The purpose of identifiers is to allow uniquely identifying NGSI-LD elements (Entities, Context Subscriptions or Context Source Registrations) within an NGSI-LD system. This annex is intended to clarify the different issues around the design of identifiers in NGSI-LD.
In order to enable the participation of NGSI-LD in linked data scenarios, all Entities are identified by URIs. If those URIs are expected to participate in external linked data relationships they should be dereferenceable.
It is noteworthy that the identifier from the point of view of NGSI-LD is different from the inherent identifier that a specific Entity may have. For instance, an NGSI-LD Entity of Type "Vehicle" may have a Property named licencePlateNumber, which it is actually a unique identifier from the point of view of the Entity domain, as it uniquely identifies the specific vehicle instance. However, from the point of view of the NGSI-LD system, it may have another identifier which might or might not include such licence plate number identifier.
NGSI-LD defines a specific URN [9] namespace intended to help API users to design readable, clean and simple identifiers. As it is based on URNs, the usage of this identification approach is not recommended when dereferenceable URIs are needed (fully-fledged linked data scenarios).
The referred namespace is defined as follows (to be registered with IANA):
EntityTypeName shall be an Entity Type name which can be expanded to a URI as per the @context.
EntityIdentificationString shall be a string that allows uniquely identifying the subject Entity in combination with the other items being part of the NSS.
It is recommended that applications use this URN namespace when applicable.
In general, the URN specification defines namespace equivalence in a case-insensitive manner, however it is assumed that context-broker implementations shall always use lowercase letters in namespaces where they have a choice in case, unless there is a strong reason otherwise. Restricting the namespace prefix to lower case urn:ngsi-ld: can improve caching and retrieval, since this ensures since alphabetic characters within the namespace specific string are always consistent.