Advanced Traveler Information Systems (ATIS) require efficient information retrieval and updating in a dynamic environment at different geographical scales. ATIS applications are useful in yielding a better utilization of the limited costly transportation arteries and providing value-added traveler information. Many ATIS applications are built on the functionalities provided by Geographical Information Systems (GIS), which often cannot meet extra requirements like real-time response. We investigate GIS-based systems in ATIS and propose a system architecture based on GIS and distributed database technology. Issues on data modeling, data representation, storage and retrieval, data aggregation, and parallel processing of queries are discussed. This paper introduces a distributed system architecture for ATIS based on recent technology. It presents new data models for information representation and proposes data shipping for efficient query processing and function shipping for reducing communication overhead. The paper also examines the use of a network of computers for solving complex problems more timely and privacy protection for sensitive data.

Shared objects are used in the distributed environment for synchronization, date sharing and service provision. They are usually replicated in a distributed system for higher availability and fault tolerance. Coherent or strongly consistent implementation of replicated objects has been studied extensively in the literature. This paper concentrates on a general method for specifying and implementing replicated objects that may require weaker consistency conditions. The weakness of the objects is left open and users are allowed to specify their objects by defining how concurrent sets of operations should be performed. The implementation of a replicated object consists of two modules: one module is an algorithm to identify concurrent sets from a distributed computation and the other module is a user-defined procedure to process the concurrent sets. We show examples of weaker objects that can be defined and implemented more efficiently because of the less stringent requirements imposed on these objects. Several objects, including a shared dictionary are used to demonstrate our scheme and we show that non-blocking implementations of some of these objects are possible.

A token based algorithm for distributed mutual exclusion is presented. It uses a distributed counter to tolerate faults due to site failures and communication failures. This eliminates the need for expensive election protocols that are commonly employed in existing token-based algorithms. As in many existing fault-tolerant mutual exclusion algorithms, timeouts are used to detect failures. Our algorithm has the unique property that even if timeout periods are incorrectly assumed, the safety requirement of mutual exclusion is still guaranteed. It is, therefore, suitable for highly asynchronous distributed environments. Performance analysis shows that the algorithm is also efficient in terms of the average number of messages required per critical section, and the average time delay to enter the critical section. Finally, we show how to obtain a bounded algorithm from the basic unbounded algorithm.

Shared objects are usually replicated in a distributed environment for higher availability and fault tolerance. Coherent or strongly consistent implementation of replicated objects have been studied extensively in the literature. This paper concentrates on a general method for specifying and implementing replicated objects that only require weaker consistency conditions. The weakness of the objects is left open and users are allowed to specify their objects by defining how concurrent sets of operations should be performed. The implementation of a replicated object consists of two modules: one module is an algorithm to identify concurrent sets from a distributed computation, and the other module is a user defined procedure to process the concurrent sets. We show examples of weaker objects that can be defined and implemented efficiently because of the less stringent requirements imposed on these objects. In particular, the ordered set dictionary problem is used to demonstrate our scheme and we show that non-blocking implementation of the dictionary is possible.

The efficiency of channel assignment in a cellular telephone system is considered using the measures of successful channel assignment ratio as well as response time. Existing paradigms of mutual exclusion and dining philosophers from distributed systems are used to synthesize new algorithms that optimize both measures. The results are verified by extensive simulations.

The fault-tolerance of distributed algorithms is investigated in asynchronous message passing systems with undetectable process failures. Two specific synchronization problems are considered, the dining philosophers problem and the binary committee coordination problem. The abstraction of a bounded doorway is introduced as a general mechanism for achieving individual progress and good failure locality. Using it as a building block, optimal fault-tolerant algorithms are constructed for the two problems.

Recoveryby checkpointing on distributed shared memory systems is investigated in this paper.The no-

Solutions to resource allocation problems and other related synchronization problems in distributed systems are examined with respect to the measures ofresponse time, message complexity,andfailure locality. Response time measures the time it takes for an algorithm to respond to the requests of a process; message complexity measures the number of messages sent and received by a process; and failure locality characterizes the size of the network that is affected by the failure of a single process. An algorithm for the resource allocation problem that achieves a constant failure locality of four along with a quadratic response time and a quadratic message complexity is presented. Applications of the algorithm to other process synchronization problems in distributed systems are also demonstrated.

Maya is a platform for investigating the impact of different memory coherence protocols on parallel architectures. We present the implementations of several weak memory protocols, together with some new primitives dedicated to weak memories using Maya. The results of some user applications are summarized and the impact of weak memories on the efficiency of these parallel programs is discussed.>

Algorithms for mutual exclusion that adapt to the current degree of contention are developed. Afilter and a leader election algorithm form the basic building blocks. The algorithms achieve system response times that are independent of the total number of processes and governed instead by the current degree of contention. The final algorithm achieves a constant amortized system response time.

Solutions to resource allocation problems in distributed systems are examined with respect to the measures of response time, message complexity, and failure locality. Response time measures the time it takes for an algorithm to respond to the requests of a process, message complexity measures the number of messages sent and received by a process, and failure locality characterizes the size of the network that is affected by the failure of a single process. An algorithm that achieves a constant failure locality of four along with a quadratic response time and a quadratic message complexity is presented.