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Date: Fri, 2 Mar 2007 07:53:51 -0800 (PST)
From: "Michael McCandless (JIRA)" <jira@apache.org>
To: java-dev@lucene.apache.org
Subject: [jira] Updated: (LUCENE-710) Implement "point in time" searching
 without relying on filesystem semantics
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     [ https://issues.apache.org/jira/browse/LUCENE-710?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

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

    Attachment: LUCENE-710.patch

OK, I've attached a patch to implement "commit on close" and "custom
deletion policies".  The design is exactly what's described above.

There are no changes to the file format.

All tests pass and I've added additional tests for this new
functionality.

Summary of the external changes:

  * For "commit on close":

    - Added new IndexWriter constructors that take "autoCommit"
      boolean: if it's false, then readers will not see any actions
      done by this writer (no new segments_N is written) until
      writer.close() is called.

    - Added IndexWriter.abort() which closes the writer without
      committing, cleaning up any temp files it had added to the
      index.

  * For "custom deletion policies":

    - Created IndexDeletionPolicy interface and added constructors to
      IndexReader/IndexWriter allowing you to specify a deletion
      policy.

    - Created IndexCommitPoint interface: this is passed to the
      deletion policy to represent each commit.  The policy calls the
      delete method on this interface to remove a commit.

    - Created one deletion policy (KeepOnlyLastCommitDeletionPolicy)
      and made that the default policy.  (The unit test for this has
      other "interesting" policies like "delete by age since this
      commit was obsoleted" initially discussed on java-dev.)

Summary of internal changes:

  * Created "files()" method in SegmentInfo (and changed
    SegmentReader.files() to use it).

  * Changed IndexFileDeleter to use reference counting to keep track
    of which files are deletable because no commit(s) (nor the
    in-memory SegmentInfos) reference them.

    This is a nice simplification of IndexFileDeleter: previously it
    had detailed knowledge about which files, extensions, etc., to
    look for and delete.  Now it has far less of that because it
    relies entirely on SegmentInfo.files() to compute that.

  * Changed IndexReader/IndexWriter to not directly delete files and
    instead notify IndexFileDeleter when there has been a change to
    the in-memory SegmentInfos.  The deleter then incref/decref's to
    determine what files can safely be deleted.

    This is also a nice simplification for the same reason as above:
    now the writers just make changes to SegmentInfo(s) without having
    to compute/track the consequences to specific index files.

  * Simplified the fix for LUCENE-702 (addIndexes corrupts index on
    disk full) to just temporarily set autoCommit=false if it's not
    already.

  * Added get/setDefaultInfoStream to IndexWriter so you could see
    things that happen during IndexWriter constructor.

  * No longer store/propogate persistent IndexFileDelter inside
    IndexReader (removed protected method get/setDeleter).  This is a
    nice simplification because the deleter is now only needed briefly
    during "commit()".

  * Reworked the toplevel javadoc for IndexWriter.

  * Added try/finally to remove a partially written segments_N if we
    hit IOException when trying to write it.

  * Other small change (small refactoring, fixes to javadocs, fixed
    spelling, etc).


> 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: LUCENE-710.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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