We meet an scenario when system starts up, application foo need to know if application bar has restored its configuration completely then do the next step initialization.
The reason is that application foo need to access some data created after application bar loading its own configuration.
Does ConfD provide any API or notification that can let application bar inform application foo everything has ready in this scenario?
No API calls but I suppose you could use stream event notifications sent from bar to foo when bar is done reading its configuration. Although, especially if foo and bar run on the same node there are much easier ways to achieve the same thing, (the simplest is maybe if bar touch a file known to both applications and foo removes the file when it has seen it).
If you want to read more about notifications, see Chapter 12, the section about NOTIFICATION STREAMS in confd_lib_dp(3) and confd_lib_events(3). We also have examples that show how to send and receive notification e.g. examples.confd/netconf_notification and misc/notifications resp.