A. El Abbadi

Advanced transaction models in workflow contexts

In recent years, numerous transaction models have been proposed to address the problems posed by advanced database applications, but only a few of these models are being used in commercial products. In this paper, we make the case that such models may be too centered around databases to be useful in real environments. Advanced applications raise a variety of issues that are not addressed at all by transaction models. These same issues, however, are the basis for existing workflow systems, which are having considerable success as commercial products in spite of not having a solid theoretical foundation. We explore some of these issues and show that, in many aspects, workflow models are a superset of transaction models and have the added advantage of incorporating a variety of ideas that have so far remained outside the scope of traditional transaction processing.

Efficient view maintenance at data warehouses

We present incremental view maintenance algorithms for a data warehouse derived from multiple distributed autonomous data sources. We begin with a detailed framework for analyzing view maintenance algorithms for multiple data sources with concurrent updates. Earlier approaches for view maintenance in the presence of concurrent updates typically require two types of messages: one to compute the view change due to the initial update and the other to compensate the view change due to interfering concurrent updates. The algorithms developed in this paper instead perform the compensation locally by using the information that is already available at the data warehouse. The first algorithm, termed SWEEP, ensures complete consistency of the view at the data warehouse in the presence of concurrent updates. Previous algorithms for incremental view maintenance either required a quiescent state at the data warehouse or required an exponential number of messages in terms of the data sources. In contrast, this algorithm does not require that the data warehouse be in a quiescent state for incorporating the new views and also the message complexity is linear in the number of data sources. The second algorithm, termed Nested SWEEP, attempts to compute a composite view change for multiple updates that occur concurrently while maintaining strong consistency.

Maintaining availability in partitioned replicated databases

In a replicated database, a data item may have copies residing on several sites. A replica control protocol is necessary to ensure that data items with several copies behave as if they consist of a single copy, as far as users can tell. We describe a new replica control protocol that allows the accessing of data in spite of site failures and network partitioning. This protocol provides the database designer with a large degree of flexibility in deciding the degree of data availability, as well as the cost of accessing data.

Exotica/FMQM: A Persistent Message-Based Architecture for Distributed Workflow Management

In the past few years there has been an increasing interest in workflow applications as a way of supporting complex business processes in modern corporations. Given the nature of the environment and the technology involved, workflow applications are inherently distributed and pose many interesting challenges to the system designer. In most cases, a client/server architecture is used in which knowledge about the processes being executed is centralized in one node to facilitate monitoring, auditing, and to simplify synchronization. In this paper, we explore a novel distributed architecture, Exotica/FMQM, for workflow systems in which the need for such a centralized database is eliminated. Instead, we use persistent messages as the means to store the information relevant to the execution of a business process. Our approach is to completely distribute the execution of a process so individual nodes are independent. The advantages of this approach are increased resilience to failures and greater scalability and flexibility of the system configuration.

Epidemic algorithms for replicated databases

We present a family of epidemic algorithms for maintaining replicated database systems. The algorithms are based on the causal delivery of log records where each record corresponds to one transaction instead of one operation. The first algorithm in this family is a pessimistic protocol that ensures serializability and guarantees strict executions. Since we expect the epidemic algorithms to be used in environments with low probability of conflicts among transactions, we develop a variant of the pessimistic algorithm which is optimistic in that transactions commit as soon as they terminate locally and inconsistencies are detected asynchronously as the effects of committed transactions propagate through the system. The last member of the family of epidemic algorithms is pessimistic and uses voting with quorums to resolve conflicts and improve transaction response time. A simulation study evaluates the performance of the protocols.

A peer-to-peer framework for caching range queries

Peer-to-peer systems are mainly used for object sharing although they can provide the infrastructure for many other applications. We extend the idea of object sharing to data sharing on a peer-to-peer system. We propose a method, which is based on the multidimensional CAN system, for efficiently evaluating range queries. The answers of the range queries are cached at the peers and are used to answer future range queries. The scalability and efficiency of our design is shown through simulation.

Approximate nearest neighbor searching in multimedia databases

Develops a general framework for approximate nearest-neighbor queries. We categorize the current approaches for nearest-neighbor query processing based on either their ability to reduce the data set that needs to be examined, or their ability to reduce the representation size of each data object. We first propose modifications to well-known techniques to support the progressive processing of approximate nearest-neighbor queries. A user may therefore stop the retrieval process once enough information has been returned. We then develop a new technique based on clustering that merges the benefits of the two general classes of approaches. Our cluster-based approach allows a user to progressively explore the approximate results with increasing accuracy. We propose a new metric for evaluation of approximate nearest-neighbor searching techniques. Using both the proposed and the traditional metrics, we analyze and compare several techniques with a detailed performance evaluation. We demonstrate the feasibility and efficiency of approximate nearest-neighbor searching. We perform experiments on several real data sets and establish the superiority of the proposed cluster-based technique over the existing techniques for approximate nearest-neighbor searching.

The generalized tree quorum protocol: an efficient approach for managing replicated data

In this paper, we present a low-cost fault-tolerant protocol for managing replicated data. We impose a logical tree structure on the set of copies of an object and develop a protocol that uses the information available in the logical structure to reduce the communication requirements for read and write operations. The tree quorum protocol is a generalization of the static voting protocol with two degrees of freedom for choosing quorums. In general, this results in significantly lower communication costs for comparable data availability. The protocol exhibits the property of graceful degradation, i.e., communication costs for executing operations are minimal in a failure-free environment but may increase as failures occur. This approach in designing distributed systems is desirable since it provides fault-tolerance without imposing unnecessary costs on the failure-free mode of operations.

Convergence Rates of Distributed Average Consensus With Stochastic Link Failures

We consider a distributed average consensus algorithm over a network in which communication links fail with independent probability. In such stochastic networks, convergence is defined in terms of the variance of deviation from average. We first show how the problem can be recast as a linear system with multiplicative random inputs which model link failures. We then use our formulation to derive recursion equations for the second order statistics of the deviation from average in networks with and without additive noise. We give expressions for the convergence behavior in the asymptotic limits of small failure probability and large networks. We also present simulation-free methods for computing the second order statistics in each network model and use these methods to study the behavior of various network examples as a function of link failure probability.

Epidemic algorithms in replicated databases (extended abstract)

1 Introduction We present a family of epidemic algorithms for maintaining replicated data in a transactional framework. The algorithms are based on the causal delivery of log records where each record corresponds to one transaction instead of one operation. The fist algorithm in this family is a pessimistic protocol that ensures serializability and guarantees strict executions. Since we expect the epidemic algorithms to be used in environments with low probability of conflicts among transactions, we develop a variant of the pessimistic algorithm in which locks are released as soon as transactions finish their execution locally. However, this optimistic releasing of locks introduces the possibility of cascading aborts while ensuring serializable executions. The last member of thii family of epidemic algorithms is motivated from the need for asynchronous replication solutions that are being increasingly used in commercial systems. The protocol is optimistic in that transactions commit as soon as they terminate locally and inconsistencies are detected asynchronously as the effects of committed transactions propagate through the system. With the proliferation of computer networks, PCs, and workstations, new models for workplaces are emerging. In particular, organizations need to provide ready access to corporate information to users who may or may not always be connected to the database. One way to provide access to such data is through replication. However, traditional synchronous solutions for managing replicated data [Sto79, Tho79, Gif79] can not be used especially in such a distributed, mobile, and disconnected environment. As the need for replication grows, several vendors have adopted asynchronous solutions for managing repli-cate data pBH+88, Ora]. For example, Lotus Notes uses value-base& replication in which updates are performed locally and a propagation mechanism is provided to apply these updates to other replica sites. In addition, a version number is used to detect inconsistencies. Resolution of inconsistencies is left to the users. Although the Lotus approach works reasonably well for single object updates (i.e., environments such as file-systems), it fails when multiple objects are involved in a single update (i.e., transaction oriented environments). In particular, more formal mechanisms are needed for update propagation and conflict detection in the context of asynchronous replication.