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The Attribute Registry service is a new addition to V4 that provides a more advanced way to configure the relationship between the internal IdPAttribute objects that are (for the most part) produced by the Attribute Resolver and the way the data is represented in the protocols supported by the software like SAML, CAS, or in the future OIDCOpenID.
In the case of SAML, at least, the actual XML representations and the names used are decoupled from the internal state. The Attribute Registry was designed to address the mapping of data between formats independently of the Attribute Resolver, which is chiefly concerned with how to get data and not how to encode it.
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