Generally, when there is a need to update what the visitor sees, or to make changes to
the database on the server, we accomplish this by requesting a new page. However, there are
times that it makes sense not to do so, but only to send changes to the server, or update
an existing page without reloading it. There are many terms that refer to such techniques,
such as RDA, DSO, and
HTTP Tunneling, but they all mean pretty much the same thing:
instead of loading a page again, we open up a separate connection to the server and only
transfer the data that is changed.
This technique is particularly valuable when pages are large or there are a great many
changes to be done with one screen of data. This is because the data that needs to be
changed is almost always significantly smaller than the entire page, so the process can
be completed quicker. This is a useful technique which cannot be accomplished with normal
client-side scripting. It requires more sophisticated
technologies such as Java.
This page uses RDA (more specifically, HTTP Tunneling) to accomplish data transfer, using
a database table of planetary data. It uses an invisible Javaapplet to open a connection to the server each time the
visitor changes the drop-down. A PHP script on the server
extracts the information from the database and sends it back to the applet as an
XML data stream. Client-side scripting acts together with
the applet to change the appropriate parts of the page. Notice that the page is not reloaded;
only the data changes.
