In architecture a file-server network division of dialogue components PS and PL is absent, and the computer is used for functions of the information display. It facilitates construction of the graphic interface. The file-server only takes data from files so additional users and appendices slightly increase loading by the central processor. Each new client adds computing capacity to a network.

Objects of development in a file-server the application are the components of the application defining logic of dialogue PL, and also processing BL logic and data DL managements. The developed application is realized in the form of the finished loading module, or in the form of a special code for interpretation.

However such architecture has essential lack. At performance of some inquiries to a database great volumes of data can be transferred the client, loading a network and leading to unpredictability of reaction time. The big network traffic affects the organization of the removed access to databases on a file-server. One of this lack elimination variants is the removed management a file-server the application in a network. Thus in a local network the server of applications combined with a server of access is placed. In the environment of access server are carried out usual a file-server of the application. Feature consists that dialogue input-output acts from the removed clients through data links. Applications should not be too complex, the probability of an overload of a server differently is great or very powerful platform for a server of applications is necessary.

The client-server architecture is intended for resolution of problems a file-server of applications by division of the application components and their accommodation there where they will function most effectively. Feature of the client-server architecture is presence of the allocated servers of the databases which are understanding inquiries on Structured Query Language (SQL) and carrying out search, sorting and aggregation of the information.

Distinctive feature of a DB servers - presence of the data directory in which are written down structure of a DB, restrictions of data integrity, formats and even server procedures of data processing on a call or on events in the program. Objects of development in such applications, besides dialogue and logic of processing, are, first of all, the relational model of data and the set of SQL-operators connected by it for typical inquiries to a database.

The majority of configurations the client-server uses two-level model in which the client addresses to services of a server. It is supposed, that dialogue components PS and PL are placed on the client that allows to realize the graphic interface. Components of management and FS are placed by data DS on a server, and dialogue (PS, PL) and logic (BL, DL) - on the client. In two-level definition of architecture the client-server is used this variant: the application works on the client, Data Base - on a server.

As this scheme makes the least demands to a server, it possesses the best scalability. However the complex applications actively cooperating from a DB, can rigidly load both the client, and a network. Results of SQL-inquiry should return to the client for processing because the logic of decision-making there is realized. Such scheme leads to additional complication of the applications administration scattered on various client units.

For reduction of loading on a network and component BL can be placed simplifications of applications administration on a server. Thus all logic of decision-making is made out in the form of saved procedures and carried out on a server of a DB.

Saved procedure - procedure with SQL-operators for access to a DB, caused by name with transfer of demanded parameters and carried out on a server of a DB. Saved procedures can be compiled, that raises speed of their performance and reduces loading to a server.

Saved procedures improve integrity of applications and a DB, guarantee a urgency of collective operations and calculations. Support of such procedures, and also safety (there is no direct access to data) improves.

Creation of the client-server architecture probably and on the basis of multiterminal system. In this case in the multitask environment of application programs server of users are carried out, client units fallout and are presented by terminals. The similar scheme of information system is characteristic for Unix.

Now the architecture the client-server was recognized also a wide circulation as a way of the applications organization for working groups and information systems of a corporate level. The similar organization of work raises efficiency of applications performance due to use of a DB server opportunities, unloading of a network and maintenance of the data integrity contro.

Two-level schemes of the client-server architecture can lead to some problems in complex information applications with set of users and the confused logic. The decision of these problems can become application of multilevel architecture.

The multilevel architecture became development of the client-server architecture and in the classical form consists of three levels:

• The bottom level represents the applications of clients allocated for performance of functions and representations logic PS and PL and having the program interface for a call of the application on an average level;
• The average level represents a server of appendices on which applied logic BL is carried out and from which the logic of data processing DL carries out operations with database DS;
• The top level represents the removed specialized server of a database allocated for services of data processing DS and file operations FS (without use saved procedures).

The three-level architecture allows to balance even more loading on different units and a network. This architecture promotes specialization of tools for development of applications and the client-server eliminates lacks of two-level model. Logic centralization of the application simplifies administration and support. Platforms and tools for realization of the interface and applied logic that allows to realize with the greatest feedback to their experts of a narrow structure are precisely divided.

But as borders between components PL, BL and DL are washed away, the applied logic can be realized at all three levels. The server of applications by means of the transaction provides monitor the interface with clients and other servers, can operate transactions and guarantee integrity of the distributed database. Means of the removed call of procedures most correspond to idea of the distributed calculations. They provide a call of the applied procedure located on other unit, transfer of the parameters, the removed processing and return of results from any unit of a network.

With growth of the client-server systems necessity of three levels becomes more and more obvious. Products for three-level architecture, so-called monitors of transactions, are rather new. These tools basically are focused on Unix environment, however applied servers it is possible to build on the basis of Microsoft Windows NT with a call of the removed procedures for the organization of clients communication with a server of applications.

In view of global communications the architecture can have more than three levels. Now there were new tool means for flexible segmentation of applications the client-server on various units of a network. Thus, the multilevel architecture of the distributed applications allows to raise an overall performance of corporate information system and to optimize distribution of its hardware-software resources.