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From Michael McCandless <luc...@mikemccandless.com>
Subject Re: Instantiating a RAMDirectory from a mutating directory
Date Fri, 06 Mar 2009 11:19:21 GMT

This is an interesting challenge!  Responses below...

Kieran Topping wrote:

> Hello,
>
> I would like to be able to instantiate a RAMDirectory from a  
> directory that an IndexWriter in another process might currently be  
> modifying.
>
> Ideally, I would like to do this without any synchronizing or  
> locking. Kind-of like the way in which an IndexReader can open an  
> index in a directory, even if it's currently being modified by an  
> IndexWriter.
>
> However, simply calling:
> RAMDirectory rd = new RAMDirectory("/path/to/index");
> Will not work. It will periodically fail with a  
> FileNotFoundException. It's fairly obvious why this happens:  
> Directory.copy() gets a list of the files it needs to copy, and then  
> copies them into the RAMDirectory instance one-by-one. If, in the  
> meantime, the IndexWriter deletes one of these files, a  
> FileNotFoundException occurs.
>
> One thought that I had was that I would take advantage of the fact  
> that it's possible to open an IndexReader on the mutating directory,  
> and then use the "addIndexes()" method, as follows:
>
>  // 1. create RAMDirectory.
>  RAMDirectory ramDirectory = new RAMDirectory();
>  // 2. create an index in the RAMDirectory.
>  IndexWriter writer = new IndexWriter(ramDirectory, null/ 
> *analyzer*/, true /*create*/) ;
>  // 3. open the (possibly mutating) source index.
>  IndexReader reader = IndexReader.open("/path/to/index");
>  // 4. copy the source index into the RAMDirectory index.
>  writer.addIndexes(new IndexReader [] {reader});
>
> However ... there is a fairly unambiguous warning in  
> IndexWriter.addIndexes()'s documentation:
>
> >>   NOTE: the index in each Directory must not be changed (opened  
> by a writer) while this method is running. This method does not  
> acquire a write lock in each input Directory, so it is up to the  
> caller to enforce this.
>
> I'm slightly confused by this warning though, as IndexReader's  
> documentation implies that it is OK to open an IndexReader in this  
> fashion.

Actually, I believe that NOTE only applies to the two addIndexes  
methods that take Directory.  So I think this approach will work fine  
in general.  Have you hit any problems in testing it?  I'll update the  
javadocs.

The one big downside to this approach is performance: it's a rather  
slow way to copy an index into RAM.  But maybe your indexes are small  
enough that this doesn't matter.

> I'm wondering whether anyone knows the internals of  
> IndexWriter.addIndexes() well enough to know whether my proposed  
> solution will work reliably?
>
> Or, indeed, whether there might be another way of instantiating a  
> RAMDirectory from a directory which might currently be being  
> modified by an IndexWriter?

If you could communicate w/ the separate process doing the writing,  
you could use SnapshotDeletionPolicy (in the writer process) to  
protect a particular point-in-time commit.  This is exactly how a hot  
backup of a Lucene index is done; you would have to  then communicate  
the filenames that IndexCommit (in the writer process) exposes over to  
your 2nd reader process, and copy those files, and then release the  
snapshot back in the writer process.

Alternatively, you could simply use SegmentInfos class (NOTE: it's  
package private, so you'd need code in org.apache.lucene.index  
package, and these APIs can change release-to-release) to open the  
current commit, and then simply copy the files directly (this is the  
API that IndexReader.open does).  To do this, you should subclass the  
FindSegmentsFile class, and override run() to open all referenced  
files, and probably return these open file handles to the code that  
actually does the copying.  You'd need to take some care to handle a  
FileNotFoundException (meaning you need to retry on the next segments  
file), to close any files you had succeeded in opening, else you'll  
leak file descriptors...

Mike

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