Mark Cameron Little

Transactions and Web services

Helping to realize the full potential of e-commerce.

Fixing the “broken-link” problem: the W3Objects approach

Abstract One of most serious problems plaguing the World Wide Web today is that of broken hypertext links, which are a major annoyance to browsing users and also a cause of tarnished reputation and possible loss of opportunity for information providers. The root of the problem lies in the current Web architectures lack of support for referential integrity. This paper presents a model for the provision of referential integrity for Web resources which supports resource migration and tolerates site and communication failures. The approach is object-oriented, highly flexible, completely distributed, and does not require any global administration. An attractive feature of our design is the provision of a lightweight mechanism which provides referential integrity, and which may be customised on a per resource basis to provide increased fault-tolerance and performance. Our system follows an evolutionary approach, supporting parallel operation with the existing Web, allowing users to gain the additional benefits of referential integrity while allowing continued access through trusted software components.

Coordinating business transactions on the Web

The Organization for the Advancement of Structured Information Standards (OASIS) has developed the business transaction protocol (BTP) to meet the requirements of Web-based long-running collaborative business applications. BTP is designed to support interactions that cross application and administrative boundaries, thus requiring extended transactional support beyond classical ACID. We examine the drivers behind BTP and the way it might be applied in a potential use case.

Maintaining information about persistent replicated objects in a distributed system

Presents a general model for persistent replicated object management and identify what metainformation about objects needs to be maintained by a naming and binding service to ensure that objects named by application programs are bound to only those object replicas which are in a mutually consistent state. These ideas are developed within the framework of a distributed system in which application programs are composed of atomic actions (atomic transactions) manipulating persistent (long-lived) objects.<<ETX>>

Design and implemantation of a CORBA fault-tolerant object group service

Many fault-tolerant distributed applications can be structured as one or more groups of objects that cooperate by multicasting invocations on member objects. The building of group based applications is considerably simplified if the members of a group can multicast reliably and have a mutually consistent view of the order in which events (such as invocations, host machine failures) have taken place. With this observation in mind, this paper describes the design and implementation of a CORBA middleware service for managing object groups. The object group service is portable and intended for a wide variety of applications; objects can simultaneously belong to many groups, group size could be large, and objects could be geographically widely separated. The service can provide causality preserving total order delivery to members of a group, ensuring that total order delivery is preserved even for multi-group objects. Both symmetric and asymmetric total order protocols are supported, permitting a member to use say symmetric version in one group and asymmetric version in another group simultaneously. The service is both dynamic and fault-tolerant: ordering and liveness is preserved even if membership changes occur due to (real or suspected) member failures, voluntary member departures and new group formations.

Replicated K-resilient objects in Arjuna

The design of an object replication scheme for the Arjuna distributed system is described. The design supports K-resiliency, where, in the absence of network partitions, K out of a total of K+1 replica failures can be tolerated before an object becomes unavailable. The scheme chosen uses active replication, in which each and every functioning replica of an object carries out processing. Computations are structured as atomic actions (atomic transactions). The details of how object groups are created and terminated, how a group can be evoked, and how object replicas are inserted and removed in a consistent manner in the presence of node failures are presented.<<ETX>>

The Replica Management System: a scheme for flexible and dynamic replication

The actual gains achieved by replication are a complex function of the number of replicas, the placement of those replicas, the replication protocol, the nature of the transactions performed on the replicas, and the availability and performance characteristics of the machines and networks composing the system. This paper describes the design and implementation of the Replica Management System, which allows a programmer to specify the quality of service required for replica groups in terms of availability and performance. From the quality of service specification, information about the replication protocol to be used, and data about the characteristics of the underlying distributed system, the RMS computes an initial placement and replication level. As machines and communications systems are detected to have failed or recovered, or performance characteristics change, the RMS can be re-invoked to compute an updated mapping of replicas which preserves the desired quality of service. The result is a flexible, dynamic and dependable replication system.<<ETX>>

Computing replica placement in distributed systems

The authors investigate the design of a replica management system (RMS) which allows a programmer to specify the quality of service required for individual replicated objects in terms of availability and performance. From the quality of service specification, information about the replication protocol to be used, and data about the characteristics of the underlying distributed system, the RMS computes an initial placement and replication level for the object. As machines and communications systems are detected to have failed or recovered, the RMS can be reinvoked to compute an updated mapping of replicas which preserves the desired quality of service. Preliminary simulation of the authors RMS shows that its placement algorithm gives consistent improvements in the availability of a replicated service than simply placing replicas on nodes at random, as is done in most distributed systems.<<ETX>>

Supporting highly manageable Web services

This paper focuses on the management aspects of Web service provision. We argue that support for manageability has to be considered at the design stage if services are to be capable of delivering high levels of quality of service for their users. Examples of the problems caused by lack of manageability include maintenance operations that necessitate service downtime, or difficulties in ensuring consistency of information. We categorise management issues into those concerning a site as a whole and those pertaining to individual services. Our approach to site management supports the arbitrary distribution of services to machines, allowing the optimum cost/performance configuration to be selected. Services can be easily migrated between machines, resulting in sites that scale, both in terms of the number of services and the number of users. Service management issues may be generalised as supporting evolution, for example, supporting changes to the functionality, the presentation logic, and the overall look and feel of a service. Our approach, based on the separation of functionality and presentation, allows such changes to be performed on-line and ensures that updates are reflected consistency across the various pages of a service, or across services. This approach also facilitates the development of services that utilise dynamic content for service customisations, such as tailoring a service to match the profile of users. Furthermore, all management operations are available through Web-based interfaces, making them accessible to a broad range of users, not only specialist system administrators.

The CORBA activity service framework for supporting extended transactions

Although it has long been realized that ACID (atomicity, consistency, isolation, durability) transactions by themselves are not adequate for structuring long‐lived applications and much research work has been done on developing specific extended transaction models, no middleware support for building extended transactions is currently available and the situation remains that a programmer often has to develop application specific mechanisms. The CORBA Activity Service Framework described in this paper is a way out of this situation. The design of the service is based on the insight that the various extended transaction models can be supported by providing a general purpose event signalling mechanism that can be programmed to enable activities—application specific units of computations—to coordinate each other in a manner prescribed by the model under consideration. The different extended transaction models can be mapped onto specific implementations of this framework, permitting such transactions to span a network of systems connected indirectly by some distribution infrastructure. The framework described in this paper is an overview of the OMGs (Object Management Group) Additional Structuring Mechanisms for the OTS standard. Through a number of examples the paper shows that the framework has the flexibility to support a wide variety of extended transaction models. Although the framework is presented here in CORBA specific terms, the main ideas are sufficiently general, so that it should be possible to use them in conjunction with other middleware. Copyright