Andrew File System

The Andrew File System (AFS) is a distributed file system which uses a set of trusted servers to present a homogeneous, location-transparent file name space to all the client workstations. It was developed by Carnegie Mellon University as part of the Andrew Project. Originally named "Vice", AFS is named after Andrew Carnegie and Andrew Mellon. Its primary use is in distributed computing.

AFS has several benefits over traditional networked file systems, particularly in the areas of security and scalability. One enterprise AFS deployment at Morgan Stanley exceeds 25,000 clients. AFS uses for authentication, and implements access control lists 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 server crash or a network outage.

AFS uses the Weak Consistency model. 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 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.

A consequence of the file locking strategy is that AFS does not support large shared databases or record updating within files shared between client systems. This was a deliberate design decision based on the perceived needs of the university computing environment. It leads, for example, to the use of a single file per message in the original email system for the Andrew Project, the Andrew Message System, rather than a single file per mailbox (i.e., maildir instead of mbox). See AFS and buffered I/O Problems for handling shared databases

...
Wikipedia