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.

Increasingly complex requirements, coupled with with tighter economic and organizational constraints, are making it hard to build complex distributed real-time embedded (DRE) systems entirely from scratch. The proportion of DRE systems made up of commercial-off-the-shelf (COTS) hardware and software is therefore increasing significantly. There are relatively few systematic empirical studies, however, that illustrate how suitable COTS-based hardware and software have become for mission-critical DRE systems. This paper provides the following contributions to the study of real-time quality of service (QoS) assurance and performance in COTS-based DRE systems: (1) it presents evidence that flexible configuration of COTS middleware mechanisms, and the operating system settings they use, allows DRE systems to meet critical QoS requirements over a wider range of load and jitter conditions than statically configured systems, (2) it shows that in addition to making critical QoS assurances, non-critical QoS performance can be improved through flexible support for alternative scheduling strategies, and (3) it presents an empirical study of three canonical scheduling strategies---specifically the conditions that predict success of a strategy for a production-quality DRE avionics mission computing system. Our results show that applying a flexible scheduling framework to COTS hardware, operating systems, and middleware improves real-time QoS assurance and performance for mission-critical DRE systems.

End-to-end predictability of remote operations is essential for many fixed-priority distributed real-time and embedded (DRE) applications, such as command and control systems, manufacturing process control systems, large-scale distributed interactive simulations, and testbeam data acquisition systems. To enhance predictability, the Real-time CORBA specification defines standard middleware features that allow applications to allocate, schedule, and control key CPU, memory, and networking resources necessary to ensure end-to-end quality of service support.

Advanced distributed interactive simulations have stringent quality of service (QoS) requirements for throughput, latency, and scalability, as well as requirements for a flexible communication infrastructure to reduce software lifecycle costs. The CORBA Event Service provides a flexible model for asynchronous communication among distributed and collocated objects. However, the standard CORBA Event Service specification lacks important features and QoS optimizations required by distributed interactive simulation systems.

Distributed object computing forms the basis of nextgeneration communication software. At the heart of distributed object computing are Object Request Brokers (ORBs), which automate many tedious and error-prone distributed programming tasks. Like much communication software, conventional ORBs use statically configured designs, which are hard to port, optimize, and evolve. Likewise, conventional ORBs cannot be extended without modifying their source code, which forces recompilation, relinking, and restarting running ORBs and their associated application objects. This paper makes two contributions to the study of extensible ORB middleware. First, it presents a case study illustrating how a pattern language can be used to develop dynamically configurable ORBs that can be customized for specific application requirements and system characteristics. Second, we quantify the impact of applying this pattern language to reduce the complexity and improve the maintainability of common ORB tasks, ...

Middleware is increasingly used as the infrastructure for applications with stringent quality of service (QoS) requirements, including scalability. One way to improve the scalability of distributed applications is to use adaptive middleware to balance system processing load dynamically among multiple servers.

Next-generation distributed interactive simulations have stringent quality of service (QoS) requirements for throughput, latency, and scalability, as well as requirements for a flexible communication infrastructure to reduce software lifecycle costs. The CORBA Event Service provides a flexible model for asynchronous communication among distributed and collocated objects. However, the standard CORBA Event Service specification lacks important features and QoS optimizations required by distributed interactive simulation systems.

To be an effective platform for performance-sensitive realtime systems, commercial-off-the-shelf (COTS) distributed object computing (DOC) middleware must support application quality of service (QoS) requirements end-to-end. However, conventional DOC middleware does not provide this support, which makes it unsuited for applications with stringent latency, determinism, and priority preservation requirements. It is essential, therefore, to develop standards-based, COTS DOC middleware that permits the specification, allocation, and enforcement of application QoS requirements end-to-end. The Real-time CORBA and Messaging specifications in the forthcoming CORBA 3.0 standard are important steps towards defining standards-based, COTS DOC middleware that can deliver end-to-end QoS support at multiple levels in distributed and embedded real-time systems. However, these specifications still lack sufficient detail to portably configure and control processor, communication, and memory resources fo...

Many distributed applications require a scalable event-driven communication model that decouples suppliers from consumers and simultaneously supports advanced quality of service (QoS) properties and event filtering mechanisms. The CORBA Notification Service provides a publish/subscribe mechanism that is designed to support scalable event-driven communication by routing events efficiently between many suppliers and consumers, enforcing various QoS properties (such as reliability, priority, ordering, and timeliness), and filtering events at multiple points in a distributed system. This paper provides several contributions to research on scalable notification services. First, we present the CORBA Notification Service architecture and illustrate how it addresses limitations with the earlier CORBA Event Service. Second, we explain how we addressed key design challenges faced when implementing the Notification Service in TAO, which is our high-performance, real-time ORB. Finally, we discuss the optimizations used to improve the scalability of TAO's Notification Service. 1

As network-centric computing becomes more pervasive and applications become more distributed, the demand for greater scalability and dependability is increasing. Distributed system scalability can degrade significantly, however, when servers become overloaded by the volume of client requests. To alleviate such bottlenecks, load balancing middleware mechanisms can be used to distribute system load equitably across object replicas residing on multiple servers. This paper describes the key design challenges we faced when adding this load balancing service to our CORBA ORB (TAO) and outline how we resolved the challenges by applying patterns. Keywords: Middleware, patterns, CORBA, load balancing. 1