This paper factors out the common denominator underlying these variants: full decoupling of the communicating entities in time, space, and synchronization. We use these three decoupling dimensions to better identify commonalities and divergences with traditional interaction paradigms. The many variations on the theme of publish/subscribe are classified and synthesized. In particular, their respective benefits and shortcomings are discussed both in terms of interfaces and implementations

Many real-time application domains can benefit from flexible and open distributed architectures, such as those defined by the CORBA specification. CORBA is an architecture for distributed object computing being standardized by the OMG. Although CORBA is well-suited for conventional request/response applications, CORBA implementations are not yet suited for real-time applications due to the lack of key quality of service (QoS) features and performance optimizations. This paper makes three contributions to the design of realtime CORBA systems. First, the paper describes the design of TAO, which is our high-performance, real-time CORBAcompliant implementation that runs on a range of OS platforms with real-time features including VxWorks, Chorus, Solaris 2.x, and Windows NT. Second, it presents TAO’s realtime scheduling service that can provide QoS guarantees for deterministic real-time CORBA applications. Finally, the paper presents performance measurements that demonstrate the effects of priority inversion and non-determinism in conventional CORBA implementations and how these hazards are avoided in TAO. 1

To be an effective platform for performance-sensitive real-time and embedded applications, off-the-shelf CORBA middleware must preserve the communication-layer quality of service (QoS) properties of applications end-to-end. However, the standard CORBA GIOP/IIOP interoperability protocols are not well suited for applications that cannot tolerate the message footprint size, latency, and jitter associated with general-purpose messaging and transport protocols. It is essential, therefore, to develop standard pluggable protocols frameworks that allow custom messaging and transport protocols to be configured flexibly and used transparently by applications. This paper provides three contributions to research on pluggable protocols frameworks for performance-sensitive distributed object computing (DOC) middleware. First, we outline the key design challenges faced by pluggable protocols developers. Second, we describe how we resolved these challenges by developing a pluggable protocols framewo...

With the widespread deployment and use of wireless data communications in the mobile computing domain the need for middleware that interconnects the components that comprise a mobile application in distributed and potentially heterogeneous environments arises. Middleware utilizing an event-based communication model is well suited to address the requirements of the mobile computing domain, as it requires a less tightly coupled communication relationship between application components compared to the traditional client/server communication model. This is particularly useful with the use of wireless technology, where communication relationships amongst application components are established very dynamically during the lifetime of the components. Recent research in the area of eventbased middleware for the mobile computing domain focuses on infrastructure network models for wireless data communication. In this paper, we present STEAM, an event-based middleware service that has been specifically designed for wireless local area networks utilizing the ad hoc network model. We argue that an implicit event model is best suited for the envisaged ad hoc environment and present our approach of exploiting a novel combination of three different types of event filter to address the problems related to the dynamic reconfiguration of the network topology as well as their impact on the scalability of a system and the timely delivery of events.

To be an effective platform for performance-sensitive real-time systems, distributed object computing middleware must support application quality of service (QoS) requirements end-toend. This article describes how the OMG's Real-time CORBA specification defines standard policies and mechanisms that permit the specification and enforcement of end-to-end QoS.

Distributed object computing forms the basis for nextgeneration application middleware. At the heart of distributed object computing are Object Request Brokers (ORBs), which automate many tedious and error-prone distributed programming tasks. This article presents a case study of key design patterns needed to develop ORBs that can be dynamically configured and evolved for specific application requirements and system characteristics.

There is increasing demand to extend object-oriented middleware, such as OMG CORBA, to support applications with stringent quality of service (QoS) requirements. However, conventional CORBA Object Request Broker (ORB) implementations incur high latency and low scalability when used for performance-sensitive applications. These inefficiencies discourage developers from using CORBA for mission/lifecritical applications such as real-time avionics, telecom call processing, and medical imaging. This paper provides two contributions to the research on CORBA performance. First, we systematically analyze the latency and scalability of two widely used CORBA ORBs, VisiBroker and Orbix. These results reveal key sources of overhead in conventional ORBs. Second, we describe techniques used to improve latency and scalability in TAO, which is a high-performance, real-time implementation of CORBA. Although conventional ORBs do not yet provide adequate QoS guarantees to applications, our research resu...

Events are an emerging paradigm for composing applications in an open, heterogeneous distributed world. In Cambridge we have developed scalable event handling based on a publish-register-notify model with event object classes and server-side filtering based on parameter templates. After experience in using this approach in a home-built RPC system we have extended CORBA, an open standard for distributed object computing, to handle events in this way. In this paper, we present the design of COBEA - a COrba-Based Event Architecture. A service that is the source of (parameterised) events publishes in a Trader the events it is prepared to notify, along with its normal interface specification. For scalability, a client must register interest (by invoking a register method with appropriate parameters or wild cards) at the service, at which point an access control check is carried out. Subsequently, whenever a matching event occurs, the client is notified. We outline the requirements on the...

Efficient and predictable demultiplexing is necessary to provide real-time support for distributed object computing applications developed with CORBA. This paper presents two contributions to the study of demultiplexing for real-time CORBA endsystems. First, we present an empirical study of four CORBA request demultiplexing strategies (linear search, perfect hashing, dynamic hashing, and active demultiplexing) for a range of target objects and operations. Second, we describe how we are using the perfect hashing and active demultiplexing strategies to develop a highperformance, real-time ORB called TAO. Keywords: Communication software, Real-time CORBA, Demultiplexing. 1 Introduction CORBA is a distributed object computing middleware standard defined by the Object Management Group (OMG)[14]. CORBA is designed to allow clients to invoke operations on remote objects without concern for where the object resides or what language the object is written in. In addition, CORBA shields applica...

This paper will appear in the Communications of the ACM Special Issue on CORBA edited by Krishnan Seetharaman, October 1998, Vol. 41, No. 10. Introduction