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From Michael Busch <busch...@gmail.com>
Subject Flexible indexing design (was Re: Pooling of posting objects in DocumentsWriter)
Date Wed, 09 Apr 2008 13:35:31 GMT
Thanks for your quick answers.

Michael McCandless wrote:
> Hi Michael,
> 
> I've actually been working on factoring DocumentsWriter, as a first
> step towards flexible indexing.
> 

Cool, yeah separating the DocumentsWriter into multiple classes 
certainly helped understanding the complex code better.

> I agree we would have an abstract base Posting class that just tracks
> the term text.
> 
> Then, DocumentsWriter manages inverting each field, maintaining the
> per-field hash of term Text -> abstract Posting instances, exposing
> the methods to write bytes into multiple streams for a Posting in the
> RAM "byte slices", and then read them back when flushing, etc.
> 
> And then the code that writes the current index format would plug into
> this and should be fairly small and easy to understand.  For example,
> frq/prx postings and term vectors writing would be two plugins to the
> "inverted terms" API; it's just that term vectors flush after every
> document and frq/prx flush when RAM is full.
> 

I think this makes sense. We also need to come up with a good solution 
for the dictionary, because a term with frq/prx postings needs to store 
two (or three for skiplist) file pointers in the dictionary, whereas e. 
g. a "binary" posting list only needs one pointer.

> Then there would also be plugins that just tap into the entire
> document (don't need inversion), like FieldsWriter.
> 
> There are still alot of details to work out...

Definitely. For example, we should think about the Field APIs. Since we 
don't have global field semantics in Lucene I wonder how to handle 
conflict cases, e. g. when a document specifies a different posting list 
format than a previous one for the same field. The easiest way would be 
to not allow it and throw an exception. But this is kind of against 
Lucene's way of dealing with fields currently. But I'm scared of the 
complicated code to handle conflicts of all the possible combinations of 
posting list formats. KinoSearch doesn't have to worry about this, 
because it has a static schema (I think?), but isn't as flexible as Lucene.

> 
>> The DocumentsWriter does pooling of the Posting instances and I'm 
>> wondering how much this improves performance.
> 
> We should retest this.  I think it was a decent difference in
> performance but I don't remember how much.  I think the pooling can
> also be made generic (handled by DocumentsWriter).  EG the plugin
> could expose a "newPosting()"  method.
> 

Yeah, but for code simplicity let's really figure out first how much 
pooling helps at all.

> Mike
> 
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