Tom Winch created SOLR-7341:
-------------------------------
Summary: xjoin - join data from external sources
Key: SOLR-7341
URL: https://issues.apache.org/jira/browse/SOLR-7341
Project: Solr
Issue Type: New Feature
Components: search
Affects Versions: 4.10.3
Reporter: Tom Winch
Priority: Minor
Fix For: Trunk
h2. XJoin
The "xjoin" SOLR contrib allows external results to be joined with SOLR results
in a query and the SOLR result set to be filtered by the results of an external
query. Values from the external results are made available in the SOLR results
and may also be used to boost the scores of corresponding documents during the
search. The contrib consists of the Java classes XJoinSearchComponent and
XJoinValueSourceParser, which must be configured in solrconfig.xml, and the
interfaces XJoinResultsFactory and XJoinResults, which are implemented by the
user to provide the link between SOLR and the external results source. External
results and SOLR documents are matched via a single configurable attribute (the
"join field"). The contrib JAR solr-xjoin-4.10.3.jar contains these classes and
interfaces and should be included in SOLR's class path from solrconfig.xml, as
should a JAR containing the user implementations of the previously mentioned
interfaces. For example:
{code:xml}
<config>
..
<!-- XJoin contrib JAR file -->
<lib dir="${solr.install.dir:../../..}/dist/" regex="solr-xjoin-\d.*\.jar" />
..
<!-- user implementations of XJoin interfaces -->
<lib path="/path/to/xjoin_test.jar" />
..
</config>
{code}
h2. Java classes and interfaces
h3. XJoinResultsFactory
The user implementation of this interface is responsible for connecting to the
external source to perform a query (or otherwise collect results). Parameters
with prefix "<component name>.external." are passed from the SOLR query URL to
pararameterise the search. The interface has the following methods:
* void init(NamedList args) - this is called during SOLR initialisation, and
passed parameters from the search component configuration (see below)
* XJoinResults getResults(SolrParams params) - this is called during a SOLR
search to generate external results, and is passed parameters from the SOLR
query URL (as above)
For example, the implementation might perform queries of an external source
based on the 'q' SOLR query URL parameter (in full, <component
name>.external.q).
h3. XJoinResults
A user implementation of this interface is returned by the getResults() method
of the XJoinResultsFactory implementation. It has methods:
* Object getResult(String joinId) - this should return a particular result
given the value of the join attribute
* Iterable<String> getJoinIds() - this should return the join attribute values
for all results of the external search
h3. XJoinSearchComponent
This is the central Java class of the contrib. It is a SOLR search component,
configured in solrconfig.xml and included in one or more SOLR request handlers.
It has two main responsibilities:
* Before the SOLR search, it connects to the external source and retrieves
results, storing them in the SOLR request context
* After the SOLR search, it matches SOLR document in the results set and
external results via the join field, adding attributes from the external
results to documents in the SOLR results set
It takes the following initialisation parameters:
* factoryClass - this specifies the user-supplied class implementing
XJoinResultsFactory, used to generate external results
* joinField - this specifies the attribute on which to join between SOLR
documents and external results
* external - this parameter set is passed to configure the XJoinResultsFactory
implementation
For example, in solrconfig.xml:
{code:xml}
<searchComponent name="xjoin_test"
class="org.apache.solr.search.xjoin.XJoinSearchComponent">
<str name="factoryClass">test.TestXJoinResultsFactory</str>
<str name="joinField">id</str>
<lst name="external">
<str name="values">1,2,3</str>
</lst>
</searchComponent>
{code}
Here, the search component instantiates a new TextXJoinResultsFactory during
initialisation, and passes it the "values" parameter (1, 2, 3) to configure it.
To properly use the XJoinSearchComponent in a request handler, it must be
included at the start and end of the component list, and may be configured with
the following query parameters:
* listParameter - external join field values will be placed in this query
parameter for reference by local query parameters
* results - a comma-separated list of attributes from the XJoinResults
implementation (created by the factory at search time) to be included in the
SOLR results
* fl - a comma-separated list of attributes from results objects (contained in
an XJoinResults implementation) to be included in the SOLR results
For example:
{code:xml}
<requestHandler name="/xjoin" class="solr.SearchHandler" startup="lazy">
<lst name="defaults">
..
<bool name="xjoin_test">true</bool>
<str name="xjoin_test.listParameter">xx</str>
<str name="xjoin_test.results">test_count</str>
<str name="xjoin_test.fl">id,value</str>
</lst>
<arr name="first-components">
<str>xjoin_test</str>
</arr>
<arr name="last-components">
<str>xjoin_test</str>
</arr>
</requestHandler>
{code}
h3. XJoinQParserPlugin
This query parser plugin constructs a query from the resulting join ids from
the external search, and is very similar to the TermsQParserPlugin. It takes no
parameters, and the value to be parsed is the name of the XJoin search
component that contains the results.
h3. XJoinValueSourceParser
This class provides a SOLR function that may be used, for example, in a boost
function to weight the result score from external values. The function returns
an attribute value from the external result with matching join attribute. The
external attribute returned is specified by the argument of the function
specification in the SOLR query URL (see below). The parameters for
configuration in solrconfig.xml are:
* xJoinSearchComponent - the name of the XJoin search component containing the
external results
* defaultValue - if the external result has no such attribute, then this value
is returned
For example:
{code:xml}
<valueSourceParser name="test_fn"
class="org.apache.solr.search.xjoin.XJoinValueSourceParser">
<str name="xJoinSearchComponent">xjoin_test</str>
<double name="defaultValue">1.0</double>
</valueSourceParser>
{code}
h3. Mapping between attributes and Java methods
Java method names are converted into attribute (field) names by stripping the
initial "get" or "is" and converting the remainder from CamelCase to
lowercase-with-underscores, and vice versa. For example, getScore() <-> score
or getFooBar() <-> foo_bar.
The field list parameter of XJoinSearchComponent (fl) can be given as *, in
which case all methods beginning 'get' or 'is' are converted into fields in the
SOLR result for the document.
h2. Putting it together - the SOLR query URL
Here is an example SOLR query URL to perform an xjoin:
{noformat}
http://solrserver:8983/solr/collection1/xjoin?defType=edismax&q=*:*&xjoin_test.external.q=foobar&fl=id,score&fq={!xjoin}xjoin_test&bf=test_fn(value)
{noformat}
This might result in the following SOLR response:
{code:xml}
<?xml version="1.0" encoding="UTF-8"?>
<response>
<lst name="responseHeader">
<int name="status">0</int>
<int name="QTime">346</int>
<lst name="params">
..
</lst>
</lst>
<result name="response" numFound="2" start="0" maxScore="58.60105">
<doc>
<str name="id">document1</str>
<float name="score">58.60105</float>
</doc>
<doc>
<str name="id">document2</str>
<float name="score">14.260552</float>
</doc>
</result>
<lst name="xjoin_test">
<int name="test_count">145</int>
<lst name="doc">
<str name="id">document1</str>
<double name="value">7.4</double>
</lst>
<lst name="doc">
<str name="id">document2</str>
<double name="value">2.3</double>
</lst>
</lst>
</response>
{code}
Notes:
* The actual 'join' is specified by the fq parameter. See XJoinQParserPlugin
above.
* The function test_fn is used in the bf score-boost function. Since the
argument is value2, that attribute of the external results is used as the score
boost.
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