Ask 10 people what "Web middleware" means and you may get 10 different answers. But a basic working definition is "anything that helps deliver information from back-end servers to 'thin,' browser-type clients."

"Middleware" is not so much a functional description as a broad concept that covers many products. You'll often find that the only common ground among middleware products is that they all sit between two other things.

This loose framework makes choosing a Web middleware product a difficult proposition, even after you've identified your business needs. And yet there are compelling reasons to make the effort to employ Web middleware in the enterprise.

Using Web browsers or other thin clients as your client/presentation layer is a mixed bag. On the plus side, a browser is a familiar interface that you need not develop, it has built-in Internet/intranet connectivity and it can scale applications for use across the enterprise with relative ease. A browser-based three-tier system also makes it easier to deploy applications to remote users. Distributing disks containing the client software becomes unnecessary--users simply access a Web page, perhaps downloading a Java applet or two as they go, and your middle tier handles the rest.

The challenges begin with the inherent lack of state and session management on the Web--a problem you can solve by choosing the right Web middleware product. Most computer systems maintain state--that is, they know where the users are in the system, where they have been and probably even where they're going. Every operation has a context. For example, an order-entry system knows when the user is entering a product order, and it probably has programmatical constraints that guide the user as he or she completes the required fields on the order form, submits it for processing and then goes to the next function. A user can't easily jump from one business function to another unless the program is specifically designed to allow it. Each user has a clearly identified session, and the system can easily tell who's who, and what each user is doing.

By contrast, accessing a Web page through a browser is usually "stateless": The browser doesn't know what the user has done or what he or she is doing. All it knows is that the user is looking at a particular Web page; he or she can easily jump to another at any time. Sticking with the order-entry example, the user can be filling out an order on a Web page, suddenly decide to jump over to CNN to read the news, then return to the order-entry form. He or she can then hit the "back" button on the browser and revisit a previous page...and so on and so on.

This creates serious difficulties for a system programmer trying to guide the user through a logical process. However, using cookies (bits of data written to small temporary files maintained on users' computers) or other methods of preserving state, a middle-tier application server makes it easier to keep track of a user's whereabouts and activities on a Web-based system. Cookies are the most common choice for state management, but be sure to ask the vendor what happens to the product if a user decides not to accept cookies, which is an option on most popular Web browsers.

Narrowing Your Search The products listed in our Interactive Buyer's Guide (www.buyersguide.nwc.com) cover many specific functional areas. Several are full-fledged application servers, including Bluestone Software's Sapphire/Web, NetDynamics' (now owned by Sun Microsystems) NetDynamics 4 and Oracle Corp.'s Application Server. These products typically enable native connectivity and/or ODBC (Open Database Connectivity) to multiple database back ends, have functionality to preserve state, can be integrated with various programming languages and may include Web site development tools. Many products, such as BEA Systems' TUXEDO, IBM Corp.'s Transaction Server (Encina/ CICS), Microsoft Corp.'s Transaction Server and Sybase's Jaguar, enable connectivity to transaction processing (TP) products. TP systems simplify the processing and monitoring of data requests and updates from multiple front-end clients to multiple back-end data sources. They ensure, for instance, that database updates occur properly, and that transactions are totally completed or rolled back as necessary under a failure condition. Some kind of TP product is a must for a strong three-tier enterprise system.

If your environment has unreliable, poor or intermittent network connections, such as in a remote office, or if you're supporting mobile workers, look for products that directly support or can interface with message-queueing products such as IBM's MQSeries and Microsoft's MSMQ. Message queuing, especially when used in conjunction with transaction processing, allows for support of asynchronous data transfer within three-tier systems. For instance, it lets you place database update requests in managed queues until a database connection is available. It then provides reliable means to ensure that the transaction was completed, and returns the result of the update (success or failure) to the remote user's process that originally attempted the transaction. Meanwhile, the user originating the process is free to initiate and conduct additional transactions without waiting for the first one to be completed.

If your needs include Web site development but you don't require a heavy-duty application server, look at products that have roots in Web site development environments, such as Allaire Corp.'s Cold Fusion and EveryWare Development's Tango. The former, for example, has powerful extensions to HTML similar to Microsoft's Active Server Page and a sophisticated IDE (Integrated Development Environment) for Web page development, and it can serve applications. But Cold Fusion's load-balancing and failover features are not as sophisticated as those in Bluestone's Sapphire/Web. And Cold Fusion has no native drivers for connectivity to back-end databases; ODBC support is included instead. Tango has an excellent IDE for developers that targets high-end application servers with multithreaded application-server capabilities and thread pooling capability. But Tango lacks the kind of sophisticated load-balancing and failover support you'd want for a heavily used system.

If you need a heavy-duty application server, there are a number of mature, powerful products from which to choose, such as Sapphire/Web, NetDynamics and SilverStream Software's Web Application Platform. Sapphire/ Web is supported on numerous platforms due to its pure Java architecture, and has both native driver and ODBC/ JDBC (Java Database Connectivity) support for many databases. It also includes a robust server, with capable failover, load-balancing, state management and fault-tolerance features. It also has back-end integration capabilities to LDAP, BEA's TUXEDO, IBM's CICS and MQSeries, PeopleSoft, SAP and others. Sapphire/Web is one of the few products that supports two-phase database commits without a third-party transaction processor. Like most of the high-end products, however, Sapphire/Web is difficult to learn and is best suited for large-scale projects by an enterprise with a strong developer team.

If your interest is strictly in publishing database information on the Web, products such as Showbase's Collaborative and Extra take data in a variety of database formats and translate it into forms that are easily served over a variety of Web servers.

Specialized products focus on connectivity with mainframes and legacy data sources. Unikix Technologies, with its KixORB Server and 3270 ScreenBean, is geared toward CICS and other legacy data-integration tools, enabling access via Java and CORBA (Common Object Request Broker Architecture); StarQuest Software's StarSQL provides access to DB2 databases from any ODBC-compliant application; and Simba Technologies offers several products allowing OLE DB, ODBC and JDBC connectivity to various data sources.

Another specialty area appears in Level 8 Systems' upcoming e-commerce Broker, which will integrate strong messaging features with the ability to handle the high transaction volumes a large e-commerce site can expect.

A Model World Your choice of Web middleware may be affected by the component model(s) you wish to support. Some products support multiple models, though it's often through a bridge or wrapper that can affect performance and introduce complexity.

CORBA runs on many platforms, though there are so many variants that interoperability can still be a serious concern--especially among different vendors' implementations.

Microsoft's DCOM (Distributed Component Object Model) is a fairly robust object model that several Web middleware packages support. However, while some COM support is available for non-Windows platforms, DCOM is primarily a Windows-based model and the upcoming COM+ model (to be included with Windows NT 5.0) is strictly limited to Windows.

Enterprise JavaBeans, the new object model from Sun Microsystems, is relatively immature, but shows promise for cross-platform interoperability.

The more flexible your object model support is now, the less likely you'll end up with vertical, non-interoperable systems later.

Send your comments on this article to Richard Hoffman at rhoffman@nwc.com.