Using real options to select stable Middleware-induced software architectures

The requirements that force decisions towards building distributed system architectures are usually of a non-functional nature. Scalability, openness, heterogeneity, and fault-tolerance are examples of such non-functional requirements. The current trend is to build distributed systems with middleware, which provide the application developer with primitives for managing the complexity of distribution, system resources, and for realising many of the non-functional requirements. As non-functional requirements evolve, the `coupling' between the middleware and architecture becomes the focal point for understanding the stability of the distributed software system architecture in the face of change. It is hypothesised that the choice of a stable distributed software architecture depends on the choice of the underlying middleware and its flexibility in responding to future changes in non-functional requirements. Drawing on a case study that adequately represents a medium-size component-based distributed architecture, it is reported how a likely future change in scalability could impact the architectural structure of two versions, each induced with a distinct middleware: one with CORBA and the other with J2EE. An option-based model is derived to value the flexibility of the induced-architectures and to guide the selection. The hypothesis is verified to be true for the given change. The paper concludes with some observations that could stimulate future research in the area of relating requirements to software architectures

Unification of Transactions and Replication in Three-Tier Architectures Based on CORBA

In this paper, we describe a software infrastructure that unifies transactions and replication in three-tier architectures and provides data consistency and high availability for enterprise applications. The infrastructure uses transactions based on the CORBA object transaction service to protect the application data in databases on stable storage, using a roll-backward recovery strategy, and replication based on the fault tolerant CORBA standard to protect the middle-tier servers, using a roll-forward recovery strategy. The infrastructure replicates the middle-tier servers to protect the application business logic processing. In addition, it replicates the transaction coordinator, which renders the two-phase commit protocol nonblocking and, thus, avoids potentially long service disruptions caused by failure of the coordinator. The infrastructure handles the interactions between the replicated middle-tier servers and the database servers through replicated gateways that prevent duplicate requests from reaching the database servers. It implements automatic client-side failover mechanisms, which guarantee that clients know the outcome of the requests that they have made, and retries aborted transactions automatically on behalf of the clients

Multicast-Based Interactive-Group Object-Replication For Fault Tolerance

Distributed systems are clusters of computers working together on one task. The sharing of information across different architectures, and the timely and efficient use of the network resources for communication among computers are some of the problems involved in the implementation of a distributed system. In the case of a low latency system, the network utilization and the responsiveness of the communication mechanism are even more critical. This thesis introduces a new approach for the distribution of messages to computers in the system, in which, the Common Object Request Broker Architecture (CORBA) is used in conjunction with IP multicast to implement a fault-tolerant, low latency distributed system. Fault tolerance is achieved by replication of the current state of the system across several hosts. An update of the current state is initiated by a client application that contacts one of the state object replicas. The new information needs to be distributed to all the members of the distributed system (the object replicas). This state update is accomplished by using a two-phase commit protocol, which is implemented using a binary tree structure along with IP multicast to reduce the amount of network utilization, distribute the computation load associated with state propagation, and to achieve faster communication among the members of the distributed system. The use of IP multicast enhances the speed of message distribution, while the two-phase commit protocol encapsulates IP multicast to produce a reliable multicast service that is suitable for fault tolerant, distributed low latency applications. The binary tree structure, finally, is essential for the load sharing of the state commit response collection processing

The CORBA object group service:a service approach to object groups in CORBA

Distributed computing is one of the major trends in the computer industry. As systems become more distributed, they also become more complex and have to deal with new kinds of problems, such as partial crashes and link failures. To answer the growing demand in distributed technologies, several middleware environments have emerged during the last few years. These environments however lack support for "one-to-many" communication primitives; such primitives greatly simplify the development of several types of applications that have requirements for high availability, fault tolerance, parallel processing, or collaborative work. One-to-many interactions can be provided by group communication. It manages groups of objects and provides primitives for sending messages to all members of a group, with various reliability and ordering guarantees. A group constitutes a logical addressing facility: messages can be issued to a group without having to know the number, identity, or location of individual members. The notion of group has proven to be very useful for providing high availability through replication: a set of replicas constitutes a group, but are viewed by clients as a single entity in the system. This thesis aims at studying and proposing solutions to the problem of object group support in object-based middleware environments. It surveys and evaluates different approaches to this problem. Based on this evaluation, we propose a system model and an open architecture to add support for object groups to the CORBA middle- ware environment. In doing so, we provide the application developer with powerful group primitives in the context of a standard object-based environment. This thesis contributes to ongoing standardization efforts that aim to support fault tolerance in CORBA, using entity redundancy. The group architecture proposed in this thesis — the Object Group Service (OGS) — is based on the concept of component integration. It consists of several distinct components that provide various facilities for reliable distributed computing and that are reusable in isolation. Group support is ultimately provided by combining these components. OGS defines an object-oriented framework of CORBA components for reliable distributed systems. The OGS components include a group membership service, which keeps track of the composition of object groups, a group multicast service, which provides delivery of messages to all group members, a consensus service, which allows several CORBA objects to resolve distributed agreement problems, and a monitoring service, which provides distributed failure detection mechanisms. OGS includes support for dynamic group membership and for group multicast with various reliability and ordering guarantees. It defines interfaces for active and primary-backup replication. In addition, OGS proposes several execution styles and various levels of transparency. A prototype implementation of OGS has been realized in the context of this thesis. This implementation is available for two commercial ORBs (Orbix and VisiBroker). It relies solely on the CORBA specification, and is thus portable to any compliant ORB. Although the main theme of this thesis deals with system architecture, we have developed some original algorithms to implement group support in OGS. We analyze these algorithms and implementation choices in this dissertation, and we evaluate them in terms of efficiency. We also illustrate the use of OGS through example applications

Contributions aux systèmes répartis en environnements ubiquitaires : adaptation, sensibilité au contexte et tolérance aux fautes

D'années en années, nous observons l'arrivée sur le marche d'ordinateurs personnels de plus en plus petits pour des utilisateurs de plus en plus nombreux, ainsi des assistants personnels numériques et des objets dits connectés, en passant par les téléphones mobiles. Tous ces dispositifs tendent à être interchangeables du point de vue des ressources en mémoire, en calcul et en connectivité : par exemple, les téléphones mobiles sont devenus des équipements informatiques de moins en moins spécialisés ou de plus en plus universels et font dorénavant office en la matière de portails d'accès aux capteurs présents dans l'environnement immédiat de l'utilisateur. L'enjeu abordé dans nos travaux est la construction de systèmes répartis incluant ces nouveaux dispositifs matériels. L'objectif de mes recherches est la conception des paradigmes d'intermédiation génériques sous-jacents aux applications réparties de plus en plus ubiquitaires. Plus particulièrement, la problématique générale de mes travaux est la définition du rôle des intergiciels dans l'intégration des dispositifs mobiles et des objets connectés dans les architectures logicielles réparties. Ces architectures logicielles reposaient très majoritairement sur des infrastructures logicielles fixes au début des travaux présentés dans ce manuscrit. Dans ce manuscrit, je décris mes travaux sur trois sujets : 1) l'adaptation des applications réparties pour la continuité de service pendant les déconnexions, 2) la gestion des informations du contexte d'exécution des applications réparties pour leur sensibilité au contexte, et 3) les mécanismes de détection des entraves dans les environnements fortement dynamiques tels que ceux construits avec des réseaux mobiles spontanés. Sur le premier sujet, nous fournissons une couche intergicielle générique pour la gestion des aspects répartis de la gestion des déconnexions en utilisant une stratégie d'adaptation collaborative dans les architectures à base d'objets et de composants. Sur le deuxième sujet, nous étudions les paradigmes architecturaux pour la construction d'un service de gestion de contexte générique, afin d'adresser la diversité des traitements (fusion et agrégation, corrélation, détection de situation par apprentissage, etc.), puis nous adressons le problème de la distribution des informations de contexte aux différentes échelles de l'Internet des objets. Enfin, sur le troisième sujet, nous commençons par la détection des modes de fonctionnement pour l'adaptation aux déconnexions afin de faire la différence, lorsque cela est possible, entre une déconnexion et une défaillance, et ensuite nous spécifions et construisons un service de gestion de groupe partitionnable. Ce service est assez fort pour interdire la construction de partitions ne correspondant pas à la réalité de l'environnement à un instant donné et est assez faible pour être mis en oeuvre algorithmiquemen

Integrating legacy mainframe systems: architectural issues and solutions

For more than 30 years, mainframe computers have been the backbone of computing systems throughout the world. Even today it is estimated that some 80% of the worlds' data is held on such machines. However, new business requirements and pressure from evolving technologies, such as the Internet is pushing these existing systems to their limits and they are reaching breaking point. The Banking and Financial Sectors in particular have been relying on mainframes for the longest time to do their business and as a result it is they that feel these pressures the most. In recent years there have been various solutions for enabling a re-engineering of these legacy systems. It quickly became clear that to completely rewrite them was not possible so various integration strategies emerged. Out of these new integration strategies, the CORBA standard by the Object Management Group emerged as the strongest, providing a standards based solution that enabled the mainframe applications become a peer in a distributed computing environment. However, the requirements did not stop there. The mainframe systems were reliable, secure, scalable and fast, so any integration strategy had to ensure that the new distributed systems did not lose any of these benefits. Various patterns or general solutions to the problem of meeting these requirements have arisen and this research looks at applying some of these patterns to mainframe based CORBA applications. The purpose of this research is to examine some of the issues involved with making mainframebased legacy applications inter-operate with newer Object Oriented Technologies

A middleware service for fault-tolerant group communications

PhD ThesisMany distributed applications require multicast group communication services, enabling an entity to interact with a group of other entities. Providing the reliability and ordering guarantees required by group based applications is not a trivial task in distributed systems where computation and communication delays might not be known accurately. Furthermore, the approaches available to support these guarantees are diverse. The choice of approach may significantly effect the performance of an application and/or may not be suitable for some application types. Nowadays, distributed applications are frequently built as a Middleware service. The Thesis develops techniques for providing group communication support in Middleware environments. A group communication service has been designed and implemented in such a way as not to hinder the interoperability/portability of applications built using it. The service provides a variety of functions that may be tailored to suit many different types of applications. Group communication protocols are presented that ensure reliability and ordering guarantees. Furthermore, the reliability and ordering guarantees of such protocols may be tailored to suit a wide variety of applications. Mechanisms that provide a variety of approaches to inter-member and inter-group interactions that are suitable for satisfying the requirements of many different types of applications (e.g., fault- tolerant, collaborative) are also supported. The service can work over local and wide area networks (Internet).Hewlett Packard laboratories Engineering and Physical Science Research Counci