[jira] Commented: (SOLR-303) Federated Search over HTTP

["Yonik Seeley (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/SOLR-303?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12512983 ] 

Yonik Seeley commented on SOLR-303:
-----------------------------------

Thanks for kicking this off Sharad!

> "Index view consistency between multiple requests" requirement is relaxed in this implementation. 

Do you have plans to remedy that?  Or do you think that most people are OK with inconsistencies that could arise?

> Load-balancing and Fail-over taken care by VIP as usual

In a static configuration, this works OK, but it might be nice to support a more dynamic environment where extra shards could be easily added.  It might also be the case that a custom partitioning function could be implemented (such as improving caching by partitioning queries, etc) or it may be more efficient to do the second phase of a query on the same shard copy as the first phase.
In that case it might make sense load balancing across shards from Solr . The  VIP solution would map to the simplest case of a single copy of each shard, thus a LB could still be used if desired.

> STEP 1: The query is built, terms are extracted. 

Where are terms extracted from (some queries require index access)?  This should be delegated to the shards, no?  It can be the same step that gets the docFreqs from the shards (pass the query, *not* the terms).  Step 1 should also be optional for those that can make do with local idf factors.

In order to facilitate custom logic in a distributed environment,
I think we should base the solution on something like
https://issues.apache.org/jira/browse/SOLR-281
With additional hooks for distributed search.
This should allow relatively independent parts of query processing to piggyback in the same network request (for example, the first steps to querying and faceting can be added to a single request, and highlighting and stored field retrieval can be done in conjunction).

Any thoughts on RMI vs HTTP for the searcher-subsearcher interface?  



> Federated Search over HTTP
> --------------------------
>
>                 Key: SOLR-303
>                 URL: https://issues.apache.org/jira/browse/SOLR-303
>             Project: Solr
>          Issue Type: New Feature
>          Components: search
>            Reporter: Sharad Agarwal
>            Priority: Minor
>         Attachments: fedsearch.patch
>
>
> Motivated by http://wiki.apache.org/solr/FederatedSearch
> "Index view consistency between multiple requests" requirement is relaxed in this implementation.
> 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 solrconfig.xml
> - Global weight calculation is done by querying the terms' doc frequencies from all shards.
> - 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
> OPEN ISSUES;
> -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. 

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