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From Shai Erera <ser...@gmail.com>
Subject Atomic optimize() + commit()
Date Thu, 02 Apr 2009 12:22:42 GMT
Hi

I've run into a problem in my code when I upgraded to 2.4. I am not sure if
it is a real problem, but I thought I'd let you know anyway. The following
is a background of how I ran into the issue, but I think the discussion does
not necessarily involve my use of Lucene.

I have a class which wraps all Lucene-related operations, i.e., addDocument,
deleteDocument, search and optimize (those are the important ones for this
email). It maintains an IndexWriter open, through which it does the
add/delete/optimize operations and periodically opens an IndexReader for the
search operations using the reopen() API.

The application indexes operations (add, delete, update) by multiple
threads, while there's a manager which after the last operation has been
processed, calls commit, which does writer.commit(). I also check from time
to time if the index needs to be optimized and optimizes if needed (the
criteria for when to do it is irrelevant now). I also have a unit test which
does several add/update/delete operations, calls optimize and checks the
number of deleted documents. It expects to find 0, since optimize has been
called and after I upgraded to 2.4 it failed.

Now ... with the move to 2.4, I discovered that optimize() does not commit
automatically and I have to call commit. It's a good place to say that when
I was on 2.3 I used the default autoCommit=true and with the move to 2.4
that default has changed, and being a good citizen, I also changed my code
to call commit when I want and not use any deprecated ctors or rely on
internal Lucene logic. I can only guess that that's why at the end of the
test I still see numDeletedDocs != 0 (since optimize does not commit by
default).

So I went ahead and fixed my optimize() method to do: (1) writer.optimize()
(2) writer.commit().

But then I thought - is this fix correct? Is it the right approach? Suppose
that at the sime time optimize was running, or just between (1) and (2)
there was a context switch, and a thread added documents to the index. Upon
calling commit(), the newly added documents are also committed, without the
caller intending to do so. In my scenario this will probably not be too
catastrophically, but I can imagine scenarios in which someone in addition
to indexing updates a DB and has a virtual atomic commit, which commits the
changes to the index as well as the DB, all the while locking any update
operations. Suddenly that someone's code breaks.

There are a couple of ways I can solve it, like for example synchronizing
the optimize + commit on a lock which all indexing threads will also
synchronize (allowing all of them to index concurrently, but if optimize is
running all are blocked), but that will hold all my indexing threads. Or, I
can just not call commit at the end, relying on the workers manager to
commit at the next batch indexing work. However, during that time the
readers will search on an unoptimized index, with deletes, while they can
search on a freshly optimized index with no deletes (and less segments).

The problem with those solutions is that they are not intuitive. To start
with, the Lucene documentation itself is wrong - In IndexWriter.commit() it
says: "Commits all pending updates (added & deleted documents)" - optimize
is not mentioned (shouldn't this be fixed anyway?). Also, notice that the
problem stems from the fact that the optimize operation may be called by
another thread, not knowing there are update operations running. Lucene
documents that you can call addDocument while optimize() is running, so
there's no need to protect against that. Suddenly, we're requiring every
search application developer to disregard the documentation and think to
himself "do I want to allow optimize() to run concurrently with
add/deletes?". I'm not saying that it's wrong, but if you're ok with it, we
should document it.

I wonder though if there isn't room to introduce an atomic optimize() +
commit() in Lucene. The incentive is that optimize is not the same as
add/delete. Add/delete are operations I may want to hide from my users,
because they change the state of the index (i.e., how many searchable
documents are there). Optimize just reorganizes the index, and is supposed
to improve performance. When I call optimize, don't I want it to be
committed? Will I ever want to hold that commit off (taking out edge cases)?
I assume that 99.9% of the time that's what we expect from it.

Now, just adding a call to commit() at the end of optimize() will not solve
it, because that's the same as calling commit outside optimize(). We need
the optimize's commit to only commit its changes. And if there are updates
pending commit - not touch them.

BTW, I've scanned through the documentation and haven't found any mention of
such thing, however I may have still missed it. So if there is already a
solution to that, or such an atomic optimize+commit, I apologize in advance
for forcing you to read such a long email (for those of you who made it thus
far) and appreciate if you give me a reference.

Shai

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