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One approach is replication, which essentially means maintaining a PC database copy of the mainframe data. Replication refreshes the PC database, which clients access as if it were the original. You can then use one of the tools described in this article to Web-enable the PC database.

However, you may not want PC-based copies of the mainframe database lying around. Really large databases don't lend themselves to storage on desktop computers. More important, you may have the sort of database design that's usable only when the data is preprocessed by a nontrivial amount of business logic--logic you have painstakingly programmed and have running on the mainframe. In fact, mainframe databases whose design assumes access only through a given set of business logic aren't all that uncommon. If the business logic assumes access via 3270 terminals but you want it available through a Web browser, don't despair. You can leave the business logic in place, untouched, and simply redirect the data to Web browser clients, even though the mainframe application assumes terminal access.

The two methods of redirecting 3270 data streams to a Web browser are conversion to other than 3270 format and a combination of screen scraping, screen interpretation and reformatting. The straight conversion method is mainframe-based and intercepts data streams before they're processed by a terminal emulator. The second method is client-based and examines the 3270 data after it's placed in a screen buffer by a terminal emulator (hence the screen scraping). PLATINUM technology's InfoSession uses the first method while Attachmate Corp.'s Host Publishing System uses the latter.

Attachmate Corp. Host Publishing System
Kudos to the Attachmate developers for the creative way they've elevated screen scraping to a new level of functionality. Using the familiar Visual Basic development environment as a graphical design substructure, the Host Publishing System (HPS), typically priced at $25,000 per server, tur ns 3270 screen buffer data into HTML for display b y a Web browser. After connecting to the host, typically via a TN3270 session, you use the QuickApp Recorder component of HPS to monitor and record the screen and keyboard activity for that session. You then start Visual Basic version 4.0 or later, load the HPS controls into your project and use Visual Basic controls (text boxes, list boxes, pushbuttons and drop-down list boxes, for instance) to design what will become your Web pages. During the design phase, HPS uses the previously recorded screen and keyboard activity to automatically code the program steps that will navigate the 3270 screens. The mainframe application accesses the database and emits 3270 screens. HPS lets you present the result as Web pages in a browser.

The next step is the surprising one: HPS turns VB's resulting form definition file (FRM) into HTML and the VB program statements into an in-process OLE server Dynamic Link Library (DLL). You can even embed HTML in the captions of form controls because HPS turns these captions into HTML tags. Attachmate supplies a Common Gateway Interface (CGI) executable you reference in the URL identifying your Web page, and the DLL's procedures and functions become available to the CGI process. When the Web server (any flavor that supports CGI and runs on Windows NT) sees the URL's CGI executable reference, it relinquishes control to the CGI process. With the help of the VB program's DLL code, the CGI process logs on to the host, displays individual Web pages (through the Web server), interacts with the user and produces its output report as dynamically built HTML. HPS also works with Netscape's NSAPI and Microsoft's ISAPI native interfaces.

HPS represents an extraordinary and imaginative level of direction between a 3270 session and a Web browser. Its use of VB's graphical design studio feels somehow appropriate; VB is sophisticated enough to let nonprogrammers create computer programs with drag-and-drop ease. For mainframe applications that display da tabase information on 3270 screens, HPS is a ti me machine into a Web-based future.

HPS requires TCP/IP and works with Web servers that offer CGI.

PLATINUM technology InfoSession
Intercepting 3270 data streams within the mainframe itself--before they travel through controllers to and from 3270 devices--is InfoSession's approach to providing data. The interception occurs similarly to the way TSRs and virtual device drivers redirect events (such as I/O operations that become file server request messages) on a PC. A VTAM application, running in its own address space under MVS on the mainframe, uses specifications you supply in a client Windows design session to store 3270 screen data in a virtual database structure.

The Windows design session, which PLATINUM calls the Workbench, lets users navigate 3270 screens and indicate those data elements to be made available via InfoSession's virtual database. The virtual database, residing in auxiliary MVS data spaces, responds to SQL instructions as if it were a database. To r ender the resulting query responses in HTML, you use one of the desktop computer tools mentioned in this article. InfoSession doesn't provide its own mechanism for emitting HTML.

InfoSession lets you avoid assigning a systems-level programmer the job of screen scraping to transform 3270 data into something users can access more easily. SQL query tools are much cheaper than the bare-metal programming effort EHLLAPI requires. InfoSession's VTAM-based approach reduces network traffic; only the requested data flows across the network. And because the virtual database resides centrally on the mainframe at all times, distributing changes becomes a non-issue.

InfoSession, typically priced about $52,000, supports LU2, LU6.2 and TCP/IP protocols for the design-time PC-to-host communications, and it offers two popular call-level programming APIs for access by query tools--ODBC 2.0 and an X/Open-compliant command-line interface.