WSO2 registry has APP support , so any operation that we can do with UI can do with APP as well. In addition to that there is something called RemoteRegistry , where we can point to a registry and do operations using any kind of Java application , there the communicate taking place using APP. To support APP WSO2 registry uses Apache Abdera an APP implementation (I would say that is cool and easy to work with).
I thought that by default Abdera generate valid feed , but when I do a validation though W3C atom validator I found that we had a number of issues with the generated feed. Then I realized that it is our job to populate Abdara correctly , so I fixed all the issue and was able to generate valid atom feed from WSO2 Registry.