AJAX - The Complete Reference

PART III

Chapter 10: Web Services and Beyond 517

Approach Description Comments

Fast poll Calls the server very rapidly using a

standard XHR call to see if changes

are available.

Uses standard HTTP request to a Web server.

Not really a push but if continuous enough

appears as instantaneous.

Significant burden on server and network with

numerous requests.

No way for server to initiate the data transfer.

Long poll Uses an XHR, but we hold the

connection open for an extended

period of time say 20-30 seconds.

After the time threshold is reached,

the connection is shut down and

re-established by the client. The

server may push data down the

held connection at any time and

thus shut the connection, which the

browser will immediately re-open.

Uses standard Web server with HTTP

connections.

Server can push data to browser assuming there

is a held connection open.

Held connections and some Web server-

application server architectures may not get

along well.

Gap of no connectivity when browser re-establishes

connection after data transfer or timeout.

Slow load Uses an iframe that points to a

never finishing URL. The URL in

question is a program that pushes

data when needed to the iframe,

which then can call upward into the

hosting page to provide the newly

available data.

Does not use an XHR and thus lacks some

networking and script control, though as an

iframe it works in older browsers.

Continuous load can present some disturbing

user interface quirks such as a never finishing

loading bar.

Tends to result in growing browser memory

consumption and even crashes if connection

held open for a very long time.

Binary

bridge

Uses Flash or Java applet to make a

socket connection to the server. As

two-way communication, the socket

provides full push possibilities.

Received data is made available via

JavaScript from the communications

helper binary.

Relies on binary that may not be installed.

Piping between JavaScript and binary may be

problematic.

Very flexible in terms of communication methods

and data formats.

Native

browser

access

In some browsers like Opera 9

you should be able to subscribe

to server events that will wake

the browser when data is made

available.

Uses native browser facilities.

Apparently works similarly to an endless iframe

from a network point of view.

Not widely implemented as of yet.

TABLE 10-2 Summary of Push-style Communications Approaches