(d) AFS uses callback to ensure cache consistency. How does AFS deal with the risk that callback messages may be lost?(f) What is the mechanism used in NFS and AFS, respectively, to maintain the consistency of client caches? Describe how each mechanism operates. Which one is more scalable and why?(g) To what extent does Sun NFS deviate from one-copy file update semantics? Construct a scenario in which two user-level processes sharing a file would operate correctly in a single UNIX host but would observe inconsistencies when running in different hosts.