As we approach nation-wide integration of computer systems, it is clear that le replication will play a key role, both to improve data availability in the face of failures, and to improve performance by locating data near where it will be used. We expect that future le systems will have an extensible, modular structure in which features such as replication can be \slipped in" as a transparent layer in a stackable layered architecture. We introduce the Ficus replicated le system for NFS and show how it is layered on top of existing le systems. The Ficus le system di ers from previous le replication services in that it permits update during network partition if any copy of a le is accessible. File and directory updates are automatically propagated to accessible replicas. Con icting updates to directories are detected and automatically repaired; con icting updates to ordinary les are detected and reported to the owner. The frequency of communications outages rendering inaccessible some replicas in a large scale network and the relative rarity of con icting updates make this optimistic scheme attractive. Stackable layers facilitate the addition of new features to an existing le system without reimplementing existing functions. This is done in a manner analogous to object-oriented programming with inheritance. By structuring the le This work was sponsored by DARPA under contract number F29601-87-C-0072. y This author is also associated with Locus Computing Corporation. system as a stack of modules, each with the same interface, modules which augment existing services can be added transparently. This paper describes the implementation of the Ficus le system using the layered architecture.