what is relationship between the kv_app(KVWorker, KVServer) and Apps(BCD, LBFGS, SGD)

Hi, guys:
As has been introduced in the paper '_DiFacto — Distributed Factorization Machines_', Difacto's communication relies on the Parameter Server. From the project's dictionary, we see that kv_app.h encapsulates the van logic to supply some higher but simplistic interface Pull, Push;
However we don't see any function call of Push and Pull in apps such as BCD, LBFGS, SGD etc., which is called by store_local's Pull through the function Get. So, I am puzzeled what is relationship between the kv_app(KVWorker, KVServer) and Apps(BCD, LBFGS, SGD). Can anybody help me ?