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From Leo Galambos <Le...@seznam.cz>
Subject Re: High Capacity (Distributed) Crawler
Date Tue, 10 Jun 2003 12:38:06 GMT
Otis Gospodnetic wrote:

>>What interface do you need for Lucene? Will you use PUSH (=the robot 
>>will modify Lucene's index) or PULL (=the engine will get deltas from
>>
>>the robot) mode? Tell me what you need and I will try to do all my
>>best.
>>    
>>
>
>I'd imagine one would want to use it in the PUSH mode (e.g. the crawler
>fetches a web page and adds it to the searchable index).
>How does PULL mode work?  I've never heard of web crawlers being used
>in the PULL mode.  What exactly does that mean, could you please
>describe it?
>  
>
It is a long story, so I will assume, that everything runs on a single 
box - it is the most simple case.
"[x]" will denote points, where Lucene may have problems with a fast 
implementation, I guess.

Crawler: The crawler stores meta and body of all documents. If you want 
to retrieve the document meta or body (knowing its URI), it costs O(1) 
(2 seeks and 2 read requests in auxiliary data structures). After this 
retrieval you also get a direct handle to meta and body - then the price 
of retrieval becomes O(1), but no extra seeks in any structures. The 
handle is persistent and is related to URI. The meta and body is updated 
as soon as the crawler fetches a new fresh copy.

Engine: engine stores the handle for each document. Moreover it knows 
the last (highest) handle, which is stored in the main index. So the 
trick is this:
1) build up an auxiliary index from new documents. The new documents are 
documents which have their handle greater than the last handle which is 
known to the engine, thus you can iterate them easily - this process can 
run in a separate thread
2) consult the changes. You read meta, which are stored in index, and 
test if they are obsolete (note: you have already got the handle, so it 
smokes). If so, you denote the respective document as "deleted" and its 
new version (if any) is appended to another index - the index of 
changes. The insertion to the index runs in a separate thread, so the 
main thread is not blocked. BTW: [x] The documents, which are not 
modified, may modify their ranks (depthrank, pagerank, frequencyrank 
etc) in this round.

[x] The two auxiliary indices are then merged with the main index.

Obviously, the weak point is the test if anything is changed. This can 
be easily solved with the index dynamization I use. Despite Lucene, I 
order barrels (segments in your terminology) by their size. I do not 
want to describe all the details - I hate long e-mails ;-), but the 
dynamization guarantees that:
a) the query time is never worse than 8x, comparing with 
fully-optimalized index (if you buy 8x faster HW, you overcome this easily)
b) the documents, which are often modified, are stored in small barrels 
of the main index. It means, that their actualization is fast.

So, I process only the small barrels once a day, and the larger ones 
less often. If we say, that 5M of docs are updated daily, PULL mode can 
handle this load in few minutes. Unfortunately, the slowest point is the 
HTML parser which may run few hours :-(.

If you want to actualize other 10^10 crap pages once a month, it can be 
done too, but it is out of my first assumption above ;-).

-g-


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