The Arjuna transaction system began life in the mid 1980s as an academic project to examine the use of object-oriented techniques in the development of fault-tolerant distributed systems. Twenty five years later, it is an integral part of the JBoss application sever middleware from Red Hat. This journey from an academic to a commercial environment has been neither easy nor smooth but it has been interesting from many different perspectives. This paper gives an overview of this journey and discusses key lessons learned.
We consider a computation model in which application programs manipulate persistent (long-lived) objects under the control of atomic actions (atomic transactions). The Common Object Request Broker Architecture (CORBA) together with its services is a well known example of a distributed object model that will support the above model. At the basic level CORBA consists of the Object Request Broker (ORB) that enables distributed objects to interact with each other. At the next level a number of system level services have been specified. These services include persistence, concurrency control and Object Transaction Service (OTS). The OTS is a protocol engine intended to guarantee that transactional behaviour is obeyed, relying on other system level services to support the ACID properties [1]. Such a structure has the advantage that an application can be bound to any given set of compliant system services for obtaining transactional behaviour. There is therefore considerable scope for customising a transactional application (say for performance improvements) by choosing the most appropriate implementations of individual services. In this paper we explore this idea in detail and present the design of a transaction framework (a distributed object transaction support system) that permits a transactional application to be customised (configured) to a degree that has not been possible before. A very modular approach is presented that enables individual objects to be made atomic, with each object bound to concurrency control, persistence and recovery services in an application specific manner. Furthermore, these bindings can be changed at run time. So for example, it is possible for an object to switch from pessimistic to optimistic concurrency control at run time. The framework to be presented here is based on our experience of designing and implementing a number of transaction services for distributed objects. These include the Arjuna system [2], the OTS version of the Arjuna system that we have recently completed and a Java transaction system [3]. This experience has enabled us to design a set of ‘ implementation neutral’ service interfaces that permit considerable scope in the provision of a wide variety of implementations, all conforming to the same interface specification. 2. Diversity of requirements
The Web frequently suffers from failures which can affect both the performance and consistency of applications running over it. For example, if a user purchases a cookie (a token) granting access to a newspaper site, it is important that the cookie is deli vered and stored if the user’s account is debited; a failure could prevent either from occurring, and leave the system in an indeterminate state. For resources such as documents, failures may simply be annoying to users; for commercial services, they can result in loss of revenue and credibilit y.
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ABSTRACT An emerging trend in multimedia applications such as distance education and corporate training is to service clients according to their convenience in terms of start time of the multimedia content and the quality of reception. Examples include clients requesting for a movie to start at a convenient time specified by (t+di) where t is the current time and di is the delay tolerance acceptable to client i. Such applications typically involve a Closed User Group network that exhibits heterogeneous characteristics, where a Content Service Provider (CSP) services requests from geographically dispersed clients synchronously. It is important for a CSP to utilize resources such as buffers, transcoders, and caches judiciously in order to minimize costs while serving clients with their required quality in order to maximize revenues. We approach the problem in the following steps: (i) Determine the optimal quality deliverable to the clients while satisfying their delay tolerance assuming static network characteristics for the duration of the play out (ii) Since the playback need to start at the requested time, determine the optimal placement of buffers, caches, and transcoders such that resource utilization is maximized and client device constraints are satisfied (iii) Use admission control and scheduling to consider the trade off in revenue if clients were admitted (dynamic arrivals) while satisfying the admitted clients’ requirements by maximizing resource utilization. We have developed an optimization-based approach to determine the best quality that can be delivered to the clients using resources such as buffers and transcoders. Simulation results demonstrate the usefulness of exploiting client delay tolerance specifications for delivering enhanced Quality of Service (QoS) with little or no additional resources.
Thank you very much for that warm introduction. I must say, there are few things that make me more nervous than speaking in front of Joel Seligman. There is perhaps no other person who knows more about the history of our securities markets and the SEC than Joel. I’ve often thought that if the wing of the Library of Congress containing the nation’s historical documents on our markets was to burn down, the first thing our government would do is call up Joel, with pen in hand, and ask politely, “Could you just start from the beginning?” But when Dean Seligman looks back on these formative times for our markets, I think it’s safe to say that there will be plenty for him to write about. Today, our capital markets are experiencing change at a pace and on a scale we’ve never seen before—driven by the forces of technology and competition. New market entrants, new ways to invest, and new investors are changing the face and form of America’s markets Seven months ago, I talked about a regulatory framework that I believe will continue to give competition the space and sustenance to flourish. In it, multiple market centers compete with one another to produce faster and cheaper executions of securities transactions. A few months later, I addressed a broker’s duty of best execution, which serves as a guardian of competition among our markets. Last week in Chicago, I talked about the two remaining pillars of our National Market System: transparency and linkages. Today I want to revisit the components of best execution, transparency, and linkages, but I want to approach them from a slightly different angle. Often, when we discuss concepts such as “trade execution” or “price visibility” or “order interaction,” it’s easy to get mired in highly technical, even puzzling details. These are not necessarily straightforward concepts, and meaningful debates about their impact on our markets often demand rigorous analysis. It’s easy to lose sight of what’s at the very core of these principles—and that’s serving the investor interest. Execution quality takes some effort to consider, but it is more than worth it for most investors. Best execution is the duty of brokers to obtain the best execution reasonably available for their customers. Put another way, it is a pledge by
A compact, light and highly efficient gas regenerator unit for a positive pressure closed circuit rebreathing apparatus having an improved carbon dioxide scrubber contained within an annular frame to the lower open portion of which a spring loaded flexible diaphragm is sealingly connected around its periphery and across the upper portion of which a cover plate is releasably attachable with the interior of the unit within the frame being divided by a transversely extending partition into a gas conditioning chamber, in which the CO2 of the wearer's expelled breath gases are removed by passing through chemicals compressively retained in the canister of a scrubber extending across the span of the gas conditioning chamber, and a variable volume gas chamber above diaphragm from which the reconditioned gases that have been enriched with a predetermined amount of oxygen are again inhaled. A valving arrangement operated by movement of the spring loaded diaphragm will maintain a positive pressure within the system and add additional oxygen to the variable volume chamber when excess oxygen is being consumed by the wearer or vent excess gases from the chamber when oxygen consumption decreases. An anti-anoxia valve prevents the wearer from breathing into the unit until oxygen is being released into the unit. The design of the reconditioning gas chambers and passages, including the CO2 scrubber, establishes a uniform flow of breathing gases through the scrubber and the unit with a minimum pressure drop.
