PL (Presentation Logic) - logic of representation. Operates interaction between the user and the COMPUTER. Processes actions of the user at the choice of alternative of the menu, on pressing of the button or at an element choice from the list.
Dialogue environments support as text for DOS, and the graphic interface of the user for Windows. Introduction of the graphic interface led to development of object properties of tools, means of visual generation of programs and the event mechanism of appendices.
The architecture the client server is intended for solution of problems the file server applications by division of components of the appendix and their placement where they will function more effectively. The client server is feature of architecture use of the allocated servers of the databases which are understanding inquiries in language of the structured inquiries of SQL and carrying out search, sorting and aggregation of information on a place without excessive transfer of data on workstations.
In the centralized system, characteristic for Unix, the terminal realizes only functions of data presentation of PS whereas other functions are provided by the central knot. The center has to react to each inquiry of the user (PL), carry out logic of the appendix (BL, DL) and take given from a DB (DS, FS). There are two serious problems for the centralized scheme: it is difficult to provide the graphic interface; each additional user and the appendix bring essential load of the server, scalability is lost.
Other distinctive feature of the DB servers - existence of a slovar of data in which are written down structure of a DB, restrictions of integrity of data, formats and even server procedures of data processing for a call or for events in the program. Objects of development in such appendices besides dialogue and logic of processing are first of all the relational model of data and the related set of SQL operators for standard inquiries for this DB.
The file server applications for the personal computers local networks is a development basis a tool environment of various "personal" DBMS: FoxPro, Clipper, Paradox, Clarion, Paradox, dBase, etc. Such means are, as a rule, realized in the form of the dialogue integrated environment, the providing three levels of access:
From the moment of an exit significantly more copies of Delphi, than Visual Basic are sold. Application of a product possibly for creation of settlement and analytical programs, for development of DLL, for maintenance and development of the development executed on Turbo and Borland Pascal and also for fast prototyping of future appendices. In some cases solving for a choice will be moderate requirements of the Delphi application to system technically providing.
DBMS for the MS Access personal computer can be used for creation of scalable single and group information supplements and for development of client part of appendices the client server and also as an office-work automation equipment as a part of MS-Office.
As this scheme imposes the smallest requirements to the server, it possesses the best scalability. However the difficult appendices causing big interaction with a DB can rigidly load both the client, and a network. Results of SQL inquiry have to return to the client for processing because there is a logic of decision-making. Such scheme assigns additional burden of administration of the appendices scattered on various client knots.
However such architecture has two main shortcomings: some inquiries to a DB can pump over all DB to the client, loading a network and having unpredictable time of reaction, thereby, creating the considerable network schedule, and also the arising problem of "the fat client" - the Windows-interface, codes of the appendix and DBMS can overload even the powerful personal computer.
On the basis of multiterminal system as an application server also possibly creation of architecture the client server (drawing). In this case in the multitask environment of an application server programs of users are carried out, and client knots are degenerated and presented by terminals.