Proposed Structure for an Active File System for UNIX

We describe ongoing work on a mechanism for incorporating active les, i e les whose contents are computed dynamically, in a UNIX le system. The main advantage of our method is that it does not require any reconnguration of the kernel, but instead uses the existing Network File System and Remote Procedure Call facilities. In many cases one would like for the contents of a le to be recomputed dynamically, either because they are based on changing data or because the le would be very large but is needed only occasionally. To allow this we would like to extend the semantics of the le system in exible ways without having to do kernel modiications. UNIX pipes and shell scripts enable a user to easily recompute output from diierent sources, but in many cases pipes cannot be used, as for example in les being included in other documents. This problem has been addressed in many ways, such as Watchdogs 1], the Intentional File System 2] and the BSD Portal File system 3]. These however require the kernel to be extended and some amount of programming expertise from a user to be able to use them. We decided that in the majority of cases there would be no reason to write anything to the les, so they could, in the manner of the Alex le system 4] be implemented as a read-only le system. We decided to call our le system Ander, in honour of Alex. Ander gives a new view of the le system, interpreting the les as scripts to run and not only as streams of bytes as the ordinary le system does. The scripts need not be specially written for Ander, any script writing its result on standard output can be used as an active le.