For Identity Broker 3.* reference attributes are "forward (read) only". It is a standard IdM connector requirement to be able to export to reference attributes. While work-arounds are often (but not always) possible they are clumsy at best, and require double-processing of data (e.g. once for a reference flow, and once or more - in the case of multi-part keys - for a string flow). In most scenarios this requires storing of data redundantly (denormalisation) on the object being exported, and when this is necessary it usually creates additional overhead in deriving this data (e.g. for FIM Portal => FIM Metaverse => Identity Broker for a SQL connected system), e.g. through the use of FIM custom workflow activities. This creates not only inefficiency, but also potential loss of data integrity if there are workflow errors.
This idea came from this issue raised for DEEWR
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