Robust, Distributed References and Acyclic Garbage Collection 1

We propose eecient, programming language-independent, location-transparent references as a substitute for pointers in distributed applications. These references provide the semantics of normal pointers for both local and distributed, transient and persistent objects. They may be passed in messages between and within nodes using a low-overhead presentation-layer protocol. The programmer remains free to create, delete or migrate objects at will. Sending references (or migrating objects) may cause references to be chained together across any number of spaces; we provide a short-circuit protocol to optimize access through such chains. Integrated with these references, we provide eecient, distributed, garbage collection of acyclic data structures. Even in the presence of network failures such as lost messages, duplicated messages, out of order messages and site failures, the correctness of GC is guaranteed. The protocol assumes the existence of local garbage collectors of the tracing family. The protocol combines: (i) local tracing (from a conservative root); (ii) conservative distributed reference counting; (iii) periodic tightening of the counts; and (iv) allowance for messages in transit during GC. The protocol uses only information local to each site, or exchanged between pairs of sites; no global mechanism is necessary. It is parallel and should scale to very large systems, e.g. tens of thousands of nodes connected using both local and wide-area networks.