XML in a Nutshell, 3rd Edition

Document Permanence

XML documents that are intended for computers to read are often transitory. For instance, a SOAP document that represents a request to a Windows server running .NET exists for just as long as it takes the client to send it to the server and for the server to parse it into its internal data structures. After that’s done, the document will be discarded. It probably won’t be around for two minutes, much less two years. It’s an ephemeral communication between two systems, with no more permanence than any of billions of other messages that computers exchange on a daily basis, most of which are never even written to disk, much less archived for posterity.

Some applications do store more permanent computer-oriented data in XML. For instance, XML is the native file format of the Gnumeric spreadsheet. On the other hand, this format is really only understood by Gnumeric and perhaps the other Gnome applications. It’s designed to meet the specific needs of that one program. Exchanging data with other applications, including ones that haven’t even been invented yet, is a secondary concern.

XML documents meant for humans tend to be more permanent and less software bound, however. If you encode the Declaration of Independence in XML, you want people to be able to read it in 2, 200, or 2,000 years. You also want them to be able to read it with any convenient tool, including ones not invented yet. These requirements have some important implications for both the XML applications ...