From Cooperative Scans to Predictive Buffer Management

In analytical applications, database systems often need to sustain workloads with multiple concurrent scans hitting the same table. The Cooperative Scans (CScans) framework, which introduces an Active Buffer Manager (ABM) component into the database architecture, has been the most effective and elaborate response to this problem, and was initially developed in the X100 research prototype. We now report on the the experiences of integrating Cooperative Scans into its industrial-strength successor, the Vectorwise database product. During this implementation we invented a simpler optimization of concurrent scan buffer management, called Predictive Buffer Management (PBM). PBM is based on the observation that in a workload with long-running scans, the buffer manager has quite a bit of information on the workload in the immediate future, such that an approximation of the ideal OPT algorithm becomes feasible. In the evaluation on both synthetic benchmarks as well as a TPC-H throughput run we compare the benefits of naive buffer management (LRU) versus CScans, PBM and OPT; showing that PBM achieves benefits close to Cooperative Scans, while incurring much lower architectural impact.Comment: VLDB201

Assumption-based reasoning in dynamic normative agent systems

In this paper we address dynamic assumption-based reasoning in open agent systems, where, unavoidably, agents have incomplete knowledge about their environment and about other agents. The interactions among agents in such systems are typically subject to norms, which stipulate what each agent is obliged, permitted, prohibited, empowered etc. to do, while it participates in the system. In such environments agents need to resort to assumptions, in order to establish what actions are appropriate to perform, and they need to do so dynamically, since the environment, the agents that exist in it, the information that is exchanged between them, and the normative relations between them change over time. In earlier work, we had proposed Default Theory construction to support dynamic assumption-based reasoning. We argued that in this way, agents could perform both assumption identification and employment dynamically, contrary to other approaches to assumption-based reasoning, which catered for either one or the other. A shortcoming of this early proposal of ours, though, is that Default Theory construction seems to require proof, which is notably computationally expensive. In this paper we present a computational technique that can be used for this construction in an incremental manner that does not depend on proof, and a prototype tool that we developed for experimentation. In a nutshell, depending on their current knowledge at any given time, agents can identify appropriate candidate assumptions in an ad hoc manner. When such choices need to be revised, agents can reconstruct their view of the possible world in which they find themselves, and establish their revised assumption requirements at run-time. © 2010 - IOS Press and the authors. All rights reserved

Normative conflicts:Patterns,detection and resolution

The analysis, representation and management of normative conflicts have been the focus of much research in recent years in commercial and business applications. In this paper we are concerned with normative conflicts that arise for agents engaging in electronic contracting. First, we identify a set of primitive conflict patterns and present some patterns that have not been identified in other proposals. Secondly, we use a representation of e-contracts as Default Theories, which afford us both detection and resolution of such conflict patterns

Normative conflicts in electronic contracts

We are concerned with normative conflicts that arise for agents engaging in electronic contracting within an electronic marketplace. We identify a set of primitive conflict patterns, examine other analyses of conflicts found in the literature of distributed systems, legal reasoning and multi-agent interaction, and show how these may be seen as instances of primitive patterns. We also present additional patterns that have not been identified previously. Moreover, this work proposes and illustrates the representation of e-contracts in default logic, and argues that such a representation of contract norms as default rules facilitates the detection, prediction and resolution of normative conflicts. (C) 2010 Elsevier B. V. All rights reserved

How can agents know what to assume when?

The work in this paper is motivated from the need for assumption-based reasoning in normative systems, where, realistically, agents will have incomplete knowledge about their environment, and about other agents. The question we seek to address is whether it is possible for agents to identify appropriate assumptions dynamically, in order to fill in informational gaps. We discuss and illustrate our proposal with reference to an e-commerce example. In our previous work, we argued that e-contracts could be represented as Default Theories and proposed a theoretical way in which such theories could be constructed automatically from initial Event Calculus representations. That proposal relied on determining what information could be proved from the agent's knowledge base, in order to decide whether it would serve as an assumption or not. In this paper we present an incremental technique that can be used for this construction that enables the dynamic and ad hoc identification of candidate assumptions, without resorting to proof. © 2008 IEEE

Defeasible reasoning with e-contracts

We propose a mapping from a contract representation in Event Calculus to Default Logic. The resulting representation allows for defeasible reasoning with e-contracts, which is useful in order to determine the normative state of a business exchange in the presence of incomplete or inaccurate knowledge. More importantly, we may use this representation in Default Logic coupled with priorities, in order to manage normative conflicts dynamically. © 2006 IEEE

The role of assumption identification in autonomous agent reasoning

The concept of autonomy is central to the notion of agenthood and has been addressed by many researchers in the multi-agent systems community. We argue that the degree to which an agent's reasoning is autonomous is affected by the degree to which it is able to choose its assumptions autonomously. We discuss a technique that enables agents to identify appropriate assumptions dynamically, by employing representations of norms in Default Logic. Copyright © 2009, International Foundation for Autonomous Agents and Multiagent Systems

The representation of e-contracts as default theories

It is widely acknowledged that a temporal representation of e-contracts is essential in order to support e-contract execution and performance monitoring. One possibility that has been explored by many researchers is to represent e-contracts in Event Calculus. Although such representations are intuitive and facilitate temporal reasoning about actions/events and their factual and normative effects, they fall short in situations where domain knowledge cannot be assumed to be complete. Moreover, it is not clear how dynamic normative conflict resolution can be achieved, without resorting to unintuitive representations for conflict resolution strategies. In order to maintain the benefits of an underlying Event Calculus representation, and incorporate assumption-based reasoning and dynamic conflict management capability, we propose a representation of e-contracts as Default Theories, which are constructed by translating Event Calculus representations dynamically. Finally, we discuss how the resulting Default Theory representation enables a software agent to address various reasoning problems. © Springer-Verlag Berlin Heidelberg 2007