DCE RPC NSI is an RPC-based interface that uses the DCE Cell Directory Service (CDS) as its database. The NSI routines do not constitute a general interface into CDS as such; they are a set of specialized routines whose purpose is simply to provide ways for RPC servers to advertise themselves to RPC clients, and for clients to find and bind to them.
In fact there is no public general application programming interface (API) to CDS. There is a general CDS interface that is used internally by the DCE components, but applications normally access CDS through NSI. Applications can get full access to CDS, if necessary, by using the XDS interface.
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