Handbook of Local Area Networks, 1998 Edition:LAN-based Application Development Issues and Solutions
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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, SunSoft’s ToolTalk Message, IBM MQSeries, and Covia’s Communications Integrator.
Exhibit 6-3-8. Interprocess Communications (IPC) Based on Message Queuing
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 Microsoft’s Open DataBase Connectivity and Borland’s Integrated Database Application Program Interface (see Exhibit 6-3-9). Apple’s Data Access Language (DAL), the ISO/ANSI RDA specification,and IBM’s Distributed Remote Data Access (DRDA) are others.
Exhibit 6-3-9. Data Base Connectivity Products
RPC Architecture
IPC Architecture
Procedural-like
Nonprocedural
Synchronous
Synchronous or asynchronous
Implied request/reply or client/server model, and a transfer of control
No implied structure and does not imply transfer of control
One-to-one communications
Many-to-many communications
No explicit multithreading
Multi-threading supported
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:
• Accept statements that use a well-specified grammar (usually SQL) from the client application.
• Translate the statements into a specific data base format.
• Send the translated statements to the data base engine for execution.
• Translate the results returned by the data base into a well-defined format for the calling application.
• Return the data and any status information to the client application.
Gateway products are available from a number of vendors. For example:
• The Information Builders Enterprise Data Access/SQL line of products provides SQL-based access to both relational and nonrelational data sources on networked multivendor systems.
• Database Gateway from Micro Decisionware is an OS/2-based product that accepts SQL transactions from client applications and redirects them to DB2, SQL/DS, or Teradata DBC/1012 data bases for processing.
• Microsoft’s SQL Bridge is a protocol gateway for building distributed client/server systems in a Windows, DOS, Os/2, Macintosh, VAX, or Unix environment. It is designed primarily for use with Microsoft’s SQL Server products and supports OS/2 Named Pipes, TCP/IP, and DECnet protocols on LAN Manager and Digital Pathworks networks.
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