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From Michael McCandless <>
Subject Re: Realtime Search for Social Networks Collaboration
Date Thu, 11 Sep 2008 12:29:52 GMT

Right, there would need to be a snapshot taken of all terms when  
IndexWriter.getReader() is called.

This snapshot would 1) hold a frozen int docFreq per term, and 2) sort  
the terms so TermEnum can just step through them.  (We might be able  
to delay this sorting until the first time something asks for it).   
Also, it must merge this data from all threads, since each thread  
holds its hash per field.  I've got a rough start at coding this up...

The costs are clearly growing, in order to keep the "point in time"  
feature of this RAMIndexReader, but I think are still well contained  
unless you have a really huge RAM buffer.

Flushing is still tricky because we cannot recycle the byte block  
buffers until all running TermDocs/TermPositions iterations are  
"finished".  Alternatively, I may just allocate new byte blocks and  
allow the old ones to be GC'd on their own once running iterations are  


Jason Rutherglen wrote:

> Hi Mike,
> There would be a new sorted list or something to replace the
> hashtable?  Seems like an issue that is not solved.
> Jason
> On Tue, Sep 9, 2008 at 5:29 AM, Michael McCandless
> <> wrote:
>> This would just tap into the live hashtable that DocumentsWriter*  
>> maintain
>> for the posting lists... except the docFreq will need to be copied  
>> away on
>> reopen, I think.
>> Mike
>> Jason Rutherglen wrote:
>>> Term dictionary?  I'm curious how that would be solved?
>>> On Mon, Sep 8, 2008 at 3:04 PM, Michael McCandless
>>> <> wrote:
>>>> Yonik Seeley wrote:
>>>>>> I think it's quite feasible, but, it'd still have a "reopen"  
>>>>>> cost in
>>>>>> that
>>>>>> any buffered delete by term or query would have to be  
>>>>>> "materialiazed"
>>>>>> into
>>>>>> docIDs on reopen.  Though, if this somehow turns out to be a  
>>>>>> problem,
>>>>>> in
>>>>>> the
>>>>>> future we could do this materializing immediately, instead of
>>>>>> buffering,
>>>>>> if
>>>>>> we already have a reader open.
>>>>> Right... it seems like re-using readers internally is something we
>>>>> could already be doing in IndexWriter.
>>>> True.
>>>>>> Flushing is somewhat tricky because any open RAM readers would  
>>>>>> then
>>>>>> have
>>>>>> to
>>>>>> cutover to the newly flushed segment once the flush completes,  
>>>>>> so that
>>>>>> the
>>>>>> RAM buffer can be recycled for the next segment.
>>>>> Re-use of a RAM buffer doesn't seem like such a big deal.
>>>>> But, how would you maintain a static view of an index...?
>>>>> IndexReader r1 = indexWriter.getCurrentIndex()
>>>>> indexWriter.addDocument(...)
>>>>> IndexReader r2 = indexWriter.getCurrentIndex()
>>>>> I assume r1 will have a view of the index before the document was
>>>>> added, and r2 after?
>>>> Right, getCurrentIndex would return a MultiReader that includes
>>>> SegmentReader for each segment in the index, plus a "RAMReader"  
>>>> that
>>>> searches the RAM buffer.  That RAMReader is a tiny shell class  
>>>> that would
>>>> basically just record the max docID it's allowed to go up to (the  
>>>> docID
>>>> as
>>>> of when it was opened), and stop enumerating docIDs (eg in the  
>>>> TermDocs)
>>>> when it hits a docID beyond that limit.
>>>> For reading stored fields and term vectors, which are now flushed
>>>> immediately to disk, we need to somehow get an IndexInput from the
>>>> IndexOutputs that IndexWriter holds open on these files.  Or,  
>>>> maybe, just
>>>> open new IndexInputs?
>>>>> Another thing that will help is if users could get their hands  
>>>>> on the
>>>>> sub-readers of a multi-segment reader.  Right now that is hidden  
>>>>> in
>>>>> MultiSegmentReader and makes updating anything incrementally
>>>>> difficult.
>>>> Besides what's handled by MultiSegmentReader.reopen already, what  
>>>> else do
>>>> you need to incrementally update?
>>>> Mike
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