The Coda file system [21] supposes that mobile computing will take place in the form of ``disconnected operation,'' and describes in [11] a method in which the user specifies how to ``stash'' (read/write) files before disconnection and then, upon reconnection, have the file service run an algorithm to detect version skew. Coda can be taken as a point of contrast to our system, since the idea of disconnection is antithetical to our philosophy. We believe trends in wireless communication point to the ability to be connected any time, anywhere. Users may decide not to connect (e.g., for cost reasons) but will not be forced not to connect (e.g., because the network is unreliable or not omnipresent). We call this mode of operation elective connectivity.
An obvious alternative to our NFS-based effort is to employ a file system designed for wide-area and/or multi-domain operation. Such file systems have the advantages of a cache consistency protocol and a security model that recognizes the existence of many administrative domains. Large scale file systems include AFS [6] and its spinoffs, Decorum [10] and IFS (Institutional File System) [5]. Experiments involving AFS as a ``nation-wide'' file service have been going on for years [23]. This effort has focused on stitching together distinct administrative domains so as to provide a single unified naming and protection space. However, some changes are needed to the present authentication model in order to support the possibility of a mobile client relocating in a new domain. In particular, if the relocated client will make use of local services, then there should be some means whereby one authentication agent (i.e., that in the new domain) would accept the word of another authentication agent (i.e., that in the client's home domain) regarding the identity of the client.
The IFS project has also begun to investigate alterations to AFS in support of mobile computers [4]. Specifically, they are investigating cache pre-loading techniques for disconnected operation and transport protocols that are savvy about the delays caused by ``cell handoff'' -- the time during which a mobile computer moves from one network to another.
Plan 9's bind command has been designed to make it easy to mount new file systems. In particular, file systems can be mounted ``before'' or ``after'' file systems already mounted at the same point. The before/after concept replaces the notion of a search path. Plan 9 also supports the notion of a ``union mount'' [17]. The Plan 9 bind mechanism is a more elegant alternative to our double mounting plus comparison. However, a binding mechanism -- even an unusually flexible one such as that of Plan 9 -- addresses only part of the problem of switching between file systems. The harder part of the problem is determining when to switch and what to switch to.