From: Mobile and disconnected computing

File access in a mobile computing environment, where the communication
link to a file server is not guaranteed, has been a major area of
study.  Coda [Satyanarayan, 90], a descendant of the Andrew File
system (AFS), pioneered support for disconnected operations in
file-systems.  Coda increases file availability by replicating a
single volume at multiple server locations.  Disconnected operations
occur when the set of accessible servers for a particular volume
becomes null.  Coda supports disconnected operations by pre-caching
the files a user is most likely to need and then allowing all
operations on cached copies of these files, while disconnected.  Upon
reconnection, reintegration occurs through reconciliation of the
cached copy with the now-reachable server's copy, through the use of a
replay log maintained during the disconnection.

Disconnected operations have also been implemented for AFS [Huston,
93].  The highly available peer-to-peer based Ficus [Page, 91] file
system achieves similar results, although mobile computing was not one
its initial applications.  Caching issues are beginning to predominate
the open research topics in this area.  In between connected and
disconnected states, there are many states of expensive, intermittent
and unreliable connections.  Adapting caching to these varying
situations is a necessity.  More importantly, as introduced by the
Hoarding scheme of Coda, user control over some caching behavior is
extremely beneficial, and this need for user input becomes even more
important when the server connection is weak.