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Representing business logic of the application, the REST APIs comprise the set of functionality needed to completely support the client user interface. They are well organized-functionally and do not ever refer to each other. Thus, each service can stand on its own as a collection of functionality to suport some major feature set of the application (e.g. "refset editing", or "release process").
For each REST API there is also a fully-functional client that can be used for integration testing against that service.
The REST APIs were designed with some general principles in mind
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