Problem of the multiuser access and parallel data processing in automated information system

The Natural effect of the development DBMS is a problem to organizations of collaboration several users with one and same data collection, or, briefly, problems of the multiuser access to data.

We Stop in more detail on the main aspect of this problem. First of all situation of division one and same data collection between several users can bring about origin conflict (the attempt of the lump change one and same record, coincidence operation reading and removing to information and t. d.). The Separate place when work with DBMS occupy the questions of the prevention collision, which can appear in the event of uncoordinated change the structure of the tables, the forms have given the reports by one user, when with they work others.

With standpoint of the organizations of the joint access to given on the part of several users state of working with they are divided on mode monopoly (exclusive) of the access and mode general (prepared) of the access.

The Mode of the monopoly access to database provides that only one of the users (the programme processes) can work with her, but possibility of her(its) opening other user (the process) is blocked. Opening database in monopoly mode, as a rule, is used for performing operation on change the structure of the tables and relationships between them, export big amount to information, execution official operation with given (conservation, reconstruction, compression) and t. p.
Accordingly, in mode of the prepared access immediately several users can work with database. For prevention possible conflict at attempt on the part of different users to change one and same record in DBMS is used mechanism of the blocking. Blocking that or other object in the event of work with him some user means the prevention of any other attempts to change this object, but is herewith saved possibility of his(its) reading. Thereby, mechanism of the blocking gives the more flexible possibilities for manipulate with given in contrast with cut monopoly access.

For different DBMS concrete technical decisions on realization of the device of the blocking greatly differ. In MS Access, in particular, when change record by one user by default occurs her(its) automatic blocking up to moment of the termination to operations. When making the forms, the reports or request in Access is provided possibility of the task a parameter lock mode. On fig. is shown process of the change characteristic Blocking the notes for the form.

As can be seen from drawing, characteristic Blocking the record can take importances:

• Is absent - is allowed simultaneous change the record on the part of several users. Herewith if two users try the сохранись made changes to one and same record, that second user is removed warning message, on base which he can or abandon to the further action, or will substitute change, made by first user, having saved own variant. Obviously that in such mode is saved maximum leeway of the users, "board" for which are a possible conflicts in view of incoordination their action.
• All record - occurs blocking of all record in the source given under his(its) opening of one of the users with the result that he can without hindrance change him(it). The Other users have an access only on reading (viewing).
• Changeable record - one of the users finds way on change necessary him record, but the other users can only read the bing kept data in her. The Givenned mode superimposes the minimum restrictions to collaboration. Follows to add that technically in Access are blocked not record as such, but so named pages - a blocks of the file database by size 2048 bytes, containing necessary record.

Cancelling the blocking in Access occurs then, when user, earlier blocked record, or will save maded change, or will abandon to them. That change, produced by one user, became are seen others, through determined time lag is provided automatic renovation of the contents of the tables, the forms and reports. Importance of the period of the renovation is assigned from menu Service > Parameters, вкладка Others, field Period renovations.

Other essential question, which must be solved for ensuring the normal operation multiuser DBMS, is an organization of the system администрирования data. Amongst problems администрирования can be named:

• making the system of the users and division of the rights of the access of the different users to object DBMS
• organization and maintenance of the system of reserve keeping to information and her(its) reconstruction in the event of programme and hardware malfunctions
• monitoring programme and hardware resource, used for ensuring the work DBMS, and acceptance on his(its) base of the decisions on optimization of their use.
  Some questions connected with organization of the system of the users DBMS.

The First multiuser DBMS had a site architecture and were based on greater computer or mini-COMPUTER. The Worker of the place of the users were situated on terminal, connected to central computer, ria which were executed all processes on manipulates with data. However with spreading personal computer special urgency have gained DBMS, realizing technologies portioned data processing that is to say such technologies, which allow to lead the simultaneous work with several comparatively limited on hardware possibility of the machines, united in ceть. In this case one part function DBMS is executed on computer-client, but the other - on computer-server moreover their interaction is realized through a certain coordinated protocol.

Historically first technology of the portioned work with data has got the name a file-server (FS-model). In her(its) frame is expected that one of the computer in network is a file server and gives their own facility on processing the files other computer, on he also is situated the vault data. There is applied software On the other computer, realizing functions of the user interface of the access to data, and copy of the processor database (DBMS). Any time applied program addresses to the base, processor data addresses to file server. In answer file server directs on network required block data, got which, DBMS realizes the actions, declaimed in applied program. The Protocol of the exchange between server and клиентскими computer presents itself set low-level call, providing interface application access to file system of the server.

The Technological defect FS-models result restrictions from intrinsically inherent her. Amongst they in the first place follow to name:

• high network traffic conditioned by need to send big amount file block from server to applications
• limited set possible action on data processing on file server, which not capable "to understand" internal logical structure database and perceives this base in the same way, either as any other file
• absence of the reliable facilities of the provision to safety of the work with given - is allowed protection at a rate of function of the network operating system only

The Enumerated problems define that DBMS, founded on technologies file-server, can be used only in much limited scale. For instance, when making the collective information systems, calculated on small quantity of the users and limited information flows. Simultaneously follows to note that FS-model prescribed in base of the architectures suppressing majority desk DBMS such as FoxPRO, Clipper, Clarion, Paradox, Access, achieved broad popularity amongst domestic developer.

The author: Y.N.Antonov Is added: 21.06.2008