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From "Jason Rutherglen" <>
Subject Re: Scaling
Date Fri, 18 Jul 2008 13:49:43 GMT
RMI search in Lucene uses[] docFreqs(Term[] terms) to obtain
the docfreqs for all terms in a the query from each server.  Which it then
turns into a globalized Weight that is submitted to all the Searchables
(servers).  Look at MultiSearcher.  This is fine for most systems even with
the network traffic.

There are ways to mitigate network traffic at the greater than 100 server
mark by locating a proxy server at each rack.  This way the main search
client does not have for example 5000 sockets open to 5000 servers, it only
has 500 sockets open to the 500 proxy servers with 100 server per proxy.
The intensive local network traffic is performed by the search proxy which
then returns the results to the main search client.

In web search a global term frequency database is used.  This does not work
for many Lucene instances where the the index is updated frequently.  It
still could be done with indexes that are updates often however it would
seem to require a lot of work with possibly little to gain, unless you want
to offer the user 0.05 second response times.

On Fri, Jul 18, 2008 at 3:49 AM, Eric Bowman <> wrote:

> Jason Rutherglen wrote:
>> The scaling per machine should be linear.  The overhead from the network
>> is
>> minimal because the Lucene object sizes are not impacting.  Google
>> mentions
>> in one of their early white papers on scaling
>> that they have sub
>> indexes which are now popularly called shards over which an individual
>> thread performs a search over.  Executed in parallel
>> (ParallelMultiSearcher
>> which does not use thread pooling) the response time will be faster than
>> using a single thread assuming part of the indexes are in the system
>> cache.
>> A query is simply an iteration so it is easy to see how parallelization
>> speeds up response times.  Queries per second should ideally be solved by
>> adding more hardware with the same indexes on each server.  Then further
>> dividing these into what can be termed cells which represent different
>> indexes on sets of servers.
>> Having a large index on a single machine does not scale well because most
>> of
>> the index will not be in the system cache.  If the index grows so does the
>> response time.  Dividing the index up into shards and cells allows for
>> efficient scaling which is proven at the big G.  It puts more of the total
>> index in the system cache of many machines.
>> The general assumption is that hardware is cheap and can be added easily,
>> search systems can take advantage of this and parallelize as much as
>> possible, per server, per application.
> One thing I have trouble understanding is how scoring works in this case.
>  Does Lucene really "just work", or are there special things we have to do
> to make sure that the scores are coherent so we can actually decide which
> was the best match?  What kind of constraints are there when breaking up the
> index into parts to make sure scoring remains coherent?
> Thanks,
> Eric
> --
> Eric Bowman
> Boboco Ltd
> +35318394189/+353872801532
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