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From "patrick o'leary" <>
Subject Re: [jira] Updated: (SOLR-303) Federated Search over HTTP
Date Wed, 19 Sep 2007 18:48:25 GMT
is this still blocked by solr-281?

Stu Hood (JIRA) wrote:
>      [
> Stu Hood updated SOLR-303:
> --------------------------
>     Attachment: fedsearch.stu.patch
> I got the rest of the DF issues resolved: please refer to the attached and ignore my
earlier comments (some of them were faulty).
> Here is a patch that is very similar to your last patch, but with my fixes included.
If you `diff fedsearch.stu.patch fedsearch.patch` you should be able to see what I did.
> The final (minor) issue I've found, is that when I strip the 'start' parameter in SecondQPhaseComponent.createSecondPhaseParams,
it gets stripped from the response that is returned to the user as well (although it is honored
in the results).
> Thanks again!
>> Federated Search over HTTP
>> --------------------------
>>                 Key: SOLR-303
>>                 URL:
>>             Project: Solr
>>          Issue Type: New Feature
>>          Components: search
>>            Reporter: Sharad Agarwal
>>            Priority: Minor
>>         Attachments: fedsearch.patch, fedsearch.patch, fedsearch.patch, fedsearch.stu.patch
>> Motivated by
>> "Index view consistency between multiple requests" requirement is relaxed in this
>> Does the federated search query side. Update not yet done.
>> Tries to achieve:-
>> ------------------------
>> - The client applications are totally agnostic to federated search. The federated
search and merging of results are totally behind the scene in Solr in request handler . Response
format remains the same after merging of results.
>> The response from individual shard is deserialized into SolrQueryResponse object.
The collection of SolrQueryResponse objects are merged to produce a single SolrQueryResponse
object. This enables to use the Response writers as it is; or with minimal change.
>> - Efficient query processing with highlighting and fields getting generated only
for merged documents. The query is executed in 2 phases. First phase gets the doc unique keys
with sort criteria. Second phase brings all requested fields and highlighting information.
This saves lot of CPU in case there are good number of shards and highlighting info is requested.
>> Should be easy to customize the query execution. For example: user can specify to
execute query in just 1 phase itself. (For some queries when highlighting info is not required
and number of fields requested are small; this can be more efficient.)
>> - Ability to easily overwrite the default Federated capability by appropriate plugins
and request parameters. As federated search is performed by the RequestHandler itself, multiple
request handlers can easily be pre-configured with different federated search settings in
>> - Global weight calculation is done by querying the terms' doc frequencies from all
>> - Federated search works on Http transport. So individual shard's VIP can be queried.
Load-balancing and Fail-over taken care by VIP as usual.
>> -Sub-searcher response parsing as a plugin interface. Different implementation could
be written based on JSON, xml SAX etc. Current one based on XML DOM.
>> HOW:
>> -------
>> A new RequestHandler called MultiSearchRequestHandler does the federated search on
multiple sub-searchers, (referred as "shards" going forward). It extends the RequestHandlerBase.
handleRequestBody method in RequestHandlerBase has been divided into query building and execute
methods. This has been done to calculate global numDocs and docFreqs; and execute the query
efficiently on multiple shards.
>> All the "search" request handlers are expected to extend MultiSearchRequestHandler
class in order to enable federated capability for the handler. StandardRequestHandler and
DisMaxRequestHandler have been changed to extend this class.
>> The federated search kicks in if "shards" is present in the request parameter. Otherwise
search is performed as usual on the local index. eg. shards=local,host1:port1,host2:port2
will search on the local index and 2 remote indexes. The search response from all 3 shards
are merged and serviced back to the client. 
>> The search request processing on the set of shards is performed as follows:
>> STEP 1: The query is built, terms are extracted. Global numDocs and docFreqs are
calculated by requesting all the shards and adding up numDocs and docFreqs from each shard.
>> STEP 2: (FirstQueryPhase) All shards are queried. Global numDocs and docFreqs are
passed as request parameters. All document fields are NOT requested, only document uniqFields
and sort fields are requested. MoreLikeThis and Highlighting information are NOT requested.
>> STEP 3: Responses from FirstQueryPhase are merged based on "sort", "start" and "rows"
params. Merged doc uniqField and sort fields are collected. Other information like facet and
debug is also merged.
>> STEP 4: (SecondQueryPhase) Merged doc uniqFields and sort fields are grouped based
on shards. All shards in the grouping are queried for the merged doc uniqFields (from FirstQueryPhase),
highlighting and moreLikeThis info.
>> STEP 5: Responses from all shards from SecondQueryPhase are merged.
>> STEP 6: Document fields , highlighting and moreLikeThis info from SecondQueryPhase
are merged into FirstQueryPhase response.
>> TODO:
>> -Support sort field other than default score
>> -Support ResponseDocs in writers other than XMLWriter
>> -Http connection timeouts
>> -Merging of facets by "top n terms of field f" 
>> Scope for Performance optimization:-
>> -Search shards in parallel threads
>> -Http connection Keep-Alive ?
>> -Cache global numDocs and docFreqs
>> -Cache Query objects in handlers ??
>> Would appreciate feedback on my approach. I understand that there would be lot things
I might have over-looked. 


Patrick O'Leary

You see, wire telegraph is a kind of a very, very long cat. You pull his tail in New York
and his head is meowing in Los Angeles.
 Do you understand this? 
And radio operates exactly the same way: you send signals here, they receive them there. The
only difference is that there is no cat.
  - Albert Einstein

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