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From Michael McCandless <>
Subject Re: More IndexDeletionPolicy questions
Date Sun, 02 Mar 2008 09:33:54 GMT
Unfortunately, there is currently no way to ask an IndexReader
(DirectoryIndexReader) which commit point it's using.

I think it makes sense to add this.  I'll open a Jira issue.

More answers below:

Tim Brennan wrote:

> Is there a direct way to ask an IndexReader what segment it is  
> pointing at?  That would make implementing custom deletion policies  
> a LOT easier.
> It seems like it should be pretty simple -- keep a list of open  
> IndexReaders, track what Segment files they're pointing to, and in  
> onCommit don't delete those segments.

This implies you have multiple readers in a single JVM?  If so, you  
should not need to make a custom deletion policy to handle this case  
-- the OS should be properly protecting open files from deletion.   
Can you  shed more light on the bigger picture here?

> Unfortunately it ends up being very difficult to directly determine  
> what Segment an IndexReader is pointing to.  Is there some  
> straightforward way that I'm missing -- all I've managed to do so  
> far is to remember the most recent one from onCommit/onInit and use  
> that one....that works OK, but makes bootstrapping a pain if you  
> try to open a Reader before you've opened the writer once.
> Also, when I use IndexReader.reopen(), can I assume that the newly  
> returned reader is pointing at the "most recent" segment?  I think  
> so...

Yes, except you have a synchronization challenge: if the writer is in  
the process of committing just as your reader opens you can't be  
certain whether the reader got the new commit or the previous one.   
If you have external synchronization to ensure reader only re-opens  
after writer has fully committed then this isn't an issue.

> Here's a sequence of steps that happen in my app:
> 0) open writer, onInit tells me that seg_1 is the most recent segment
> 1) Open reader, assume it is pointing to seg_1 from (0)
> 2) New write commits into seg_2, don't delete seg_1 b/c of (1)
> 3) Call reader.reopen() on the reader from (1) reader is  
> pointing to seg_2 now?
> 4) seg_1 stays around until the next time I open or commit a  
> writer, then it is removed.
> Does that seem reasonable?
> --tim
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