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The RPC must operate on top of some kind of transport, such as Transmission Control Protocol TCP or User Datagram Protocol (UDP). It is quite possible that the caller and the provider may be on systems of different types that have different ways of representing data. Thus, a machine-independent method must be selected for transferring data such that the provider of the service can accept data from the caller to act on, and the caller can accept returned data from the provider. Authentication is a means for the server and caller to decide whether they trust one another. Is the caller authorized to access the server? Should the caller trust that it is connected to the correct server? IPCs typically use message queuing to provide asynchronous communications. This allows data and control to be distributed through the use of messages. IPCs are also referred to as message-oriented middleware. Communications can be either connectionless or connection oriented. Addressing is handled by name resolution, dynamic binding (binding at run time), and the use of a global namespace provider such as Domain Name System (DNS), Lightweight Directory Access Protocol (LDAP), or X.500. Exhibit 6-3-8 shows two application segments running on two different systems that use message queuing for communications. There are a number of commercially available implementations of message queuing, including Peerlogic Pipes, SunSofts ToolTalk Message, IBM MQSeries, and Covias Communications Integrator.
In brief, then, remote procedure call and interprocess communications compare as follows: Data Base Connectivity Middleware Most data base connectivity products are designed to provide a common application programmer interface to data base engines from multiple vendors. By using these products, a client application can be independent of the data bases to which it must connect. Products or specifications for products that provide this type of functionality include Microsofts Open DataBase Connectivity and Borlands Integrated Database Application Program Interface (see Exhibit 6-3-9). Apples Data Access Language (DAL), the ISO/ANSI RDA specification,and IBMs Distributed Remote Data Access (DRDA) are others.
Gateway Products. Still another class of data base connectivity products are data base gateway products. They act as interface translators that move data, SQL commands, and applications from one type of data base to another. They generally have the following types of functionality:
Gateway products are available from a number of vendors. For example:
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