Extending Closed Systems
But as customers demand more avenues of access to their account information, it becomes necessary to devise ways to share data between mainframe applications and new server-based solutions. One area in which demand has far outstripped IT departments' capability to deliver is in the area of Internet access to legacy data.
Figure 10.2 shows a more complex system configuration that takes into account the presence of Internet users.
Figure 10.2. Mainframe system that incorporates Internet users.
One way to understand the problem of connecting legacy applications with modern, Internet-enabled applications is to view it in terms of the seven-layer Open Systems Interconnection (OSI) networking model. Table 10.1 lists the layers and gives a brief description of each.
Several well-established technologies handle the lower-level connectivity issues. The almost universal adoption of TCP/IP and Ethernet has greatly simplified physical and networking connection issues that previously plagued mainframe operations. Message Oriented Middleware technologies such as Microsoft's MSMQ and IBM's MQ Series provide mechanisms for transmitting arbitrary messages between PCs and mainframes. In addition, relatively new application protocols such as HTTP and SOAP are being introduced in a wide variety of mainframe environments.
Where XML can add value is in the high-level presentation and application layers. Character set issues and byte-ordering problems are answered by the requirement that XML documents be transmitted using Unicode character encoding. At the application layer, XML provides a convenient format for representing complex, hierarchical data structures across different machine architectures.
Moving to an XML messaging format provides numerous benefits to programmers on both sides of the connection. Programmers can focus on the data itself, not necessarily on the operations that need to be performed on it. This data-centric approach can greatly simplify the cross-platform development cycle.
The downside is that many of the benefits of a monolithic, mainframe-based system are lost. Data freshness and consistency become issues. If read and write access to mainframe data is required, some form of basic transaction control must be implemented to make sure that changes made on one platform don't overwrite changes made on another.