# Recoll user manual
API

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### Interface elements

A few elements in the interface are specific and and need an explanation.

udi

  An udi (unique document identifier) identifies a document. Because of limitations inside the index engine, it is restricted in length (to 200 bytes), which is why a regular URI cannot be used. The structure and contents of the udi is defined by the application and opaque to the index engine. For example, the internal file system indexer uses the complete document path (file path + internal path), truncated to length, the suppressed part being replaced by a hash value.

ipath

  This data value (set as a field in the Doc object) is stored, along with the URL, but not indexed by Recoll. Its contents are not interpreted, and its use is up to the application. For example, the Recoll internal file system indexer stores the part of the document access path internal to the container file (`ipath` in this case is a list of subdocument sequential numbers). url and ipath are returned in every search result and permit access to the original document.

Stored and indexed fields

  The `fields` file inside the Recoll configuration defines which document fields are either "indexed" (searchable), "stored" (retrievable with search results), or both.

Data for an external indexer, should be stored in a separate index, not the one for the Recoll internal file system indexer, except if the latter is not used at all). The reason is that the main document indexer purge pass would remove all the other indexer's documents, as they were not seen during indexing. The main indexer documents would also probably be a problem for the external indexer purge operation.

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