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Stateless and Call-Back Filing Systems

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Shared file systems must deal with multiple versions of files. For example, if two people request a file and make changes to it at the same time, three versions of the file exist-one per user and the original at the server. One solution is to lock a file so that only one person can edit it at a time. There are two synchronization methods that can also be used:

  • Stateless systems    The server doesn't track files sent to clients. Clients must keep track of changes made to the original on the server. NFS is a stateless system.

  • Call back systems    In this method, the server maintains state information about files by tracking who has the files and the changes made. The server uses a call back promise technique to inform clients when another client has changed a file.

AFS (Andrew File System) and CIFS (Common Internet File System) are call back systems. As clients change files, other clients holding copies of the files are called back and notified of changes. See "File Systems."

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