Federated Search over HTTP
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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
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:-
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- 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:
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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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