Towards a Grand Unified Framework for Mobile Objects

Distributed object systems (DOS) are usually implemented as middleware which repeats and undoes a signiicant amount of work performed by the underlying operating system 3]. This introduces overhead and leads to distinct, incompatible middleware implementations. Therefore, basic mechanisms are not usually shared. If mobility and replication rely on common techniques and mechanisms no matter the application domain, why are these mechanisms not shared? We believe that, behind it all, the reason is the inadequate support that existing operating systems ooer for mobility, replication, and DOS in general. The lack of proper support promotes ad-hoc solutions and discourages sharing. Many recent approaches consider support for general-purpose DOS applications. However, they do not focus on providing basic mechanisms that could be shared to implement diierent distributed object abstractions and models. We propose a diierent organization in which operating system and middle-ware cooperate to support eecient distributed object systems. A exible micro-kernel is responsible for securely multiplexing distributed hardware, following the philosophy of 1]. Adaptable middleware 4] is built on top of this minimal network microkernel and implements a exible object model that can be cus-tomized to meet application-speciic requirements. In what follows, we elaborate our proposal by discussing some concrete issues. Object naming and protection is traditionally provided by the middleware, duplicating the work performed by the OS naming system. We propose to handle logical naming entirely inside the middleware. Should the OS support names for distributed physical resources, abstract resource (i.e., object) names can be implemented without duplication of eeorts. With respect to protection, if the mid-dleware can select the protection policy for each physical resource it is granted, mobile objects can be assigned their own physical resources and protect them (even from the hosting environment). The middleware can then implement abstract resource protection based on the physical protection provided by the OS. Reference resolution and method invocation in mobile and replicated objects are more complex because traditional OS facilities are not aware of migration