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From "仇寅" <qiuyi...@software.nju.edu.cn>
Subject Re: Does Lucene support partition-by-keyword indexing?
Date Sun, 02 Mar 2008 16:09:51 GMT
Hi,

I am just doing some simple research on P2P searches. In my
assumption, all nodes in this system index their own documents and
each node will be able to search all the documents in the network
(including others'). This is just like file sharing. The simple
approach is to keep all the indices locally and to flood the query
along the network. But this may consume much network bandwidth because
too much nodes, which are not relevant to one certain query, are
involved in handling a query.

So I want to introduce DHT into my system to solve this problem. First
let me explain what DHT is if you don't know about it. Distributed
Hash Table implements a function lookup(key) on a P2P overlay. It
finds the node of nodeID "nearest"  to a certain "key" efficiently,
usually in logN hops, where N is the total number of nodes. Here the
concept "nearest" is not necessarily geographically, but is defined
the DHT implementation. For example, one implementation would define a
key is nearest to a nodeID when |nodeID-key| is the minimum value. DHT
is widely used in P2P file sharing tools, like Azureus and eMule.

In my design, why I want to partition the indices by keyword is to
well facilitate this lookup(key) function. Suppose we have two terms:
A and B. Term A's docList contains 1, 2, and 3; while term B's docList
contains 2, 3, and 4. To partition the indices, we hashes terms A and
B to two keys. We transfer the indices to the corresponding nodes
after calling lookup(key). To handle this query, we would again hash
these two terms to two keys, and locate the nodes responsible for the
terms, respectively, also using the lookup(key) function. We finally
do an intersection and get the result "2 and 3". Of course here we
only get a pointer to the actual indexed document, so we still to
fetch the document content after this.

And I don't think this scheme brings lots of single point of failures
because indices can be replicated on several nodes whose nodeID's are
near to each other. Even one node is down, the lookup(key) function
still finds a substituting node.

As I don't know how to partition the indices by keyword yet, my design
now is to keep the Lucene indices locally and writes the simplified
inverted indices to the nodes responsible for certain keywords. For
example, node X has the postings like:

termA: doc1, doc2, doc3
termB: doc2, doc3, doc4

and lookup(hash(termA)) returns node Y, lookup(hash(termB)) returns node Z.

Then in the indexing process, node X writes the info "termA:
nodeX.doc1, nodeX.doc2, nodeX.doc3" on node Y, an the info "termB:
nodeX.doc2, nodeX.doc3, nodeX.doc4" on node Z. At the final step, we
follow this info and go back to node X to get the Lucene Field values
and the real documents.

Any idea or feedback on my preliminary design?

On Sun, Mar 2, 2008 at 3:48 PM, Uwe Goetzke
<uwe.goetzke@healy-hudson.com> wrote:
> Hi,
>
>  I do not yet fully understand what you want to achieve.
>  You want to spread the index split by keywords to reduce the time to distribute indexes?
>  And you want the distribute queries to the nodes based on the same split mechanism?
>
>
>  You have several nodes with different kind of documents.
>  You want to build one index for all nodes and split and distribute the index based on
a set of keywords specific to a node. This you want to do to split the queries so "each query
involves communicating with constant number of nodes".
>
>  Do documents at the nodes contain only such keywords? I doubt.
>  So you need anyway a reference where the indexed doc can be found and retrieve it from
its node for display.
>  You could index at each node, merge all indexes from all nodes and distribute the combined
index.
>  On what criteria you can split the queries? If you have a combined index each node can
distribute the queries to other nodes on statistical data found in the term distribution.
>  You need to merge the results anyway.
>
>  I doubt that this kind of overhead is worth the trouble because you introduce a lot
of single points of failure. And the scalability seems limited because you would need to recalibrate
the whole network when a adding a new node. Why don't you distribute the complete index (we
do this after getting it locally zipped and later unzipped on the receiver node, size is less
than one third for transfering). Each node should have some activity indicator. Distribute
the complete query to the node with the smallest activiy. So you get redundancy, do not need
to split queries and merge results. OK, one "evil" query can bring a node "down" but the network
is still working.
>
>  Do you have any results using lucene on a single node for your approach? How many queries
and how many documents do you expect?
>
>  Regards
>
>  Uwe
>
>  -----Ursprüngliche Nachricht-----
>  Von: allenchue@gmail.com [mailto:allenchue@gmail.com] Im Auftrag von ??
>  Gesendet: Sonntag, 2. März 2008 03:05
>  An: java-user@lucene.apache.org
>  Betreff: Re: Does Lucene support partition-by-keyword indexing?
>
>
>
>  Hi,
>
>  I agree with your point that it is easier to partition index by document.
>  But the partition-by-keyword approach has much greater scalability over the
>  partition-by-document approach. Each query involves communicating with
>  constant number of nodes; while partition-by-doc requires spreading the
>  query a long all or many of the nodes. And I am actually doing some small
>  research on this. By the way, the documents to be indexed are not
>  necessarily web pages. They are mostly files stored on each node's file
>  system.
>
>  Node failures are also handled by replicas. The index for each term will be
>  replicated on multiple nodes, whose nodeIDs are near to each other. This
>  mechanism is handled by the underlying DHT system.
>
>  So any idea how can partition index by keyword in lucene? Thanks.
>
>  On Sun, Mar 2, 2008 at 5:50 AM, Mathieu Lecarme <mathieu@garambrogne.net>
>  wrote:
>
>  > The easiest way is to split index by Document. In Lucene, index
>  > contains Document and inverse index of Term. If you wont to put Term
>  > in different place, Document will be duplicated on each index, with
>  > only a part of their Term.
>  >
>  > How will you manage node failure in your network?
>  >
>  > They were some trial to build big p2p search engine to compet with
>  > Google, but, it will be easier to split by Document.
>  >
>  > If you have to many computers and want to see them working together,
>  > why don't use Nutch with Hadoop?
>  >
>  > M.
>  > Le 1 mars 08 à 19:16, Yin Qiu a écrit :
>  >
>  > > Hi,
>  > >
>  > > I'm planning to implement a search infrastructure on a P2P overlay. To
>  > > achieve this, I want to first distribute the indices to various nodes
>  > > connected by this overlay. My approach is to partition the indices by
>  > > keyword, that is, one node takes care of certain keywords (or
>  > > terms). When a
>  > > simple TermQuery is encountered, we just find the node associated
>  > > with that
>  > > term (with distributed hash table) and get the result. And suppose a
>  > > BooleanQuery is issued, we contact all the nodes involved in this
>  > > query and
>  > > finally merge the result.
>  > >
>  > > So my question is: does Lucene support partitioning the indices by
>  > > keywords?
>  > >
>  > > Thanks in advance.
>  > >
>  > > --
>  > > Look before you leap
>  > > -------------------------------------------
>  >
>  >
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