Catálogo de publicaciones - revistas

<jats:p>Database systems were a solution to the problem of shared access to heterogeneous files created by multiple autonomous applications in a centralized environment. To make data usage easier, the files were replaced by a globally integrated database. To a large extent, the idea was successful, and many databases are now accessible through local and long-haul networks. Unavoidably, users now need shared access to multiple autonomous databases. The question is what the corresponding methodology should be. Should one reapply the database approach to create globally integrated distributed database systems or should a new approach be introduced?</jats:p> <jats:p> We argue for a new approach to solving such data management system problems, called <jats:italic>multidatabase</jats:italic> or <jats:italic>federated</jats:italic> systems. These systems make databases interoperable, that is, usable without a globally integrated schema. They preserve the autonomy of each database yet support shared access. </jats:p> <jats:p>Systems of this type will be of major importance in the future. This paper first discusses why this is the case. Then, it presents methodologies for their design. It further shows that major commerical relational database systems are evolving toward multidatabase systems. The paper discusses their capabilities and limitations, presents and discusses a set of prototypes, and, finally, presents some current research issues.</jats:p>

<jats:p>The state machine approach is a general method for implementing fault-tolerant services in distributed systems. This paper reviews the approach and describes protocols for two different failure models—Byzantine and fail stop. Systems reconfiguration techniques for removing faulty components and integrating repaired components are also discussed.</jats:p>

<jats:p>The purpose of a distributed file system (DFS) is to allow users of physically distributed computers to share data and storage resources by using a common file system. A typical configuration for a DFS is a collection of workstations and mainframes connected by a local area network (LAN). A DFS is implemented as part of the operating system of each of the connected computers. This paper establishes a viewpoint that emphasizes the dispersed structure and decentralization of both data and control in the design of such systems. It defines the concepts of transparency, fault tolerance, and scalability and discusses them in the context of DFSs. The paper claims that the principle of distributed operation is fundamental for a fault tolerant and scalable DFS design. It also presents alternatives for the semantics of sharing and methods for providing access to remote files. A survey of contemporary UNIX-based systems, namely, UNIX United, Locus, Sprite, Sun's Network File System, and ITC's Andrew, illustrates the concepts and demonstrates various implementations and design alternatives. Based on the assessment of these systems, the paper makes the point that a departure from the extending centralized file systems over a communication network is necessary to accomplish sound distributed file system design.</jats:p>

<jats:p> Support for unusual applications such as computer-aided design data has been of increasing interest to database system architects. In this survey, we concentrate on one aspect of such support, namely, <jats:italic>version modeling</jats:italic> . By this, we mean the concepts suitable for structuring a database of complex engineering artifacts that evolve across multiple representations and over time and the operations through which such artifact descriptions are created and modified. There have been many proposals for new models and mechanisms to support such concepts within database data models in general and engineering data models in particular; here we not only describe such proposals; we also unify them. We do not propose yet another model but provide a common terminology and collection of mechanisms that underlie any approach for representing engineering design information in a database. The key remaining challenge is to construct a single framework, based on these mechanisms, which can be tailored for the needs of a given version environment. </jats:p>

<jats:p>Floating-point arithmetic is considered as esoteric subject by many people. This is rather surprising, because floating-point is ubiquitous in computer systems: Almost every language has a floating-point datatype; computers from PCs to supercomputers have floating-point accelerators; most compilers will be called upon to compile floating-point algorithms from time to time; and virtually every operating system must respond to floating-point exceptions such as overflow. This paper presents a tutorial on the aspects of floating-point that have a direct impact on designers of computer systems. It begins with background on floating-point representation and rounding error, continues with a discussion of the IEEE floating point standard, and concludes with examples of how computer system builders can better support floating point.</jats:p>

<jats:p>Distributed computations are concurrent programs in which processes communicate by message passing. Such programs typically execute on network architectures such as networks of workstations or distributed memory parallel machines (i.e., multicomputers such as hypercubes). Several paradigms—examples or models—for process interaction in distributed computations are described. These include networks of filters, clients, and servers, heartbeat algorithms, probe/echo algorithms, broadcast algorithms, token-passing algorithms, decentralized servers, and bags of tasks. These paradigms are appliable to numerous practical problems. They are illustrated by solving problems, including parallel sorting, file servers, computing the topology of a network, distributed termination detection, replicated databases, and parallel adaptive quadrature. Solutions to all problems are derived in a step-wise fashion from a general specification of the problem to a concrete solution. The derivations illustrate techniques for developing distributed algorithms.</jats:p>

<jats:p> The development of distributed operating systems and object-based programming languages makes possible an environment in which programs consisting of a set of interacting modules, or objects, may execute concurrently on a collection of loosely coupled processors. An object-based programming language encourages a methodology for designing and creating a program as a set of autonomous components, whereas a distributed operating system permits a collection of workstations or personal computers to be treated as a single entity. The amalgamation of these two concepts has resulted in systems that shall be referred to as <jats:italic>distributed</jats:italic> , <jats:italic>object-based programming systems</jats:italic> . </jats:p> <jats:p>This paper discusses issues in the design and implementation of such systems. Following the presentation of fundamental concepts and various object models, issues in object management, object interaction management, and physical resource management are discussed. Extensive examples are drawn from existing systems.</jats:p>

<jats:p>VLSI cell placement problem is known to be NP complete. A wide repertoire of heuristic algorithms exists in the literature for efficiently arranging the logic cells on a VLSI chip. The objective of this paper is to present a comprehensive survey of the various cell placement techniques, with emphasis on standard cell and macro placement. Five major algorithms for placement are discussed: simulated annealing, force-directed placement, min-cut placement, placement by numerical optimization, and evolution-based placement. The first two classes of algorithms owe their origin to physical laws, the third and fourth are analytical techniques, and the fifth class of algorithms is derived from biological phenomena. In each category, the basic algorithm is explained with appropriate examples. Also discussed are the different implementations done by researchers.</jats:p>

<jats:p>At high data transmission rates, the packet transmission time of a local area network (LAN) could become comparable to or less than the medium propagation delay. The performance of many LAN schemes degrades rapidly when the packet transmission time becomes small comparative to the medium propagation delay. This paper introduces LANs and discusses the performance degradation of LANs at high speeds. It surveys recently proposed LAN schemes designed to operate at high data rates, including their performance characteristics desirable in LAN medium access protocols are identified and discussed. The paper serves as a tutorial for readers less familiar with local computer communication networks. It also serves as a survey of the state-of-the-art LANs.</jats:p>