Solr highlighting with unexpected prefix and suffix - solr

Question

I need to customize Solr highlighting prefix and suffix like this:
<span class="highlight">text</span>
instead of the default
<em>text</em>
That's why I'm using this configuration within the solrconfig.xml for the HighlightComponent:
<searchComponent class="solr.HighlightComponent" name="highlight">
<highlighting>
<fragmentsBuilder name="simple" default="true" class="solr.highlight.SimpleFragmentsBuilder">
<lst name="defaults">
<str name="hl.tag.pre"><![CDATA[<span class="highlight">]]></str>
<str name="hl.tag.post"><![CDATA[</span>]]></str>
</lst>
</fragmentsBuilder>
</highlighting>
</searchComponent>
The following are the default parameters for my standard request handler:
<requestHandler name="standard" class="solr.SearchHandler" default="true">
<lst name="defaults">
<str name="hl">true</str>
<str name="hl.fl">body,title</str>
<str name="hl.useFastVectorHighlighter">true</str>
</lst>
</requestHandler>
When I search for the text word I do get the text word highlighted, but not always using the prefix and suffix I configured:
<lst name="highlighting">
<lst name="document_1">
<arr name="body">
<str>my <em>text</em> highlighted</str>
</arr>
<arr name="title">
<str>my <span class="highlight">text</span> highlighted</str>
</arr>
</lst>
</lst>
Does anybody know why?

Answer 1

I am guessing you are seeing this behavior behavior because you only have the prefix and suffix defined for the SimpleFragmentsBuilder and the other highlights are coming from another fragment builder.
I am using a custom prefix and suffix for my highlighting and I set this value in the formatter section of the highlighting section of the solrconfig.xml and have not had any issues as it will apply to all fragment builders.
So maybe try the following:
<highlighting>
<fragmentsBuilder name="simple" default="true"
class="solr.highlight.SimpleFragmentsBuilder"/>
<!-- Configure the standard formatter -->
<formatter name="html" class="org.apache.solr.highlight.HtmlFormatter"
default="true">
<lst name="defaults">
<str name="hl.simple.pre"><![CDATA[<span class="highlight">]]></str>
<str name="hl.simple.post"><![CDATA[</span>]]></str>
</lst>
</formatter>
</highlighting>

Answer 2

I finally found out why! I'm using fastVectorHighlighter to make highlighting faster.
At the beginning I was highlighting only the title field and everything worked fine.
When I added the body field to highlighting I forgot to enable termVectors=true.
Now that my body field looks like this
<field name="body" type="text" indexed="true" stored="true" termVectors="true" termPositions="true" termOffsets="true" />
after a full reindex highlighting is working perfectly:
<lst name="highlighting">
<lst name="document_1">
<arr name="body">
<str>my <span class="highlight">text</span> highlighted</str>
</arr>
<arr name="title">
<str>my <span class="highlight">text</span> highlighted</str>
</arr>
</lst>
</lst>
Previously the body field highlighting did work, but without fastVectorHighlighter since the field didn't have the termVectors=true parameter. That's why I got body highlighted with default prefix and suffix. Since fastVectorHighlighter is a completely different highlighting method, the configuration is different as well.
To avoid this kind of mistakes, as long the users can choose what fields to highlight with the hl.fl parameter, I'd recommend to include also the configuration for the standard highlighting (formatter element, class solr.highlight.HtmlFormatter) like this:
<searchComponent class="solr.HighlightComponent" name="highlight">
<highlighting>
<formatter name="html" default="true" class="solr.highlight.HtmlFormatter">
<lst name="defaults">
<str name="hl.simple.pre"><![CDATA[<span class="highlight">]]></str>
<str name="hl.simple.post"><![CDATA[</span>]]></str>
</lst>
</formatter>
<fragmentsBuilder name="simple" default="true" class="solr.highlight.SimpleFragmentsBuilder">
<lst name="defaults">
<str name="hl.tag.pre"><![CDATA[<span class="highlight">]]></str>
<str name="hl.tag.post"><![CDATA[</span>]]></str>
</lst>
</fragmentsBuilder>
</highlighting>
</searchComponent>
This way highlighting will work with the same prefix and suffix even for fields with termVectors disabled.