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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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Upgraded systems with an older services.xml file are configured internally to load only the existing AttributeResolverConfiguration resources in order to process the <AttributeEncoder> elements within it in order to produce a compatible set of rules to use. The default set of rules supplied with the software is not loaded in order to prevent any changes in behavior, including duplication of encoded attributes.
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The system is designed with a high degree of flexibility for defining and loading the mapping rules, but the primary mechanism defined for now is based on properties or maps keyed by some pre-defined strings. Most often the map values will also be strings, which allows use of a Java property file as as an alternative way of defining a rule without using XML.
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