Multiple-Tier Architecture

Proxies

As mentioned earlier, it is also possible to incorporate other types of HTTP servers into your WebLogic installation. This is a big selling point for BEAamong other things, it means that WebLogic server can be integrated into existing Web sites with relatively little trauma. In order to interact with WebLogic server, an HTTP server must be configured with a plug-in. Plug-ins are available for the following server classes:

• Apache

• Netscape Enterprise server

• Internet Information server

Ideally, these servers should handle only requests for static Web pages. To support legacy applications it may be necessary to permit them to serve some interpreted code, either as CGI or from a script module such as mod_perl. However, it will always be a bad idea for JSPs and servlets to be handled by anything other than WebLogic server. Note also that for administration purposes plug-in enabled HTTP servers are not really part of the cluster. In particular, they cannot be configured or invoked through the Administration Console.

In the configuration shown in Figure 4-4 there are two tiers to the application architecture. The Web tier consists of some number of plug-in enabled HTTP servers. These servers handle all requests for static Web pages. Requests involving servlets, JSPs, and EJBs are passed through the proxy plug-in to a WebLogic server cluster that incorporates servers capable of servicing the requests.

This architecture is interesting in that it provides an opportunity for additional security. To the outside world, it appears as though all requests are being handled by the HTTP servers. The WebLogic servers are invisible, shielded from hacking eyes by carefully configured HTTP servers that now stand between them and the cruel, cruel world. Only the HTTP servers should be accessible through the firewall. The truly paranoid will also be tickled by the addition of a second firewall between the WebLogic servers and the database cluster.

Note that the above architecture is essentially a variation on the basic architecture shown in Figure 4-1. The details that we are primarily concerned withEJBs, JSPs, and servletsare housed together on each server.

A more interesting configuration is shown in Figure 4-5. As you can see, this is a variation on the enterprise architecture of Figure 4-3 in the sense that there are two tiers of WebLogic servers. The presentation tier handles the generation of conventional dynamic content via JSPs, servlets, and the like, and the object tier provides a pool of WebLogic servers dedicated to handling object requests. Note also that, as in Figure 4-1, we have a layer of plug-in enabled HTTP servers between the WebLogic servers and the outside world. ^[3]

^[3] Yes, I'm familiar with the term "DMZ." I avoided using it on purpose. My feeling is that if your job does not place you in danger of attack by mortar or machine gun, the use of military acronyms is a little silly. I also don't say "niner."