Being able to use diverse Notes applications together is a great story, but only part of what people want to do when it comes to integration. There are many web applications that are not buil
with Domino. Often when companies merge they might be faced with similar solutions built on different technologies. It becomes a vital part of the business process for people to work with both, but because they are on disparate platforms it usually involves using one application and then the other one serially, with error prone cutting and pasting forming the linkage between the two. XPages gives you techniques to perform improved integration between these platforms.
If you have created an XPage in Domino Designer you probably worked mostly from the "Design" tab. If you switch to the "Source" tab, you can see all of the XSP markup language used to describe that page. If you are an advanced user, you may have worked out that you can actually edit the information in the Source tab. Certain operations, like search and replace, or cut and paste, are easier to do in the Source tab. What is not immediately obvious is that you can add in additional XHTML elements to the Source tab. These are processed along with the XSP elements and passed to the browser.
Almost anything that can be done by another application with XHTML elements can be surfaced in an XPage. This gives you the first, simple, route to integration. Say, for example, your other web application, queries a service, gets some XML data, and styles with a style sheet. That code can be placed on your XPage to do the same work.
Even better, since the XHTML code is being processed by the Domino server, it can add parameters or make alternations to it in exactly the same way as XPages code. You may notice at points on the Source tab code that looks like this: