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solrconfig.xml (Customizing Search Handlers)

Go to http://localhost:8983/solr/#/collection1/files?file=solrconfig.xml
This will open up the solrconfig.xml file whose two most important handlers are:
  1. Search Handlers
  2. Update Handlers
Some of the common search handlers are discussed below.

Search solrconfig.xml for solr.SearchHandler and this will show around 10 search handlers.
The very first one of these is:
<requestHandler name="/select" class="solr.SearchHandler">
    <!-- default values for query parameters can be specified, these
         will be overridden by parameters in the request
     <lst name="defaults">
       <str name="echoParams">explicit</str>
       <int name="rows">10</int>
       <str name="df">text</str>

    <!-- In addition to defaults, "appends" params can be specified
         to identify values which should be appended to the list of
         multi-val params from the query (or the existing "defaults").
         In this example, the param "fq=instock:true" would be appended to
         any "fq" params the user may specify.

         NOTE: there is *absolutely* nothing a client can do to prevent these
         "appends" values from being used, so don't use this mechanism
         unless you are sure you always want it.
       <lst name="appends">
         <str name="fq">inStock:true</str>

    <!-- "invariants" are a way of letting the Solr maintainer lock down
         the options available to Solr clients.  Any params values
         specified here are used regardless of what values may be specified
         in either the query, the "defaults", or the "appends" params.

         In this example, the facet.field and facet.query params would
         be fixed, limiting the facets clients can use.  Faceting is
         not turned on by default - but if the client does specify
         facet=true in the request, these are the only facets they
         will be able to see counts for; regardless of what other
         facet.field or facet.query params they may specify.

         NOTE: there is *absolutely* nothing a client can do to prevent these
         "invariants" values from being used, so don't use this mechanism
         unless you are sure you always want it.
       <lst name="invariants">
         <str name="facet.field">cat</str>
         <str name="facet.field">manu_exact</str>
         <str name="facet.query">price:[* TO 500]</str>
         <str name="facet.query">price:[500 TO *]</str>
    <!-- If the default list of SearchComponents is not desired, that
         list can either be overridden completely, or components can be
         prepended or appended to the default list.  (see below)
       <arr name="components">
solr.SearchHandler refers to the class SearchHandler somewhere in Solr code.
The prefix "solr." emphasizes that the class belongs in Solr code only, not outside.
"/select" means that any URL in Solr ending with "/select" will hit a servlet in Solr such that the request will be passed to the class mentioned in the next parameter "class" (In the above case, value for this next parameter "class" is "solr.SearchHandler")

The request parameters can be enhanced using the following options:
  1. defaults: It is a map having options specified as name-value pairs.
    It sets default parameters on the request if they are not already set by the client.

  2. appends: This key-value map "appends" the parameter to the request parameter.
    So if request parameter was "cat:shoes" and append parameter was "cat:jeans", then the final parameter would be "cat:shoes OR jeans".

  3. invariants: Options specified here do not change regardless of what is specified in the query parameters.

Solr has several classes which can be used out-of-the-box for handling queries.
The above mechanism provides a way to customize a great deal of query handling.
One can also write their own custom handlers, drop the jar files into Solr classpath and use those as well.

Search Components

One can optionally specify "first-components", "components" and "last-components" which are extra plugins that run before, during and after the query.
Some of the components already available for this purpose are:

Clustering Component and Carrot2 Algorithm

While browsing schema.xml, you might come across request-handler by the URL "/clustering".
This is useful for defining the Clustering Component which is used incarrot2-solr-integration.

Carrot2 is a clustering algorithm that groups documents into clusters based on their content.
For example, if you give Solr, product-descriptions sold on an e-commerce website, this algorithm can look for semantic similarities between these documents and cluster them into categories like phones, TVs, chairs, tables, books etc.
Its definition looks like:
<!-- Clustering Component

       You'll need to set the solr.clustering.enabled system property
       when running solr to run with clustering enabled:

            java -Dsolr.clustering.enabled=true -jar start.jar
  <searchComponent name="clustering"
                   class="solr.clustering.ClusteringComponent" >
    <lst name="engine">
      <str name="name">lingo</str>

      <!-- Class name of a clustering algorithm compatible with the Carrot2 framework.

           Currently available open source algorithms are:
           * org.carrot2.clustering.lingo.LingoClusteringAlgorithm
           * org.carrot2.clustering.kmeans.BisectingKMeansClusteringAlgorithm

           See for more information.

           A commercial algorithm Lingo3G (needs to be installed separately) is defined as:
           * com.carrotsearch.lingo3g.Lingo3GClusteringAlgorithm
      <str name="carrot.algorithm">org.carrot2.clustering.lingo.LingoClusteringAlgorithm</str>

      <!-- Override location of the clustering algorithm's resources 
           (attribute definitions and lexical resources).

           A directory from which to load algorithm-specific stop words,
           stop labels and attribute definition XMLs. 

           For an overview of Carrot2 lexical resources, see:

           For an overview of Lingo3G lexical resources, see:
      <str name="carrot.resourcesDir">clustering/carrot2</str>

    <!-- An example definition for the STC clustering algorithm. -->
    <lst name="engine">
      <str name="name">stc</str>
      <str name="carrot.algorithm"></str>

    <!-- An example definition for the bisecting kmeans clustering algorithm. -->
    <lst name="engine">
      <str name="name">kmeans</str>
      <str name="carrot.algorithm">org.carrot2.clustering.kmeans.BisectingKMeansClusteringAlgorithm</str>

  <!-- A request handler for demonstrating the clustering component

       This is purely as an example.

       In reality you will likely want to add the component to your 
       already specified request handlers. 
  <requestHandler name="/clustering"
    <lst name="defaults">
      <bool name="clustering">true</bool>
      <bool name="clustering.results">true</bool>
      <!-- Field name with the logical "title" of a each document (optional) -->
      <str name="carrot.title">name</str>
      <!-- Field name with the logical "URL" of a each document (optional) -->
      <str name="carrot.url">id</str>
      <!-- Field name with the logical "content" of a each document (optional) -->
      <str name="carrot.snippet">features</str>
      <!-- Apply highlighter to the title/ content and use this for clustering. -->
      <bool name="carrot.produceSummary">true</bool>
      <!-- the maximum number of labels per cluster -->
      <!--<int name="carrot.numDescriptions">5</int>-->
      <!-- produce sub clusters -->
      <bool name="carrot.outputSubClusters">false</bool>

      <!-- Configure the remaining request handler parameters. -->
      <str name="defType">edismax</str>
      <str name="qf">
        text^0.5 features^1.0 name^1.2 sku^1.5 id^10.0 manu^1.1 cat^1.4
      <str name="q.alt">*:*</str>
      <str name="rows">10</str>
      <str name="fl">*,score</str>
    <arr name="last-components">
This shows how a component could be used and customized with attributes in a SearchHandler.

Terms Component

This is a component to return terms and their frequency in the document.
  <searchComponent name="terms" class="solr.TermsComponent"/>

  <!-- A request handler for demonstrating the terms component -->
  <requestHandler name="/terms" class="solr.SearchHandler" startup="lazy">
     <lst name="defaults">
      <bool name="terms">true</bool>
      <bool name="distrib">false</bool>
    <arr name="components">

Query Elevation Component

This is a search component that enables you to configure the top results for a given query regardless of the normal lucene scoring.
  <searchComponent name="elevator" class="solr.QueryElevationComponent" >
    <!-- pick a fieldType to analyze queries -->
    <str name="queryFieldType">string</str>
    <str name="config-file">elevate.xml</str>

  <!-- A request handler for demonstrating the elevator component -->
  <requestHandler name="/elevate" class="solr.SearchHandler" startup="lazy">
    <lst name="defaults">
      <str name="echoParams">explicit</str>
      <str name="df">text</str>
    <arr name="last-components">

Essentially, Solr provides all the options required for performing a great search. If you ever get a requirement which does not seem to get handled by regular queries, do not get disheartened. Come to solrconfig.xml and it will have something to offer.

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