Buzz buzz buzz.
It reminds me of the Big Honey Hunt where following a single buzzing bee turned into a roundabout adventure that never quite seemed to get to the source but ended up stirring up the entire bee community.
My sentiments match those of Dougal who is simply glad enough that developers have discovered these concepts, however late in the game and by whatever name (mind you, I’m fully behind Dougal’s ARSE …umm, strike that).
So it’s time to cut through the crap and get some work done with this stuff.
AJAX is an acronym for Asynchronous JAvascript + XML. Ok, now that I’ve told you that, forget you ever heard it. Ajax (proper case) has already come to represent a wider concept. The concept is this: Load up and render a webpage, then remain at that page while scripted routines make background trips to the server for data that is used to update the page in place by re-rendering and or hiding/unhiding portions of the page.
Now that we’ve got that settled, let’s break the concept down into some functional parts to see where the convoluted discussion’s apples and oranges lie:
- Presentation
- Invocation
- Message Encoding
- Transport
The Presentation layer manipulates the display of data using whatever technology is available. DOM manipulation is the current way to do it, but older methods used the various flavours of DHTML available from the browsers at the time. Keeping up with the current set of browsers and their rendering quirks is pretty hairy.
Invocation can be object oriented or procedural depending on the API wrapped around the transport. Asynchronous or synchronous invocation depends more on the transport implementation. My JSRS library for instance only offers asynchronous communication – a limitation imposed on it by the transport layer I needed to use to work across the different browsers available at the time. Microsoft’s Java Applet based Remote Scripting could do both sync and async, and provided both an object and a procedural interface. The XMLHTTP transport offers both async and sync and you can wrap it up how you like.
I prefer to remain lightweight in my message encoding, sending strings back and forth, perhaps encoding structures into Javascript code, a trick I first saw in Microsoft’s RS (the “uneval” function that serializes a javascript obect into the code to reconstitute it). You can get some quite efficiently sized packets with low overhead. Many people prefer to use XML either because their particular development space is all about XML or in order to raise the level of abstraction at the cost of API internal complexity. XMLHTTP etc is then used for the encoding/decoding.
There are various transport methods that allow a browser to send and receive data without page refresh. The lightest one I use is to create an image object and set its source url to a server-side script that takes the querystring parameters, performs work, and returns a string via a cookie, then sends an empty document as the image. Before XMLHTTP was widely available, various hidden iframe and layer methods were used in order to actually hack functionality out of browsers that had not been designed with arbitrary data transport in mind. Other methods involve returning Javascript to the browser for execution. I use XMLHTTP now because I can use it synchronously, which was not possible in Javascript alone because of the lack of a non-blocking sleep function (you go into a tight loop to wait for a result and CPU runs to 100%). Previously would need Java or Flash or an ActiveX control to get that level of control. Now the prominent browsers have their own versions of XMLHTTP with which you can get arbitrary data in either sync or async fashion.
It should be apparent by now that we’re talking about a concept that has remained constant since as early as 1998 and a set of techniques with which to accomplish the task which have evolved over time around the capabilities of the tools at hand. The method used for any project has been selected as a result of the project’s choice of platform and design parameters.
When I get a chance, I’ll try to compile a list of links. In the meantime, use your favourite search engine to look for “Remote Scripting” for some of the historical solutions, and “Ajax” for some of the new stuff.
