Unique vs Nonunique Indexes

Unique indexes differ from nonunique indexes in enforcing uniqueness of the indexed attribute values for objects that are attached to the index. For example, you might want to populate Person objects into two indexes, a unique one on an “email” attribute, and a nonunique one on a “lastname” attribute. The lastname index allows many Person objects with the same last name to be attached. On the other hand, the AttachToIndex call that targets the email index returns a LinkNameAlreadyInUseException error if a Person with the same email attribute is already attached. Note that the error does not remove the Person object itself. Consequently, an application might create the Person, attach it to the hierarchy, and attach it to indexes, all in a single batch request. This ensures that if uniqueness is violated on any of the indexes, the object and all of its attachments are rolled back automatically.