Alejandro P. Buchmann

The HiPAC project: combining active databases and timing constraints

The HiPAC (High Performance ACtive database system) project addresses two critical problems in time-constrained data management: the handling of timing constraints in databases, and the avoidance of wasteful polling through the use of situation-action rules that are an integral part of the database and are monitored by DBMSs condition monitor. A rich knowledge model provides the necessary primitives for definition of timing constraints, situation-action rules, and precipitating events. The execution model allows various coupling modes between transactions, situation evaluations and actions, and provides the framework for correct concurrent execution of transactions and triggered actions. Different approaches to scheduling of time-constrained tasks and transactions are explored and an architecture is being designed with special emphasis on the interaction of the time-constrained, active DBMS and the operating system. Performance models are developed to evaluate the various design alternatives.

A peer-to-peer approach to content-based publish/subscribe

Publish/subscribe systems are successfully used to decouple distributed applications. However, their efficiency is closely tied to the topology of the underlying network, the design of which has been neglected. Peer-to-peer network topologies can offer inherently bounded delivery depth, load sharing, and self-organisation. In this paper, we present a content-based publish/subscribe system routed over a peer-to-peer topology graph. The implications of combining these approaches are explored and a particular implementation using elements from Rebeca and Chord is proven correct.

Encoded bitmap indexing for data warehouses

Complex query types, huge data volumes, and very high read/update ratios make the indexing techniques designed and tuned for traditional database systems unsuitable for data warehouses (DW). We propose an encoded bitmap indexing for DWs which improves the performance of known bitmap indexing in the case of large cardinality domains. A performance analysis and theorems which identify properties of good encodings for better performance are presented. We compare encoded bitmap indexing with related techniques, such as bit slicing, projection-, dynamic-, and range-based indexing.

Bubblestorm: resilient, probabilistic, and exhaustive peer-to-peer search

Peer-to-peer systems promise inexpensive scalability, adaptability, and robustness. Thus, they are an attractive platform for file sharing, distributed wikis, and search engines. These applications often store weakly structured data, requiring sophisticated search algorithms. To simplify the search problem, most scalable algorithms introduce structure to the network. However, churn or violent disruption may break this structure, compromising search guarantees. This paper proposes a simple probabilistic search system, BubbleStorm, built on random multigraphs. Our primary contribution is a flexible and reliable strategy for performing exhaustive search. BubbleStorm also exploits the heterogeneous bandwidth of peers. However, we sacrifice some of this bandwidth for high parallelism and low latency. The provided search guarantees are tunable, with success probability adjustable well into the realm of reliable systems. For validation, we simulate a network with one million low-end peers and show BubbleStorm handles up to 90% simultaneous peer departure and 50% simultaneous crash.

Technologies for E-Services

In the traditional application model, services are tightly coupled with the processes they support. For example, whenever a server’s process changes, existing clients using that process must also be updated. However, electronic commerce is moving toward e-service based interactions, where corporate enterprises use e-services to interact with each other dynamically, and a service in one enterprise could spontaneously decide to engage a service fronted by another enterprise. We clarify here the relationship between currently developing standards such as UDDI, WSDL, and WSCL, and propose a conversation controller mechanism that leverages such standards to direct services in their conversations. We can thus treat services as pools of methods, independent of the conversations they support. Even method names can be decided on independently of the conversations. Services can spontaneously discover each other and then engage in complicated interactions without the services themselves having to explicitly support conversational logic. The dynamism and flexibility enabled by this decoupling is the essential difference between applications offered over the web and e-services.

Event-based applications and enabling technologies

Event processing has become the paradigm of choice in many monitoring and reactive applications. However, the understanding of events, their composition and level of abstraction, the style of processing and the quality of service requirements vary drastically across application domains. We introduce the basic notions of event processing to create a common understanding, present the enabling technologies that are used for the implementation of event-based systems, survey a wide range of applications identifying their main features, and discuss open research issues.

Event composition in time-dependent distributed systems

Many interesting application systems, ranging from workflow management and CSCW to air traffic control, are event-driven and time-dependent and must interact with heterogeneous components in the real world. Event services are used to glue together distributed components. They assume a virtual global time base to trigger actions and to order events. The notion of a global time that is provided by synchronized local clocks in distributed systems has a fundamental impact on the semantics of event-driven systems, especially the composition of events. The well studied 2g-precedence model, which assumes that the granularity of global time-base g can be derived from a priori known and bounded precision of local clocks may not be suitable for the Internet where the accuracy and external synchronization of local clocks is best effort and cannot be guaranteed because of large transmission delay variations and phases of disconnection. We introduce a mechanism based on NTP synchronized local clocks with global reference time injected by GPS time servers. We agree that timestamps of events can be related to global reference time with bounded accuracy and propose that event timestamps are modeled using accuracy intervals. We present algorithms for event composition and event consumption which make use of accuracy interval based timestamping and illustrate the problems that arise due to inaccuracy and message transmission delays.

Rules in an Open System: The REACH Rule System

REACH is an active object system intended to control heterogeneous systems, possibly under timing constraints. When dealing with open systems in which the controlled system may execute irreversible actions, many notions of active databases must be revised and adapted to this situation. In this paper we draw the system boundaries between controlling and controlled system, present a rule system that includes events and actions both in the controlled and the controlling systems and analyze the effects of this open environment on the rule structure. We identify two new coupling modes, sequential causally dependent and exclusive causally dependent, which are necessary for handling irreversible actions in external systems and contingency plans, respectively.

Research Issues in Data Warehousing

Data warehousing is a booming industry with many interesting research problems. The database research community has concentrated on only a few aspects. In this paper, We summarize the state of the art, suggest architectural extensions and identify research problems in the areas of warehouse modeling and design, data cleansing and loading, data refreshing and purging, metadata management, extensions to relational operators, alternative implementations of traditional relational operators, special index structures and query optimization with aggregates.

Filter Similarities in Content-Based Publish/Subscribe Systems

Matching notifications to subscriptions and routing notifications from producers to interested consumers are the main problems in large-scale publish/subscribe systems.Most previously proposed distributed notification services either use flooding or, if filtering is performed, they assume that each event broker has global knowledge about all active subscriptions. Both approaches degrade the scalability of notification services as the former wastes network resources and the latter generates overly large routing tables.In this paper we describe content-based routing algorithms that exploit filter similarities in order to reduce the size of routing tables and the number of control messages that are exchanged among the brokers in order to keep the routing tables up-to-date. In particular, the proposed algorithms do not assume global knowledge about all active subscriptions. Furthermore, we describe how these optimizations can be supported if the underlying data and filter model is based on structured records.