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== Features ==
AFS<ref>{{cite journal|author =Howard, J.H.|author2 =Kazar, M.L.|author3 =Nichols, S.G.|author4 =Nichols, D.A.|author5 =Satyanarayanan, M.|author6 =Sidebotham, R.N.|author7 =West, M.J.|last-author-amp =yes|title=Scale and Performance in a Distributed File System|journal=ACM Transactions on Computer Systems|volume=6|issue=1|date=February 1988|pages=51–81|doi=10.1145/35037.35059|citeseerx =10.1.1.71.5072}}</ref> has several benefits over traditional networked [[file system]]s, particularly in the areas of security and scalability. One enterprise AFS deployment at [[Morgan Stanley]] exceeds 25,000 clients.<ref>http://www-conf.slac.stanford.edu/AFSBestPractices/Slides/MorganStanley.pdf</ref> AFS uses [[Kerberos protocol|Kerberos]] for authentication, and implements [[access control list]]s on directories for users and groups. Each client caches files on the local filesystem for increased speed on subsequent requests for the same file. This also allows limited filesystem access in the event of a [[crash (computing)|server crash]] or a [[network outage]].
AFS uses the [[Weak consistency|Weak Consistency]] model.<ref>{{Citation
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|title = Distributed Storage
|edition = Distributed Storage: Concepts, Algorithms, and Implementations
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}}</ref> Read and write operations on an open file are directed only to the locally cached copy. When a modified file is closed, the changed portions are copied back to the file server. Cache consistency is maintained by [[Callback (computer science)|callback]] mechanism. When a file is cached, the server makes a note of this and promises to inform the client if the file is updated by someone else. Callbacks are discarded and must be re-established after any client, server, or network failure, including a time-out. Re-establishing a callback involves a status check and does not require re-reading the file itself.
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