Server and Client

The RDBMS is installed and runs on a computer that functions as a database server. Any SQL commands are executed on the server by the RDBMS. Functions written in SQL or in any of the procedural languages mentioned above are also executed by the RDBMS. This has the advantage that the data tables used by these SQL commands or procedural functions are under the control of the server. This is the most efficient way to access the data. The disadvantage is that the server may have many requests to handle from many users. Another way to operate on data tables is indirectly, using a client program typically (although not necessarily) run from another computer. 

There is a smaller set of tools that are typically run on the server. Any SQL commands and any procedural language functions are run on the server. In principle, there is complete flexibility of the server side tools, since in principle any computer program can be written in any computer language. Later chapters of this book show how the RDBMS server itself can be extended using server side programming to handle chemical information. These extensions may directly solve the needs of a particular project, but more importantly they increase the flexibility of the RDBMS to handle chemical information. Client programs can use the results of chemical searches and other computations as well. 

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Pre-Qualification Activities Installation of hardware for the production system (computer system hardware, servers, and client PCs)... 

Automatic system clock synchronization of servers and clients to a known source... 

Needless to say, there are other ways to exchange data like the Common Object Request Broker Architecture (CORBA) (15). This standard provides an intermediary object-oriented layer that handles access to the data between server and client. Another recently emerging way to exchange data is web services (16), which will be described later. 

Change control (e.g., hardware servers and clients system software operating systems, DBMS, application software)... 

Hypertext Transfer Protocol (HTTP) is the application-level network protocol for transferring Web content between servers and clients. (Hypertext is a special database system of multimedia objects linked to each other.) HTTP uses the services of the TCP transport protocol. It transfers independently the objects (text, graphics, images, etc.), that build up a page. Together with each object, a special header is also passed. The header contains information alx>ut the type of the object, its size, its last modification time, and so on. HTTP also supports the use of proxy and cache servers. HTTP proxies are implemented in firewalls to pass HTTP traffic between the protected network and the Internet Object caches are used to save bandwidth on overloaded lines and to speed up the retrieval process. 

Server Software which can be invoked through a request for a service (from a so-called client) and which is dedicated to providing that service, for example a file server which provides file storage and access services. This term is often used for the computer system which provides the service, but, in fact, a server and client can be two distinct pieces of software residing on the same physical computer system. 

Tha basic architecture of the Web consists of browsers that act as clients requesting information from Web servers. Computer-to-computer communications are described in terms of protocols, and Web interactions are no exception to this rule. In order to implement the prototype Web, Berners-Lee had to define the interactions that were permissible between server and client (normally a browser, but it could in fact be any program). The HyperText Transfer Protocol (HTTP) describes these interactions. The basis of the design of HTTP was that it should make the time taken to retrieve a document from a server as short as possible. The essential interchange is Give me this document and Here it is. ... 

The server and client PCs were HP nx9030 and nx9010 portable computers, respectively, running Windows XP. They were configured to maximize the resources allocated to the present work. Batch command files were written to enable the TCP, UDP and FTP tests. The results were obtained in batch mode and written as data files to the client PC disk. Each PC had a second network adapter, to permit remote control from the official IP Unit network, via switch. 

A central system demands much more planning for administrating and performing updates. Especially, global installations only offer a few slots, where no user is working in the system, for example, performing a reboot of the server. For the installation of updates that need to be distributed on all servers and clients at the same time, several hours of even days are required, where the system is not available for the daily operation. 

The Trust ICT department, with Logica, is responsible for supporting the computer network, connections to it, most of the servers and clients attached to it, and many applications including patient records. The separate medical physics department oversees medical devices and works closely with the ICT department to ensure continuity of safety management activities. The scope of ICT has increased... 

Commercial LIMS appeared on the market in the early 1980s. These operated on then state-of-the-art minicomputers such as the 16-bit Hewlett-Packard 1000 and 32-bit Digital VAX system. By the late 1980s, several DOS-based PC LIMS operating on the primitive PC network were available. By the early 1990s, most LIMS started using commercial relational database technology and client/server systems, which operated on UNIX or the new Windows NT platform. The most advanced LIMS utilize server-based architecture to ensure system security and control. 

The first form led to client-server-styled systems, with the client combining user-interface and application logic and communicating via SQL requests with a database server that dealt with persistence, transactions, security, and so on. All communication involved database processing requests in SQL, and clients did not communicate with one another (except indirectly through shared data on the server). 

In our view, architecture is not only about Gothic-scale structures but is also about all structures and relationships used down to the level of code.1 The decision to use a four-tier structure, with a thin client, a Web server, a business application server, and a database, is architectural. But, in the extreme, we consider a consistent use of getXQ and setX(x) methods also to be part of the (detailed) architecture. This view leads to a somewhat less formal definition of architecture. 

The basic constructs of type, collaboration, and refinement support all levels of specification, architecture, and implementation. However, we also pay explicit attention to specific levels of architectural design logical and physical database mapping, technical architecture (including client-server and multitier peer-to-peer architectures), and user-interface modeling. The case study touches only on some of these aspects. 

Each component could be in a separate machine if required (as in client-server and the Web). 

A simple HTTP client program (e.g., fetch ) can be pointed at an HTTP server, accept lines of input from a standard input stream, send them as messages to an HTTP server, accept replies from that server and then write them to a standard output stream. The fetch program becomes a serial filter for any such HTTP server (which is now acting as a network compute server). 

HTTP is designed beautifully for client-server and server-client communication and the design of HTML follows suit. They were not designed for server-server communications and some deficiencies in this area show up when implementing a distributed information system on top of HTT P/HTML. Fortunately, these are easy to work around. 

Figure 15.7. Many-client many-server model for distribution on a network. Both client-server and server-server interaction is permitted, particularly in cases where a service spawns a succession of service calls.

The client/server model often allows easier integration with other network applications (eg, finance, project management, or human resources) which typically operate in the environment of the server component of the client/server system. Client/server can be gradually introduced in an existing minicomputer environment, often with litde adverse incremental impact in terms of retraining and additional cost. 

For these reasons, the desktop and client server models are expected to increase in percentage of LIMS software offerings and installed base in the... 

A three-tiered architecture includes one more node between the client and the database server—the middle tier. In a three-tiered architecture, business logic is offloaded from the client and the database server nodes to the middle tier. In fact, you can choose to further distribute the business logic among more than one middle tier node and still call it a three-tiered (or K-tiered) architecture because the idea is similar. Note that the tiers do not have to be physically separated. You can have both the middle tier server and the database server collocated on the same physical computer but running in different processes with separate memory spaces. Modem hardware architecture can partition a single hardware box into multiple virtually separate computers or domains. Typically, a three-tiered architecture supports a Web-based thin client although it can also work with a rich client. 

Both client-server and three-tired architectures can be developed using either Java or. NET technology, although. NET is a more natural choice for a rich client on Windows-based client PCs. It is also possible to take a hybrid approach—develop rich client in. NET but business logic in Java, and integrate both using SOAP-based Web services. MDL s Isentris, for example, takes the hybrid approach. 

Web-based systems offer several advantages over rich client easy to deploy and access, easy to scale (by adding more hardware resources to the application server), and shared computing resources (CPUs, memory, database connections). However, most of the information this book presents is not limited to Web-based applications. It promotes the loosely coupled Presentation Layer and Domain Layer so that business logic can be reused no matter what GUI technology is being used. 

SSL 3.0 encrypts data between the server and browser. In addition it supports client authentication lor Access Control. 

Secure Sockets Layer (SSL) is considered to be the industry-standard protocol for secure, Web-based communications. A recent version includes data encryption between the server and the browser and its support client authentication. 

Figure 1 depicts the basic scenario we consider. We are given different parties that interact with each other to offer services. A party can act both as a server and a client and each party has i) a set of services it provides and ii) a portfolio of properties (attributes) that the party enjoys. Access restrictions to the services are expressed by policies that specified the properties that a requester should enjoy to gain access to the services. The services are meant to offer certain functionalities that depend on the input parameters supplied by its users. Often input parameters must fulfill certain conditions to assure correct behavior of a... 

A complicating factor in the validation of LIMS is the wide variety in the scale of LIMS implementations. A LIMS can vary from a PC-based application in a single laboratory to a client/server-based system running across multiple sites on a company-wide area network with shared access to servers and multiple interfaces to other business systems. 

Set-up of the various system environments must be managed. Documentation must be developed to describe the hardware platform and installed software, including any network infrastructure. Hardware architecture design documentation should be prepared. A diagram should be included to illustrate the geographic distribution of any client-server hardware. Client-server software also needs to be defined. Clients are often referred to as either thick or thin, depending on whether they require substantial or minimal application-related software. Client-server software can be considered to consist of ... 

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