David Mutchler

Dynamic voting algorithms for maintaining the consistency of a replicated database

There are several replica control algorithms for managing replicated files in the face of network partitioning due to site or communication link failures. Pessimistic algorithms ensure consistency at the price of reduced availability; they permit at most one (distinguished) partition to process updates at any given time. The best known pessimistic algorithm, voting, is a “static” algorithm, meaning that all potential distinguished partitions can be listed in advance. We present a dynamic extension of voting called dynamic voting. This algorithm permits updates in a partition provided it contains more than half of the up-to-date copies of the replicated file. We also present an extension of dynamic voting called dynamic voting with linearly ordered copies (abbreviated as dynamic-linear). These algorithms are dynamic because the order in which past distinguished partitions were created plays a role in the selection of the next distinguished partition. Our algorithms have all the virtues of ordinary voting, including its simplicity, and provide improved availability as well. We provide two stochastic models to support the latter claim. In the first (site) model, sites may fail but communication links are infallible; in the second (link) model the reverse is true. We prove that under the site model, dynamic-linear has greater availability than any static algorithm, including weighted voting, if there are four or more sites in the network. In the link model, we consider all biconnected five-site networks and a wide variety of failure and repair rates. In all cases considered, dynamic-linear had greater availability than any static algorithm.

Dynamic voting

In a voting-based algorithm, a replicated file can be updated in a partition if it contains a majority of copies. In this paper, we propose an extension of this scheme which permits a file to be updated in a partition provided it contains a majority of up-to-date copies. Our scheme not only preserves mutual consistency of the replicated file, but provides improvement in its availability as well. We develop a stochastic model which gives insight into the improvements afforded by our scheme over the voting scheme.

Integrating static and dynamic voting protocols to enhance file availability

A hybrid scheme is proposed that integrates the static voting protocol and dynamic voting with linearly ordered copies. A stochastic model is used to compare the file availability afforded by the proposed hybrid scheme against the availabilities of voting, dynamic voting, and dynamic voting with linearly ordered copies. The analysis provides evidence for the conjecture that the hybrid scheme is the optimal algorithm in the context of the stochastic model.<<ETX>>

A pessimistic consistency control algorithm for replicated files which achieves high availability

A consistency control algorithm is described for managing replicated files in the face of network partitioning due to node or communication link failures. It adopts a pessimistic approach in that mutual consistency among copies of a file is maintained by permitting files to be accessed only in a single partition at any given time. The algorithm simplifies the Davcev-Burkhard dynamic voting algorithm (1985) and also improves its availability by adding the notion of linearly ordered copies. A proof that any pessimistic algorithm with fresh reads is one-copy serializable is given. >

Laptop computers in an integrated first-year curriculum

A curriculum that integrates calculus, computer science, physics, engineeringdesign, chemistry, engineering statics, and engineering graphics into a year-long sequence of three 12-credit courses was introduced at Rose-HulmanInstitute of Technology during the 1990–1991 school term. The IntegratedFirst-Year Curriculum in Science,Engineering, and Mathematics (IFYC-SEM), taught by an interdisciplinaryteam of eight faculty members [2–4],was designed from the outset withextensive availability and use of com-puters as a cornerstone [3].

A hybrid replica control algorithm combining static and dynamic voting

A hybrid scheme that integrates the static voting protocol and dynamic voting with linearly ordered copies is proposed. A stochastic model is used to compare the file availability afforded by the proposed hybrid scheme with the availabilities of voting, dynamic voting, and dynamic voting with linearly ordered copies. The hybrid scheme has the most availability of these four algorithms for all reasonable repair/failure ratios tested. >

PERFECT RECALL AND PRUNING IN GAMES WITH IMPERFECT INFORMATION

Games with imperfect information are an interesting and important class of games. They include most card games (e.g., bridge and poker) as well as many economic and political models. Here we investigate algorithms for findi ng the simplest form of a solution (a pure‐strategy equilibrium point) to imperfect information games expressed in their extensive (game tree) form. We introduce to the artificial intelligence community a classic algorithm, due to Wilson, that solves one‐player games with perfect recall. Wilsons algorithm, which we call iMP‐minimax, runs in time linear in the size of the game‐tree searched. In contrast to Wilsons result, Koller and Meggido have shown that finding a pure‐strategy equilibrium point in one‐player games without perfect recall is NP‐hard. Here, we provide another contrast to Wilsons result–we show that in games with perfect recall but more than one player, finding a pure‐strategy equilibrium point, given that such an equilibrium point exists, is NP‐hard.

The multi-player version of minimax displays game-tree pathology

Abstract It is widely believed that by searching deeper in the game tree, the decision-maker is more likely to make a better decision. Dana Nau and others have discovered pathology theorems that show the opposite: searching deeper in the game tree causes the quality of the ultimate decision to become worse, not better. The models for these theorems assume that the search procedure is minimax and the games are two-player zero-sum. This report extends Naus pathology theorem to multi-player game trees searched with maxn , the multi-player version of minimax. Thus two-player zero-sum game trees and multi-player game trees are shown to have an important feature in common.

Using multimedia and GUI programming in CS 1

This year, the ‘CS 1‘ course at Rose-HuIman Institute of Technology used Microsoft Visual C++ (MSVC) on laptop computers. We chose MSVC to provide as exciting a platform as possible for our students. MSVC is a powerful environment that permits us to do exciting laboratories. At the same time, it permits students (and us) to be sidetracked into issues not directly pertinent to a first course in computer science. We chronicle our use of MSVC, discussing the closed laboratories we developed and our efforts to make them stirring yet not overwhelming. In particular, we discuss how we used multimedia and Windows/GUI (graphical user interface) programming to create dramatic laboratories withcut obscuring

Resources for next generation introductory CS courses: report of the ITiCSE'99 working group on resources for the next generation CS 1 course

The ongoing purpose of this working group is to collect, evaluate, and foster the development of resources that might be used in next generation introductory CS courses. Such courses will integrate elements of object-oriented programming and design, program design patterns, concurrency, and event-driven programming. The product of the working group is a web repository that provides resources that might be used in such a course. The site will include sample syllabi, pedagogical patterns, programming and design patterns, course modules, projects, tools and techniques.The URL for the web site is:http://csis.pace.edu/ ~bergin/iticse99/