One year back I got a chance to help a team working on AIA PIP migration project, the team was stuck with compilation issues of processes that gets migrated to 11g. All processes were using concrete WSDL of dependent processes which were not even ready. That’s when as a best practice we replaced all concrete WSDL’s with Abstract WSDL. Similar issue happens during startup of the server. While loading processes from MDS, there used to be issues related services being unavailable due to the order of process loading. That’s when much emphasis was laid on using Abstract WSDL compared to Concrete WSDL .A nice article on why Abstract WSDL need to be used instead of using concrete WSDL during design time and how it works @ runtime.