Программное обеспечение баз данных. Архитектура клиент-сервер
Дипломная работа - Компьютеры, программирование
Другие дипломы по предмету Компьютеры, программирование
rchitecture allows efficient way to interconnect programs that are distributed efficiently across different locations. The common use of client/server model is in Computer transactions. For example, if you have to check a bank account from your computer, you have to send a request to a server program at the bank. That program process the request and forward the request to its own client program that sends a request to a database server at another bank computer to retrieve client balance information. The balance is sent back to the bank data client, which in turn serves it back to your personal computer, which displays the information of balance on your computer.
File sharing architecture PC networks were based on file sharing architectures, where the server is used to download files from the shared location to the desktop environment. The client job is then run in the desktop environment. This architectures work only if shared usage is low, update contention is low and the volume of data to be transferred is low. In the 1990s, PC LAN (local area network) computing changed because the capacity of the file sharing was overwrought as the number of online user grew. a result of these limitations of file sharing architectures, the client/server architecture emerged. /server architecture approach introduced replacement of file server by database server. User queries could be answered directly by using a relational database management system. The client/server architecture significantly decreased network traffic by providing a query response rather than total file transfer. It allows multi-user updating through a GUI front end to a shared database. Remote Procedure Calls (RPCs) or standard query language (SQL) statements are typically used to communicate between the client and server. following are the examples of client/server architectures.
) Two tier architectures two tier client/server architectures, the user interface is placed at user's desktop environment and the database management system services are usually in a server that is a more powerful machine that provides services to the many clients. Information processing is split between the user system interface environment and the database management server environment. Software vendors provide tools to simplify development of applications for the two tier client/server architecture.
) Three tier architectures three tier architecture is introduced to overcome the drawbacks of the two tier architecture. In the three tier architecture, a middleware is used between the user system interface client environment and the database management server environment. These middleware are implemented in a variety of ways such as transaction processing monitors, message servers or application servers. The middleware perform the function of queuing, application execution and database staging. In addition the middleware adds scheduling and prioritization for work in progress. The three tier client/server architecture is used to improve performance for large number of users and also improves flexibility when compared to the two tier approach. The drawback of three tier architectures is that the development environment is more difficult to use than the development of two tier applications. ) Three tier with message server. this architecture, messages are processed and prioritized asynchronously. Messages have headers that include priority information, address and identification number. The message server links to the relational DBMS and other data sources. Messaging systems are alternative for wireless infrastructures. ) Three tier with an application server architecture allows the main body of an application to run on a shared host rather than in the user system interface client environment. The application server shares business logic, computations and a data retrieval engine. In this architecture applications are more scalable and installation costs are less on a single server than maintaining each on a desktop client. /server architectures are used in industry as well as in military. They provide a versatile architecture that allows insertion of new technology more readily than earlier software designs.
Form of contract for the sale of machinetools
№ Moscow 20...Objedinenije "Rossimport", Moscow, hereinafter referred to as the "Seller" on the one part, and Messrs...., hereinafter referred to as the "Buyer" on the other part, have concluded the present contract whereby it is agreed as follows:
. SUBJECT OF THE CONTRACTSeller has sold and the Buyer has bought c.i.f, (fob., f.o.r.) ... the goods specified in the enclosed appendix, showing specifications, quantities, prices and delivery times of the goods and constituting an integral part of the present contract.
. PRICESprices are fixed in and are understood c.i.f. (f.o.b., f.o.r.) ... export packing and marking included.expenses incurred on the territory of in connection with the present contract (duties, taxes, customs, etc.) are to be paid by the Buyer.
. QUALITYquality of the goods sold under the present contract shall be in full conformity either with the State standards existing in Russia or with the technical conditions ruling at the manufacturing works.
. TIME OF DELIVERYgoods sold under this contract shall be delivered within ... months after the signing of the contract.date of the Bill of Lading (or: Railway Bill) shall be considered as the date of delivery.
. DELIVERY AND ACCEPTANCE OF GOODSgoods are considered to be delivered by the Seller and accepted by the Buyer:
)In respect of quantity - according to the number of cases and the weight asshown on the Bill of Lading (or on the Railway Bill);
)In respect of quality - according to the Manufacturer's certificate issued bymanufacturing works or by the Seller.
. TERMS OF PAYMENTfor the goods to be delivered under the present contract is to be effected in ... in accordance with the Trade Agreement between the Governments of Russia and ... : dated the ... by an irrevocable confirmed Letter of Credit to be opened by the Buyer with the Bank for; Foreign Trade of Russia Moscow, in favor of the Seller within 10 days of receipt of the Seller's notification of the readiness of the goods for shipment; The Letter of Credit to be valid ... days.payment of the credit is to be made against the following documents:.of Lading (or Railway Bill).expenses connected with the opening of the Letter of Credit are to be borne by the Buyer.shipments and transshipment are permitted.to be made to the amount of 110 per cent of the value of the goods.
. PACKING AND MARKINGgoods shall be secured and properly packed to withstand both overseas and overland transport as well as transshipments.package shall be provided with marking showing the place of destination, name of Consignee, description of goods, contract number, case number, gross and net weight.
. GUARANTEEpart or parts which may prove defective through faulty material or workmanship will be repaired or replaced by the Seller c.i.f., f.o.b. or f.o.r. free of charge within the period of тАж calendar months of the date of shipment provided the defective part or parts have been returned to the Seller's works for examination, carriage paid.guarantee will be invalidated by the Buyer making, causing or allowing any alterations or repair to the goods without the Seller's consent and also if the damage is due to negligence or improper handling on the part of the Buyer or to any other causes outside the Seller's control including damage in transit from Russia to the place of destination.wearing parts as well as parts which have suffered from natural wear are excluded from the guarantee.
9. CLAIMSwhich may arise with regard to the quality of the goods owing to their non-conformity with the standards and technical conditions provided for under clause 3 of this contract as well as claims concerning the quantity of the goods will be considered by the Seller only if submitted within 45 days from the date of arrival of the goods at the place of destination shown in the Bill of Lading and in any case not later than within 90 days of the date of delivery of the goods с if, f.o.b. or f.o.r.which arise through faulty material or workmanship as stipulated in clause 8 will be considered by the Seller only if submitted within the guarantee period.claims should be corroborated by the Buyers' reports drawn up with the assistance of competent organizations.claims put forward in respect of any consignment of the goods can be used by the Buyer as a reason for his refusal to accept the goods and to pay for them. This applies to both the consignment in respect of which a claim has been raised and to all further consignments to be delivered under the present contract.date of the postmark on the Buyer's letter containing the claim and addressed to the Seller to be considered as the date of the claim.
.CONTINGENCIESany circumstances arise which prevent the complete or partial fulfillment by any of the parties of their respective obligations under this contract, namely: fire, acts of the elements, war, military operations of any character, blockade, prohibitions of exports or imports or any other circum stances beyond the control of the parties, the time stipulated for the fulfillment of the obligations shall be extended for a period equal to that during which such circumstances will last.the above circumstances last for more than s months, each party shall have the right to refuse any further fulfillment of the obligations under the contract and in such case neither of the parties shall have the right to make a demand upon the other party for compensation for any possible losses.party, for whom it becomes impossible to meet their obligations under the contract, shall immedia