FederationGuids

Every BIS Element has an optional 128 bit Globally Unique Identifier called FederationGuid. It is really identifying the real-world Entity that the Element represents, not the Element itself. Generally it is intended that FederationGuids are assigned by external systems to federate Elements to their external meaning.

Scope of Uniqueness

Assigned FederationGuids are enforced to be unique on a single BIS Repository. However, multiple BIS Repositories can use the same FederationGuid in order to refer to the same real-world Entity.

Therefore, the Code of Elements with the same FederationGuid across multiple BIS Repositories is typically the same, unless such Code is enforced to be unique across all BIS Repositories in an iTwin.

Modeling Perspectives and Granularities

Note that a FederationGuid is implicitly representing a real-world Entity at a particular granularity and modeling-perspective. Thus, Elements that represent the same real-world Entity across different modeling perspectives are expected to get assigned different FederationGuids. That is, if a real-world Entity is modeled Physically, Functionally and Analytically, Elements that represent it on each one of those modeling perspectives are expected to get assigned a different FederationGuid.

The same expectation applies for the modeling of a real-world Entity at different granularities. That is, if the same real-world Entity is modeled as an aggregation of finer-granularity Elements, via any technique described in Information Hierarchy, each one of those Elements is expected to get assigned a different FederationGuid than the one of the Entity as a whole.

Element External Identification

When stored in an iModel, an Element receives a unique FederationGuid if it is not assigned one by its data-writer. This behavior enables the FederationGuid of an Element to be used as a stable identifier that systems external to its iModel can utilize to reference it.

This way an Element can be deleted and recreated in a BIS Repository, which will likely change its ElementId in the iModel, but it will not impact any external systems that reference it, as long as its FederationGuid is preserved.

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