This section describes several other important elements of schema.xml not covered in earlier sections.
Unique Key
The uniqueKey element specifies which field is a unique identifier for documents. Although uniqueKey is not required, it is nearly always warranted by your application design. For example, uniqueKey should be used if you will ever update a document in the index.
You can define the unique key field by naming it:
<uniqueKey>id</uniqueKey>Schema defaults and copyFields cannot be used to populate the uniqueKey field. The fieldType of uniqueKey must not be analyzed. You can use UUIDUpdateProcessorFactory to have uniqueKey values generated automatically.
Further, the operation will fail if the uniqueKey field is used, but is multivalued (or inherits the multivalue-ness from the fieldtype). However, uniqueKey will continue to work, as long as the field is properly used.
Default Search Field & Query Operator
Although they have been deprecated for quite some time, Solr still has support for Schema based configuration of a <defaultSearchField/> (which is superseded by the df parameter) and <solrQueryParser defaultOperator="OR"/> (which is superseded by the q.op parameter.
If you have these options specified in your Schema, you are strongly encouraged to replace them with request parameters (or request parameter defaults) as support for them may be removed from future Solr release.
Similarity
Similarity is a Lucene class used to score a document in searching.
Each collection has one "global" Similarity, and by default Solr uses an implicit SchemaSimilarityFactory which allows individual field types to be configured with a "per-type" specific Similarity and implicitly uses BM25Similarity for any field type which does not have an explicit Similarity.
This default behavior can be overridden by declaring a top level <similarity/> element in your schema.xml, outside of any single field type. This similarity declaration can either refer directly to the name of a class with a no-argument constructor, such as in this example showing BM25Similarity:
<similarity class="solr.BM25SimilarityFactory"/>or by referencing a SimilarityFactory implementation, which may take optional initialization parameters:
<similarity class="solr.DFRSimilarityFactory">
<str name="basicModel">P</str>
<str name="afterEffect">L</str>
<str name="normalization">H2</str>
<float name="c">7</float>
</similarity>In most cases, specifying global level similarity like this will cause an error if your schema.xml also includes field type specific <similarity/> declarations. One key exception to this is that you may explicitly declare a SchemaSimilarityFactory and specify what that default behavior will be for all field types that do not declare an explicit Similarity using the name of field type (specified by defaultSimFromFieldType) that is configured with a specific similarity:
<similarity class="solr.SchemaSimilarityFactory">
<str name="defaultSimFromFieldType">text_dfr</str>
</similarity>
<fieldType name="text_dfr" class="solr.TextField">
<analyzer ... />
<similarity class="solr.DFRSimilarityFactory">
<str name="basicModel">I(F)</str>
<str name="afterEffect">B</str>
<str name="normalization">H3</str>
<float name="mu">900</float>
</similarity>
</fieldType>
<fieldType name="text_ib" class="solr.TextField">
<analyzer ... />
<similarity class="solr.IBSimilarityFactory">
<str name="distribution">SPL</str>
<str name="lambda">DF</str>
<str name="normalization">H2</str>
</similarity>
</fieldType>
<fieldType name="text_other" class="solr.TextField">
<analyzer ... />
</fieldType>In the example above IBSimilarityFactory (using the Information-Based model) will be used for any fields of type text_ib, while DFRSimilarityFactory (divergence from random) will be used for any fields of type text_dfr, as well as any fields using a type that does not explicitly specify a <similarity/>.
If SchemaSimilarityFactory is explicitly declared with out configuring a defaultSimFromFieldType, then BM25Similarity is implicitly used as the default.
In addition to the various factories mentioned on this page, there are several other similarity implementations that can be used such as the SweetSpotSimilarityFactory, ClassicSimilarityFactory, etc…. For details, see the Solr Javadocs for the similarity factories.