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From "Michael McCandless (JIRA)" <j...@apache.org>
Subject [jira] Updated: (LUCENE-710) Implement "point in time" searching without relying on filesystem semantics
Date Thu, 15 Mar 2007 12:15:09 GMT

     [ https://issues.apache.org/jira/browse/LUCENE-710?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Michael McCandless updated LUCENE-710:
--------------------------------------

    Attachment: 710.review.take2.diff

Thanks for the review Doron!

Your added comments & improvements to the variable names are
excellent.  I especially like the new constants (YES, NO, CHECK_DIR,
etc.) in SegmentInfo.  I've tweaked your patch here and there, and
attached a modified patch (710.review.take2.diff).  If you're happy
with that then go ahead and commit it?

> IndexFileDeleter - it is nice that this became non public and somewhat
> simpler.

I especially like that this class now has very little knowledge of
what files "belong" to an index, especially compared to before.  That
knowledge has now been consolidated under SegmentInfo which I think is
the right place for it.

> I don't understand yet why we allow a deletion policy to delete
> *all* commits (including the most recent) - TestDeletionPolicy
> explains this as: "This is useful for adding to a big index w/
> autoCommit =false when you know readers are not using it." - so,
> would I risk losing the big index should uncommited additions fail?
> what does one earn by this? I first thought we should prevent
> (exception) deleting the most recent commit, but I must be missing
> something - could you elaborate on this?

The use case I was thinking of is: say you already have a large index
and then you need to add a bunch more docs to it.  If you are not
allowed to delete the starting commit, then, you will consume
substantially more disk space as you are building your index because
the large segments at the start can't be removed.  This would have
made the "autoCommit false" case unnecessarily worse than the
"autoCommit true" case.  If for a given application the developer is
concerned about safety (losing index due to crash), then the normal
default policy should be used.

> This issue moved some files logic SegmentInfo. The -1/1/0 logic and
> especially with norms is confusing, and at least I have to re-read
> the code carefully each time again and again to be convinced that it
> is correct. It would be nice when we can get rid of some of the
> backward compatibility cases here. Anyhow I added some documentation
> and also replaced the -1/1/0 with constants, I think this makes it
> easier to understand.

Yes the backwards compatibility code (for pre-2.1 indices) is complex.
The good news is by the time we release this in 2.2, most indices that
upgrade to 2.2 will be 2.1.

> Implement "point in time" searching without relying on filesystem semantics
> ---------------------------------------------------------------------------
>
>                 Key: LUCENE-710
>                 URL: https://issues.apache.org/jira/browse/LUCENE-710
>             Project: Lucene - Java
>          Issue Type: Improvement
>          Components: Index
>    Affects Versions: 2.1
>            Reporter: Michael McCandless
>         Assigned To: Michael McCandless
>            Priority: Minor
>         Attachments: 710.review.diff, 710.review.take2.diff, LUCENE-710.patch, LUCENE-710.take2.patch,
LUCENE-710.take3.patch
>
>
> This was touched on in recent discussion on dev list:
>   http://www.gossamer-threads.com/lists/lucene/java-dev/41700#41700
> and then more recently on the user list:
>   http://www.gossamer-threads.com/lists/lucene/java-user/42088
> 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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