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13.1 An Overview of Dynamic Documents
Recall from our discussion in Chapter 1 that the
client browser initiates data flow on the Web by contacting a server
with a document request. The server honors the request by downloading
the document. The client subsequently displays the
document's contents to the user. For normal web
documents, a single transaction initiated from the client side is all
that is needed to collect and display the document. Once displayed,
however, it does not change.
Dynamic documents, on the other hand, are the result of multiple
transactions initiated from either or both the server side and the
client side. A client-pull document is one that initiates
multiple transactions from the client side. When the server is the
instigator, the dynamic document is known as a
server-push document.
In a client-pull document, special HTML codes tell the client to
periodically request and download another document from one or more
servers on the network, dynamically updating the display.
Server-push documents also advance the way servers communicate with
clients. Normally, over the Web, the client stays connected with a
server for only as long as it takes to retrieve a single document.
With server-push documents, the connection remains open and the
server continues to send data to the client periodically, adding to
or replacing the previous contents.
Netscape currently is the only browser able to handle server-push
dynamic documents correctly; both Internet Explorer and Netscape
support client-pull documents. With other browsers, you might see
only part of the dynamic document, at best. At worst, the browser
completely rejects the document. Unfortunately, because dynamic
documents are client/server processes, they don't
work without an HTTP server. That means you can't
develop and test dynamic documents you have stored as local files
unless you have a server running locally as well.
13.1.1 Another Word of Caution
As always, we tell you exactly how
to use these exciting but nonstandard features, and we admonish you
not to use them unless you have a compelling and overriding reason to
do so. We are particularly strident with that admonition for dynamic
documents, not only because they aren't part of the
HTML standard, but because dynamic documents can hog the network.
They require larger, longer downloads than their static counterparts,
and they require many more (in the case of client-pull) or
longer-term (for server-push) client/server connections. Multiple
connections on a single server are limited to a few of the millions
of web users at a time. We'd hate to see your
readers miss out because you've created a jiggling
image in a dynamic document that would otherwise have been an
effective and readily accessible static document that more people
could enjoy.
