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From "Michael McCandless (JIRA)" <j...@apache.org>
Subject [jira] Created: (LUCENE-701) Lock-less commits
Date Fri, 27 Oct 2006 23:41:32 GMT
Lock-less commits
-----------------

                 Key: LUCENE-701
                 URL: http://issues.apache.org/jira/browse/LUCENE-701
             Project: Lucene - Java
          Issue Type: Improvement
          Components: Index
    Affects Versions: 2.1
            Reporter: Michael McCandless
         Assigned To: Michael McCandless
            Priority: Minor
         Attachments: lockless-commits-patch.txt

This is a patch based on discussion a while back on lucene-dev:

    http://mail-archives.apache.org/mod_mbox/lucene-java-dev/200608.mbox/%3c44E5B16D.4010805@mikemccandless.com%3e

The approach is a small modification over the original discussion (see
Retry Logic below).  It works correctly in all my cross-machine test
case, but I want to open it up for feedback, testing by
users/developers in more diverse environments, etc.

This is a small change to how lucene stores its index that enables
elimination of the commit lock entirely.  The write lock still
remains.

Of the two, the commit lock has been more troublesome for users since
it typically serves an active role in production.  Whereas the write
lock is usually more of a design check to make sure you only have one
writer against the index at a time.

The basic idea is that filenames are never reused ("write once"),
meaning, a writer never writes to a file that a reader may be reading
(there is one exception: the segments.gen file; see "RETRY LOGIC"
below).  Instead it writes to generational files, ie, segments_1, then
segments_2, etc.  Besides the segments file, the .del files and norm
files (.sX suffix) are also now generational.  A generation is stored
as an "_N" suffix before the file extension (eg, _p_4.s0 is the
separate norms file for segment "p", generation 4).

One important benefit of this is it avoids files contents caching
entirely (the likely cause of errors when readers open an index
mounted on NFS) since the file is always a new file.

With this patch I can reliably instantiate readers over NFS when a
writer is writing to the index.  However, with NFS, you are still forced to
refresh your reader once a writer has committed because "point in
time" searching doesn't work over NFS (see LUCENE-673 ).

The changes are fully backwards compatible: you can open an old index
for searching, or to add/delete docs, etc.  I've added a new unit test
to test these cases.

All units test pass, and I've added a number of additional unit tests,
some of which fail on WIN32 in the current lucene but pass with this
patch.  The "fileformats.xml" has been updated to describe the changes
to the files (but XXX references need to be fixed before committing).

There are some other important benefits:

  * Readers are now entirely read-only.

  * Readers no longer block one another (false contention) on
    initialization.

  * On hitting contention, we immediately retry instead of a fixed
    (default 1.0 second now) pause.

  * No file renaming is ever done.  File renaming has caused sneaky
    access denied errors on WIN32 (see LUCENE-665 ).  (Yonik, I used
    your approach here to not rename the segments_N file(try
    segments_(N-1) on hitting IOException on segments_N): the separate
    ".done" file did not work reliably under very high stress testing
    when a directory listing was not "point in time").

  * On WIN32, you can now call IndexReader.setNorm() even if other
    readers have the index open (fixes a pre-existing minor bug in
    Lucene).

  * On WIN32, You can now create an IndexWriter with create=true even
    if readers have the index open (eg see
    www.gossamer-threads.com/lists/lucene/java-user/39265) .


Here's an overview of the changes:

  * Every commit writes to the next segments_(N+1).

  * Loading the segments_N file (& opening the segments) now requires
    retry logic.  I've captured this logic into a new static class:
    SegmentInfos.FindSegmentsFile.  All places that need to do
    something on the current segments file now use this class.

  * No more deletable file.  Instead, the writer computes what's
    deletable on instantiation and updates this in memory whenever
    files can be deleted (ie, when it commits).  Created a common
    class index.IndexFileDeleter shared by reader & writer, to manage
    deletes.

  * Storing more information into segments info file: whether it has
    separate deletes (and which generation), whether it has separate
    norms, per field (and which generation), whether it's compound or
    not.  This is instead of relying on IO operations (file exists
    calls).  Note that this fixes the current misleading
    FileNotFoundException users now see when an _X.cfs file is missing
    (eg http://www.nabble.com/FileNotFound-Exception-t6987.html).

  * Fixed some small things about RAMDirectory that were not
    filesystem-like (eg opening a non-existent IndexInput failed to
    raise IOException; renames were not atomic).  I added a stress
    test against a RAMDirectory (1 writer thread & 2 reader threads)
    that uncovered these.

  * Added option to not remove old files when create=true on creating
    FSDirectory; this is so the writer can do its own [more
    sophisticated because it retries on errors] removal.

  * Removed all references to commit lock, COMMIT_LOCK_TIMEOUT, etc.
    (This is an API change).

  * Extended index/IndexFileNames.java and index/IndexFileNameFilter.java
    with logic for computing generational file names.

  * Changed index/IndexFileNameFilter.java to use a HashSet to check
    file extentsions for better performance.

  * Fixed the test case TestIndexReader.testLastModified: it was
    incorrectly (I think?) comparing lastModified to version, of the
    index.  I fixed that and then added a new test case for version.


Retry Logic (in index/SegmentInfos.java)

If a reader tries to load the segments just as a writer is committing,
it may hit an IOException.  This is just normal contention.  In
current Lucene contention causes a [default] 1.0 second pause then
retry.  With lock-less the contention causes no added delay beyond the
time to retry.

When this happens, we first try segments_(N-1) if present, because it
could be segments_N is still being written.  If that fails, we
re-check to see if there is now a newer segments_M where M > N and
advance if so.  Else we retry segments_N once more (since it could be
it was in process previously but must now be complete since
segments_(N-1) did not load).

In order to find the current segments_N file, I list the directory and
take the biggest segments_N that exists.

However, under extreme stress testing (5 threads just opening &
closing readers over and over), on one platform (OS X) I found that
the directory listing can be incorrect (stale) by up to 1.0 seconds.
This means the listing will show a segments_N file but that file does
not exist (fileExists() returns false).

In order to handle this (and other such platforms), I switched to a
hybrid approach (originally proposed by Doron Cohen in the original
thread): on committing, the writer writes to a file "segments.gen" the
generation it just committed.  It writes 2 identical longs into this
file.  The retry logic, on detecting that the directory listing is
stale falls back to the contents of this file.  If that file is
consistent (the two longs are identical), and, the generation is
indeed newer than the dir listing, it will use that.

Finally, if this approach is also stale, we fallback to stepping
through sequential generations (up to a maximum # tries).  If all 3
methods fail, we throw the original exception we hit.

I added a static method SegmentInfos.setInfoStream() which will print
details of retry attempts.  In the patch it's set to System.out right
now (we should turn off before a real commit) so if there are problems
we can see what retry logic had done.


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