= De-Duplication
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If duplicate, or near-duplicate documents are a concern in your index, de-duplication may be worth implementing.

Preventing duplicate or near duplicate documents from entering an index or tagging documents with a signature/fingerprint for duplicate field collapsing can be efficiently achieved with a low collision or fuzzy hash algorithm. Solr natively supports de-duplication techniques of this type via the `Signature` class and allows for the easy addition of new hash/signature implementations. A Signature can be implemented in a few ways:

* MD5Signature: 128-bit hash used for exact duplicate detection.
* Lookup3Signature: 64-bit hash used for exact duplicate detection. This is much faster than MD5 and smaller to index.
* https://cwiki.apache.org/confluence/display/solr/TextProfileSignature[TextProfileSignature]: Fuzzy hashing implementation from Apache Nutch for near duplicate detection. It's tunable but works best on longer text.

Other, more sophisticated algorithms for fuzzy/near hashing can be added later.

[IMPORTANT]
====
Adding in the de-duplication process will change the `allowDups` setting so that it applies to an update term (with `signatureField` in this case) rather than the unique field Term.

Of course the `signatureField` could be the unique field, but generally you want the unique field to be unique. When a document is added, a signature will automatically be generated and attached to the document in the specified `signatureField`.
====

== Configuration Options

There are two places in Solr to configure de-duplication: in `solrconfig.xml` and in `schema.xml`.

=== In solrconfig.xml

The `SignatureUpdateProcessorFactory` has to be registered in `solrconfig.xml` as part of an <<update-request-processors.adoc#,Update Request Processor Chain>>, as in this example:

[source,xml]
----
<updateRequestProcessorChain name="dedupe">
  <processor class="solr.processor.SignatureUpdateProcessorFactory">
    <bool name="enabled">true</bool>
    <str name="signatureField">id</str>
    <bool name="overwriteDupes">false</bool>
    <str name="fields">name,features,cat</str>
    <str name="signatureClass">solr.processor.Lookup3Signature</str>
  </processor>
  <processor class="solr.LogUpdateProcessorFactory" />
  <processor class="solr.RunUpdateProcessorFactory" />
</updateRequestProcessorChain>
----

The `SignatureUpdateProcessorFactory` takes several properties:

signatureClass::
A Signature implementation for generating a signature hash. The default is `org.apache.solr.update.processor.Lookup3Signature`.
+
The full classpath of the implementation must be specified. The available options are described above, the associated classpaths to use are:

* `org.apache.solr.update.processor.Lookup3Signature`
* `org.apache.solr.update.processor.MD5Signature`
* `org.apache.solr.update.process.TextProfileSignature`

fields::
The fields to use to generate the signature hash in a comma separated list. By default, all fields on the document will be used.

signatureField::
The name of the field used to hold the fingerprint/signature. The field should be defined in `schema.xml`. The default is `signatureField`.

enabled::
Set to *false* to disable de-duplication processing. The default is *true*.

overwriteDupes::
If true, the default, when a document exists that already matches this signature, it will be overwritten.

=== In schema.xml

If you are using a separate field for storing the signature, you must have it indexed:

[source,xml]
----
<field name="signatureField" type="string" stored="true" indexed="true" multiValued="false" />
----

Be sure to change your update handlers to use the defined chain, as below:

[source,xml]
----
<requestHandler name="/update" class="solr.UpdateRequestHandler" >
  <lst name="defaults">
    <str name="update.chain">dedupe</str>
  </lst>
...
</requestHandler>
----

This example assumes you have other sections of your request handler defined.

[TIP]
====
The update processor can also be specified per request with a parameter of `update.chain=dedupe`.
====