The contrib module, velocity, nicknamed Solritas, is a simple template engine that lets you build user interfaces directly in Solr using Apache Velocity, a very simple macro language to generate the HTML. It's similar to JSP or PHP, but with a simpler syntax consisting of just a handful of commands. It is very simple to pick up, as you can see in the following snippet of code, for rendering the HTML that displays the ID and name of an artist pulled from the first Solr document in a list of results:

#set($doc = $response.results.get(0))
#set($id = $doc.getFieldValue("id"))
<div>ID: $id</div>
<div>Name: #field('a_name')</div>

When a Velocity template is invoked, Solritas places some objects, indicated with a $ character, into a rendering context that you can use, such as $response and $request. In the preceding example, you can see that the first result in the response is assigned to the $doc object variable using the #set command. Java methods such as getFieldValue() are easily called in Velocity, allowing you to access the full power of Java within a scripting environment that is evaluated at runtime. Velocity also supports building your own functions, such as the #field() function for displaying a field from a document.

You can try out an interface optimized for searching for MusicBrainz artists by browsing to http://localhost:8983/solr/mbartists/browse. This web interface supports faceted browsing, autocompletion of queries, boosting of artists based on how recent the release is, "More Like This" based on artist name, and even "Did You Mean" spell checking!

When the browser invokes the URL, Solr hands the request off to a request handler with the name, /browse, which is a search request handler that works like any other. The point where the request takes a different turn is in rendering the response, which in Solr is configured with the wt parameter. Short for writer type, the choices are better known as response writers. Instead of letting it default to XML, it's set to velocity. The Velocity response writer uses the v.layout and v.template parameters to determine which template file to use for the overall page layout as well as what template for the specific page to render. The templates are located in conf/velocity/ relative to the Solr core, and they end in .vm. To use another directory, set the v.base_dir parameter. Note that the use of parameters to choose the template allows you to override it in the URL if desired.

<?xml version="1.0"?>
<requestHandler name="/browse" class="solr.SearchHandler">
<lst name="defaults">
  <str name="wt">velocity</str>
  <str name="v.template">browse</str>
  <str name="v.layout">layout</str>
  <str name="title">MusicBrainz</str>

  <str name="defType">edismax</str>
  <str name="mm">1</str>
  <str name="q.alt">*:*</str>
  <str name="rows">10</str>
  <str name="fl">*,score</str>
  <str name="qf">a_name^1.5 a_member_name^1.0</str>
  <str name="pf">a_name^1.5 a_member_name^1.0</str>

  <str name="mlt.qf">a_name^1.5 a_member_name^1.0</str>
  <str name="mlt.fl">a_name,a_member_name</str>
  <int name="mlt.count">3</int>
  <int name="mlt.mintf">1</int>
  <int name="mlt.mindf">2</int>
  <str name="mlt.boost">true</str>

  <str name="facet">on</str>
  <str name="facet.field">a_type</str>
  <str name="facet.field">type</str>
  <str name="facet.mincount">1</str>
  <str name="facet.range">a_release_date_latest</str>
  <str name="f.a_release_date_latest.facet.range.start">NOW/YEAR-10YEARS</str>
  <str name="f.a_release_date_latest.facet.range.end">NOW</str>
  <str name="f.a_release_date_latest.facet.range.gap">+1YEAR</str>
  <str name="f.a_release_date_latest.facet.range.other">before</str>
  <str name="f.a_release_date_latest.facet.range.other">after</str>

  <str name="spellcheck">on</str>
  <str name="spellcheck.dictionary">a_spell</str>
  <str name="spellcheck.collate">true</str>

  <str name="hl">on</str>
  <str name="hl.fl">a_name a_member_name</str>
  <str name="f.a_name.hl.fragsize">0</str>
  <str name="f.a_name.hl.alternateField">a_name</str>
</lst>
<arr name="last-components">
  <str>spellcheck</str>
</arr>
</requestHandler>