Sistamas distibuidos

Web services are a form of distributed information system. Many of the problems that web services try to solve, as well as the design constraints encountered along the way, can be understood byconsidering how distributed information systems evolved in the past . as part of this evolution process, a key aspect to keep in mind is that while the technology has changed, the problems that need to besolved are to a large extent the same. Thus, the first step toward looking at web services from the correct perspective is to develop a comprehensive understanding of distributed informationsystems.
In this chapter we introduce the most basic aspects of distributed information systems: their design, architecture,, and communication patterns. We do so in an abstract manner, to avoid thedistraction created by the myriad of details found in individual systems and products, the objective is to indentify a number of design guidelines and concepts that in subsequent chapters will help thereader compare web services with traditional information systems.
The chapter begins by addressing several aspects related to the design of an information system. Firs we discuss the different layersinvolved and how they can be designed in either a bottom-up or a top-down manner. Throughout this book we will deal almost exclusively with information systems designed bottom-up. It is thereforeimportant that we establish form the beginning what this implies in terms of how information system. We follow a historical perspective from 1tier to N-tier architectures placing special emphasis on whythey appeared and their advantages as well as elements in the evolution of distributed information systems, and the issues raided here appear again and again throughout the book. The chapter concludeswith a discussion on the differences between synchronous and asynchronous interaction as well as of the consequences of usong each of them.
1.1 design of an information system

In spite...