We propose a new design for highly concurrent Internet services, whichwe call the staged event-driven architecture (SEDA). SEDA is intended

MMCC, the multimedia conference control program, is a window-based tool for connection management. It serves as an application interface to a wide-area network packet teleconferencing system, in which it is used not only to orchestrate multisite conferences, but also to provide local and remote audio and video control, and to interact with other conference-oriented tools that support shared workspaces. In this paper we document the design, operation and continued evolution of MMCC. We present MMCC's general architecture model, its connection control protocol and its relationship to other system components. This discussion raises issues about configuration management and the impact of conferencing over the Internet. Finally, we discuss MMCC's influence on current directions for research in multimedia connection management, and on our efforts to design a scalable Internet teleconferencing architecture.

Modeling &e reliability of distributed systems requires a good understanding of the reliability of the components'. Careful modeling allows highly fault-tolerant distributed data applications to be constructed at the least cost. Failure and repair

Distributed Open Inventor is an extension to the popular Open Inventor toolkit for interactive 3D graphics. The toolkit is extended with the concept of a distributed shared scene graph, similar to distributed shared memory. From the application programmer's perspective, multiple workstations share a common scene graph. The proposed system introduces a convenient mechanism for writing distributed graphical applications based on a popular tool in an almost transparent manner. Local variations in the scene graph allow for a wide range of possible applications, and local low latency interaction mechanisms called input streams together with a sophisticated networking architecture enable high performance while saving the programmer from network peculiarities. Keywords Distributed graphics, concurrent programming, scene graph, distributed virtual environment, computer supported cooperative work, virtual reality 1.

Supermon is a flexible set of tools for high speed, scalable cluster monitoring. Node behavior can be monitored much faster than with other commonly used methods (e.g., rstatd). In addition, Supermon uses a data protocol based on symbolic expressions (Sexpressions) at all levels of Supermon, from individual nodes to entire clusters. This contributes to Supermon’s scalability and allows it to function in a heterogeneous environment. This paper presents the Supermon architecture and discuss initial performance measurements on a cluster of heterogeneous Alpha-processor based nodes. 1

The Q system provides interoperability support for multilingual, heterogeneous component-based software systems. Initial development of Q began in 1988, and was driven by the very pragmatic need for a communication mechanism between a client program written in Ada and a server written in C. The initial design was driven by language features present in C, but not in Ada, or vice-versa. In time our needs and aspirations grew and Q evolved to support other languages, such as C++, Lisp, and Prolog. As a result of pervasive usage by the Arcadia SDE research project, usage levels and modes of the Q system grew and so more emphasis was placed upon portability, reliability, and performance. In that context we identified specific ways in which programming language support systems can directly impede effective interoperability. This necessitated extensive changes to both our conceptual model, and our implementation, of the Q system. We also discovered the need to support modes of interoperabilit...

This paper describes a new message passing protocol that provides guaranteed detection of duplicate messages even when the receiver has no state stored for the sender. It also discusses how to use these messages to implement higher-level primitives such as at-most-once remote procedure calls and sequenced bytestream protocols, and describes an implementation of at-most-once RPCs using our method. Our performance measurements indicate that at-most-once RPCs can be provided at the same cost as less desirable RPCs that do not guarantee at-most-once execution. Our method is based on the assumption that clocks throughout the system are loosely synchronized. Modern clock synchronization protocols provide good bounds on clock skew with high probability; our method depends on the bound for performance but not for correctness.

Current Remote Procedure Call (RPC) services implement a variety of semantics, with many of the differences related to how communication and server failures are handled. The list increases even more when considering group RPC, a variant of RPC often used for fault-tolerance where an invocation is sent to a group of servers rather than one. This paper presents an approach to constructing group RPC in which a single configurable system is used to build different variants of the service. The approach is based on implementing each property as a separate software module called a micro-protocol, and then configuring the microprotocols needed to implement the desired service together using a software framework based on the x-kernel. The properties of point-to-point and group RPC are identified and classified, and the general execution model described. An example consisting of a modular implementation of a group RPC service is given to illustrate the approach. Dependency issues that restrict c...

Compressing protocol headers has traditionally been an attractive way of conserving bandwidth over low-speed links, including those in wireless systems. However, despite the growth in recent years in the number of end-to-end protocols beyond TCP/IP, header compression deployment for these protocols has not kept pace. This is in large part due to complexities in implementation, which often requires a detailed knowledge of kernel internals, and a lack of a common way of pursuing the general problem across a variety of end-to-end protocols. To address this, rather than defining several new protocol-specific standards, we present a unified framework for header compression. This framework includes a simple, platform-independent header description language that protocol implementors can use to describe high-level header properties, and a platform-specific code generation tool that produces kernel source code automatically from this header specification. Together, the high-level description l...

In developing a distributed file system, there are several good reasons for implementing the client file cache manager as a user-level process. These include ease of implementation, increased portability, and minimal impact on kernel size. For reasons of compatibility it is also desirable to use a standard file intercept mechanism on the client. The Sun VFS/Vnode file system interface is such a standard. However, this interface is designed for kernel-based file systems, and a user-level cache manager that used the Vnode mechanism would pay a large performance penalty due to the high number of kernel to cache manager context switches per file system call. This paper describes our solution to the problem for the Coda file system. By using a relatively small amount of kernel code to cache critical information, we are able to retain the much larger and more complex components of the Coda cache manager in a user level process. The measurements of Coda presented here confirm the performance ...