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 di erent 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 middleware 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 e orts 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 de nes 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 de nes 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 speci cation, 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 e ciency. We also illustrate the use of OGS through example applications.