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http://www.spinellis.gr/pubs/jrnl/1998-IEEESoft-CliServ/html/CliServ.html This is an HTML rendering of a working paper draft that led to a publication. The publication should always be cited in preference to this draft using the following reference:
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Software organizations began introducing the client-server system architecture in the late 1980s because CSS offered a technology-driven application design that genuinely addresses corporate and user needs. Widely promoted by software and hardware vendors who sought short-term advantages in a competitive market, CSS has always been a haphazard match of disparate, ad hoc technologies that lack the guidance of a unifying vision, purpose, and theory.
Software professionals seldom admit that the CSS architecture failed inevitably, or that technologies that claim to improve it are only addressing this failure. Our industry would be better served if we honestly appraised the situation, evaluated the lessons learned, and undertook a fresh start.
Yet MIS retained control of the mission-critical applications. These mature, legacy software systems, running in a stable operating environment with scheduled backup and maintenance procedures, ensured the longevity and reliability of the corporation's most vital information. When users needed access to MIS data from the desktop PC, a terminal emulation package opened a window on the mainframe host.
Client-server systems are not significantly cheaper than their mainframe-based ancestors; they are neither notably more user friendly nor better integrated into the modern desktop environment. In addition, client-server systems have introduced several new problems--they are less robust, efficient, and portable and, at the same time, are more difficult to implement and deploy than the mainframe-based applications they replace.
These studies compared the low acquisition, administration, and maintenance costs of PC-based autonomous end-user computing with the correspondingly higher costs of the MIS departments. However, they failed to account for many hidden costs, or to discuss the costs of managing thousands of PCs--often under the full control of their individual owners--to provide a functional, integrated, enterprise-level computing platform.
Contrary to common MIS practice, PC software maintenance is seldom budgeted, often described instead as "software upgrades." Similarly, the apparently low cost of system administration often translates into the loss of PC owners' productive time as they install new applications or juggle program patches to obtain a stable configuration.
CSS applications cannot match the user-friendliness of the typical PC spreadsheet. First, the complexities and constraints involved in CSS application development are significantly more intricate than those faced by PC application developers. CSS developers must engineer access methods and user interfaces for potentially huge data sets over limited bandwidth, deal with table cursor schemes, enable multi-user access, and use a fixed number of database handles.
In addition, commercial application vendors can use economies of scale and must, to compete, polish their products to a level of versatility beyond the reach of a typical CSS vertical or custom application developer. The executable program sizes of modern packages reveal that versatility and user friendliness result from accumulated raw effort and are not free by-products of the PC operating system or the application development process.
Further, this mostly visual integration of CSS applications with the GUI desktop has been achieved at great cost. The multivendor nature of CSS has become a curse for system integrators, administrators, developers, and end users. A CSS typically consists of the client and server platforms and their respective (often different) operating systems, a database server, database connectivity software, the client application, the application environment support libraries, and, sometimes, additional middleware and networking software components. Each of these may be supplied by a different vendor. Obtaining a functioning and stable combination of all components and keeping it working through a barrage of new releases and bug fixes is a sisyphean task.
Client-server systems force an unnatural division of application functionality that results in unneeded complexity and duplication. According to the CSS philosophy, many data validation checks can be performed on the client side. To guarantee the application reliability against malfunctioning or out-of-date client software and raw-data uploads, similar checks are often incorporated as constraints on the server side. Apart from the duplication of effort, this strategy increases application complexity because clients must also handle constraint validation errors arising from the database server. To organize this mess and provide scalable solutions for many clients and multiple servers, even more complicated multi-tier and transaction-processing-monitor architectures have been proposed and implemented.
Further technical and management difficulties stem from the distributed deployment and support of client software. A typical client installation package consists of not just an executable program but also runtime libraries, database connectivity packages, component objects, and, in many cases, changes to some operating system modules.
Installation difficulty is compounded by the inability of most desktop OSs to be remotely managed: most installation procedures require an operator's presence during installation. The installation process is complicated by the interference between installed components that other user-installed application packages share. The primitive version-checking and reference-usage-counting installation protocols, coupled with bugs in the often minimally tested installation procedures, result in shared components that can be removed without warning, regress to older versions, or be replaced with newer but incompatible versions when an application is installed. All these problems are multiplied by the number of clients that must be installed. In addition, because clients perform local processing, client updates must be performed in a single step on all desktops, and be synchronized with updates to the database structure on the server.
Further, the adoption of CSS architectures has resulted in an unfortunate loss of program portability. Popular CSS application-development environments such as Powerbuilder and Visual Basic use proprietary languages or language extensions. Version by version, the suppliers of these environments reinvent mainstream language features such as strong typing, modules, exception handling, structured data types, and OO programming. This proliferation of languages has resulted in the balkanization of training, software reuse, tool support, documentation, coding standards, and revision control. Such development environments mainly target the Microsoft Windows OS and Intel hardware architecture, resulting in a dramatic backslide in application portability. Even though we resolved these important language and software engineering issues in the middle 1970s, they are resurfacing with a vengeance 20 years later.
Industry analysts now propose several new alternatives that ostensibly improve on the CSS paradigm. A leading example, Cyrix's thin-client technology, is apparently endorsed by Microsoft. Joel Kanter (Understanding Thin-Client/Server Computing, Microsoft Press, 1998) defines a thin-client application as one in which all application processing is performed on the server; the client only transmits keyboard and mouse-movement events to the server. This approach, similar in structure to the X-Windows system, departs radically from the traditional CSS architecture by moving toward systems based on terminal emulators.
Web solutions based on HTML data exchanges stem from a similar concept, so labeling them client-server systems is misleading. Network computers that run Java clients are, however, closer in spirit to the CSS architecture. They appear to solve the problems of application deployment, implementation language functionality and portability, and installation difficulties. However, their integration with popular applications, the client-server communication protocol, and the client's user friendliness have not been satisfactorily addressed. In summary, these new approaches try to address the CSS architecture's fundamental problems in a politically correct way: they silently retreat and cover up the problem by using fashionable words and technologies.
The move toward CSS has been a large and expensive technological mistake. It has negatively affected many end users, MIS departments, balance sheets, and developers. If we view the industry's response in terms of Elisabeth Kubler-Ross's stages-of-grief model, we are currently past the denial and anger phases. As we recognize the problems with CSS, we experience the bargaining phase in which the architecture's stakeholders strive to re-establish their position with minimal loss or gain strategic advantages from the impending changes. However, after the inevitable depression about the costs and lost opportunities of the move to CSS architectures, we can now accept the situation and work toward the design of robust, user-friendly, practical, efficient, portable, cost-effective, and scalable architectures for developing MIS applications.