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The client/server model implies access by applications to remote databases— that is, databases located on another system. Further, there are diverse types of databases to be accessed, the diversity arising from any number of factors, including different decisions being made by different portions of the company, or as a side effect of company mergers; this all implies that it is desirable that access methods for databases are independent of the DBMS’s themselves. This is a real problem; although all databases provide SQL as an access method, various vendors have independently extended SQL in different manners compromising portability of applications between DBMS’s.

Various approaches have been proposed to meet the need for DBMSindependent access techniques. In this area, as in others, there are examples of scarcely-used or implemented standards, along with proprietary solutions which have the advantage of existing and working but the obvious drawback of instigating captive applications.

The access to a remote database includes the following dimensions: the interface, on the one hand, and FAP (formats and protocols), on the other hand.

As to the interface between applications and a DBMS, two approaches are distinguished:

» Embedded SQL (ESQL), which uses a precompiler to translate SQL commands embedded in the application program; with this approach, calls are fixed at application creation time

» Direct SQL calls (CLI—Call Level Interface), which do not require the application to know at creation time just which DBMS will be used

The work done within the SAG (SQL Access Group) consortium, comprising
several DBMS vendors, has resulted in a number of solutions. The first type of solution uses a CLI interface to provide access to a wide variety of DBMS; it is the ODBC (Open DataBase Connectivity) standard proposed by Microsoft.

ODBC is implemented at the workstation level. For each DBMS to be supported, it is necessary to develop a specific ODBC driver.

The principal difficulty with ODBC is that its specification is controlled by Microsoft and that it keeps changing. There is a similar technique for use in a Java environment, called JDBC, for Java DataBase Connectivity.

A second type of solution implements an open link approach, which converts application SQL commands (whether embedded or CLI) into SQL commands understood by each DBMS. In practice, this requires that the DBMS vendors support a common FAP. There are several competing solutions available of this type, including SAG’s RDA (Remote Data Access), and IBM’s DRDA (Distributed Relational Data Access) the latter provides several models for the interaction of the application with database, and also supports two-phase commit.

Source of Information : Elsevier Server Architectures 2005