I’m trying to integrating NETCONF to my old system which has its own way to manage configuration. My question is can I use tail-f product as a higher layer of my system so I can call my old system APIs (that mean I don’t have to change change my old system too much)? If so, is there any example for that?
yes, this is one of the typical use cases for ConfD. There are several approaches.
Usually you have to do:
create yang data model describing your old system (configuration and state)
implement subscriber to push configuration from ConfD (CDB database) to your old system (using your old API). To handle trasactionality, you can use two phase subscriber; see chapter 5.6. and cdb_subscribe2 API in the ConfD User Guide and subscription examples
implement data provider to fetch state data from your old system to be displayed in Netcong <get> messages; see chapter 6. in the ConfD User Guide and data provider examples.