For our SDK4SDN work we made a comparison between two SDN controllers: Ryu and Opendaylight. We focused on the Northbound APIs of the controllers and we compared the capabilities and ease of use of their respective REST APIs.
Both controllers support REST, which is based on a mix of HTTP, JSON and XML. In Ryu a WSGI web server is used to create the REST APIs, which link with other systems and browsers. In OpenDaylight, the Jersey library provides the REST APIs with both JSON and XML interfaces. Jersey also provides its own API to simplify RESTful services by extending the JAX-RS toolkit which is compliant with the Northbound API.