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The resolver service is essentially a meta-directory. It doesn't hold information but it contains a directed graph of attribute definitions and data connectors that collect information from a variety of sources, combine and transform it, and produce a final collection of IdPAttribute objects, which are an internal representation of user data not specific to SAML or any other supported identity protocol.
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This is still supported, but with a few exceptions, a better approach in V4 is to rely on a common set of encoding rules contained in the new AttributeRegistryConfiguration based on reasonable conventions for the names of the IdPAttribute objects. For example, if you just accept that you should put the subject's e-mail address into an IdPAttribute called "mail", then the registry knows how to encode that into SAML automatically using the name prescribed by the standard.
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