In general many users can share the same instance (these are not the same as OS users). In the NiFi 0.x baseline (HDF 1.2) users can have roles and roles can restrict what users can do. For example, a monitor can get a read only view of the UI and a data flow manager can change anything they want. In NiFi 1.x (HDF 2.0, not yet released) there will be a much finer grained security model where policies can be created for anything on the canvas to give specific users or groups access to an individual component (processor, process-group, etc.). A keytab is a file on the operating system controlled by the standard operating system file permissions. If the NiFi process is running as an operating system user that can access that keytab file, then any NiFi user can go into the UI and create a PutHDFS processor and type in the path to that keytab, because its not based on the user accessing NiFi, its based on the OS user NiFi is running as. The only way I could see preventing this was if files/keytabs could be defined as some kind of first-class resource in NiFi, and then policies could be created to limit which NiFi users could use those resources.
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