DISP is a practical client-server protocol for the distributed storage of immutable data objects. Unlike most other contemporary protocols, DISP permits applications to make explicit tradeoffs between total storage space, computational overhead, and guarantees of availability, integrity, and privacy on a per-object basis. Applications specify the degree of redundancy with which each item is encoded, what level of integrity checks are computed and stored with each item, and whether items are stored in an encrypted format. At one extreme, clients willing to pay the overhead are guaranteed privacy, integrity, and availability of data stored in the system as long as fewer than half the servers are Byzantine. At the other extreme, objects that do not require privacy or integrity in the face of Byzantine servers can be stored with very low computational and storage overhead.DISP is efficient in terms of message count, message size, and storage requirements: even in the worst case, the read and write protocols require a number of messages that are linear with respect to the number of servers. In terms of message size, DISP requires transferring only marginally more than L bytes to correctly read an object of size L , even in the face of Byzantine server failures. In this article we provide a description of DISP and an analysis of its fault-tolerant properties. We also analyze the complexity of the protocol and discuss several potential applications. We conclude with a description of our prototype implementation and measurements of its performance on commodity hardware.

Recent work in opening up distributed object systems to make them suitable for applications needing quality of service control has had the side effect of increasing the complexity in setting up, configuring, and initializing such applications. Configuration of distributed applications is more complicated than that of non-distributed applications, simply because of the heterogeneous and distributed nature of the application's components. CORBA and other distributed object middleware simplifies the configuration of distributed object applications, but hides much of the information and control necessary to achieve quality of service (QoS). We describe the techniques and tools that we have developed within our Quality Objects (QuO) framework for simplifying the configuration of distributed applications with QoS attributes. We describe a QuO configuration language, as well as the specific configuration needs of particular QoS properties-real-time, security, and dependability-and the support we provide for them.

As networks and the use of communications within applications continue to grow and find more uses, so too does the demand for more control and manageability of various "system properties" through middleware. An important component supporting an integrated property architecture is the concept of an object gateway, which is a quality-of-service (QoS)-aware element transparently inserted at the transport layer between clients and objects to provide managed communication behavior for the particular property being supported. In this paper, we introduce the concept of a QoS-oriented gateway to integrate a variety of QoS enforcement and implementation mechanisms controlling the underlying distributed interactions. We discuss the functions performed by such a component in achieving the desired overall end-to-end QoS, and the design considerations underlying our current implementation. We conclude with experiences to date with two variations of the gateway: one controlling managed latency and throughput using bandwidth allocation, and one controlling dependability through the coordination of object replicas.