   * Keep those "marked" as rollback points by some transaction, or
>     marked explicitly as a "snaphshot".
>   * Or, roll your own: the "policy" would be an interface or abstract
>     class and you could make your own implementation.
> I think for this issue we could just create the framework
> (interface/abstract class for "policy" and invoke it from
> IndexFileDeleter) and then implement the current policy (delete all
> but most recent segments_N) as the default policy.
> In separate issue(s) we could then create the above more interesting
> policies.
> I think there are some important advantages to doing this:
>   * "Point in time" searching would work on NFS (it doesn't now
>     because NFS doesn't do "delete on last close"; see LUCENE-673 )
>     and any other Directory implementations that don't work
>     currently.
>   * Transactional semantics become a possibility: you can set a
>     snapshot, do a bunch of stuff to your index, and then rollback to
>     the snapshot at a later time.
>   * If a reader crashes or machine gets rebooted, etc, it could choose
>     to re-open the snapshot it had previously been using, whereas now
>     the reader must always switch to the last commit point.
>   * Searchers could search the same snapshot for follow-on actions.
>     Meaning, user does search, then next page, drill down (Solr),
>     drill up, etc.  These are each separate trips to the server and if
>     searcher has been re-opened, user can get inconsistent results (=
>     lost trust).  But with, one series of search interactions could
>     explicitly stay on the snapshot it had started with.

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