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Ajax in Action: Chapter 1: A New Design for the Web Page 2

By PHP Builder Staff
on November 30, 2005

Under the hood, a lot of computation is going on at both ends of a network connection in order to send and receive data (figure 1.7). It??s this computation that slows things down, more than the physical journey along the wire. The various stages of encoding and decoding cover aspects of the communication ranging from physical signals passing along the wire (or airwaves), translation of these signals as the 1s and 0s of binary data, error checking and re-sending, to the reassembling of the sequence, and ultimately the meaning, of the binary information.


Figure 1.7 Sequence diagram of a remote procedure call. The program logic on one machine attempts to manipulate a data model on another machine.


The calling function??s request must be encoded as an object, which is then serialized (that is, converted into a linear set of bytes). The serialized data is then passed to the application protocol (usually HTTP these days) and sent across the physical transport (a copper or fiber-optic cable, or a wireless connection of some sort).

On the remote machine, the application protocol is decoded, and the bytes of data deserialized, to create a copy of the request object. This object can then be applied to the data model and a response object generated. To communicate the response to the calling function, the serialization and transport layers must be navigated once more, eventually resulting in a response object being returned to the calling function.

These interactions are complex but amenable to automation. Modern programming environments such as Java and the Microsoft .NET Framework offer this functionality for free. Nonetheless, internally a lot of activity is going on when a remote procedure call (RPC) is made, and if such calls are made too freely, performance will suffer.

So, making a call over a network will never be as efficient as calling a local method in memory. Furthermore, the unreliability of the network (and hence the need to resend lost packets of information) makes this inefficiency variable and hard to predict. The responsiveness of the memory bus on your local machine is not only better but also very well defined in comparison.

But what does that have to do with usability? Quite a lot, as it turns out.

A successful computer UI does need to mimic our expectations of the real world at the very basic level. One of the most basic ground rules for interaction is that when we push, prod, or poke at something, it responds immediately. Slight delays between prodding something and the response can be disorienting and distracting, moving the user??s attention from the task at hand to the UI itself.

Having to do all that extra work to traverse the network is often enough to slow down a system such that the delay becomes noticeable. In a desktop application, we need to make bad usability design decisions to make the application feel buggy or unresponsive, but in a networked application, we can get all that for free!

Because of the unpredictability of network latency, this perceived bugginess will come and go, and testing the responsiveness of the application can be harder, too. Hence, network latency is a common cause of poor interactivity in real-world applications.
1.1.3 Asynchronous interactions

There is only one sane response to the network latency problem available to the UI developer??assume the worst. In practical terms, we must try to make UI responses independent of network activity. Fortunately, a holding response is often sufficient, as long as it is timely. Let??s take a trip to the physical world again. A key part of my morning routine is to wake my children up for school. I could stand over them prodding them until they are out of bed and dressed, but this is a time-consuming approach, leaving a long period of time in which I have very little to do (figure 1.8).

Figure 1.8 Sequence diagram of a synchronous response to user input, during my morning routine. In a sequence diagram, the passage of time is vertical. The height of the shaded area indicates the length of time for which I am blocked from further input.

I need to wake up my children, stare out the window, and ignore the cat. The children will notify me when they are properly awake by asking for breakfast. Like server-side processes, children are slow to wake. If I follow a synchronous interaction model, I will spend a long time waiting. As long as they are able to mutter a basic ??Yes, I??m awake,?? I can happily move on to something else and check up on them later if need be.

In computer terms, what I??m doing here is spawning an asynchronous process, in a separate thread. Once they??re started, my children will wake up by themselves in their own thread, and I, the parent thread, don??t need to synchronize with them until they notify me (usually with a request to be fed). While they??re waking up, I can??t interact with them as if they were already up and dressed, but I can be confident that it will happen in due course (figure 1.9).


Figure 1.9 Sequence diagram of an asynchronous response to user input. If I follow an asynchronous input model, I can let the children notify me that they are starting to wake up. I can then continue with my other activities while the wakeup happens and remain blocked for a much shorter period of time.


With any UI, it??s a well-established practice to spawn an asynchronous thread to handle any lengthy piece of computation and let it run in the background while the user gets on with other things. The user is necessarily blocked while that thread is launched, but this can be done in an acceptably short span of time. Because of network latency, it is good practice to treat any RPC as potentially lengthy and handle it asynchronously.

This problem, and the solution, are both well established. Network latency was present in the old client/server model, causing poorly designed clients to freeze up inexplicably as they tried to reach an overloaded server. And now, in the Internet age, network latency causes your browser to ??chug?? frustratingly while moving between web pages. We can??t get rid of latency, but we know how to deal with it??by processing the remote calls asynchronously, right?

Unfortunately for us web app developers, there??s a catch.

??HTTP is a request-response protocol. That is, the client issues a request for a document, and the server responds, either by delivering the document, saying that it can??t find it, offering an alternative location, or telling the client to use its cached copy, and so on. A request-response protocol is one-way. The client can make contact with the server, but the server cannot initiate a communication with the client. Indeed, the server doesn??t remember the client from one request to the next.

The majority of web developers using modern languages such as Java, PHP, or .NET will be familiar with the concept of user sessions. These are an afterthought, bolted onto application servers to provide the missing server-side state in the HTTP protocol. HTTP does what it was originally designed for very well, and it has been adapted to reach far beyond that with considerable ingenuity. However, the key feature of our asynchronous callback solution is that the client gets notified twice: once when the thread is spawned and again when the thread is completed. Straightforward HTTP and the classic web application model can??t do this for us.

The classic web app model, as used by Amazon, for example, is still built around the notion of pages. A document is displayed to the user, containing lists of links and/or form elements that allow them to drill down to further documents. Complex datasets can be interacted with in this way on a large scale, and as Amazon and others have demonstrated, the experience of doing so can be compelling enough to build a business on.

This model of interaction has become quite deeply ingrained in our way of thinking over the ten years or so of the commercial, everyday Internet. Friendly WYSIWYG web-authoring tools visualize our site as a collection of pages. Server-side web frameworks model the transition between pages as state transition diagrams. The classic web application is firmly wedded to the unavoidable lack of responsiveness when the page refreshes, without an easy recourse to the asynchronous handler solution.

But Amazon has built a successful business on top of its website. Surely the classic web application can??t be that unusable? To understand why the web page works for Amazon but not for everyone, we ought to consider usage patterns.
1.1.4 Sovereign and transient usage patterns

It??s futile to argue whether a bicycle is better than a sports utility vehicle. Each has its own advantages and disadvantages??comfort, speed, fuel consumption, vague psychological notions about what your mode of transport ??says?? about you as a person. When we look at particular use patterns, such as getting through the rush hour of a compact city center, taking a large family on vacation, or seeking shelter from the rain, we may arrive at a clear winner.

The same is true for computer UIs.

Software usability expert Alan Cooper has written some useful words about usage patterns and defines two key usage modes: transient and sovereign. A transient?? application might be used every day, but only in short bursts and usually as a secondary activity. A sovereign application, in contrast, must cope with the user??s full attention for several hours at a time.

Many applications are inherently transient or sovereign. A writer??s word processor is a sovereign application, for example, around which a number of transient functions will revolve, such as the file manager (often embedded into the word processor as a file save or open dialog), a dictionary or spellchecker (again, often embedded), and an email or messenger program for communicating with colleagues. To a software developer, the text editor or Integrated Development Environment (IDE) is sovereign, as is the debugger.

Sovereign applications are also often used more intensely. Remember, a well-behaved UI should be invisible. A good yardstick for the intensity of work is the effect on the user??s workflow of the UI stalling, thus reminding the user that it exists. If I??m simply moving files from one folder to another and hit a two-second delay, I can cope quite happily. If I encounter the same two-second delay while composing a visual masterpiece in a paint program, or in the middle of a heavy debugging session with some tricky code, I might get a bit upset.

Amazon is a transient application. So are eBay and Google??and most of the very large, public web-based applications out there. Since the dawn of the Internet, pundits have been predicting the demise of the traditional desktop office suite under the onslaught of web-based solutions. Ten years later, it hasn??t happened. Web page??based solutions are good enough for transient use but not for sovereign use.
1.1.5 Unlearning the Web

Fortunately, modern web browsers resemble the original ideal of a client for remote document servers about as closely as a Swiss army knife resembles a neolithic flint hunting tool. Interactive gizmos, scripting languages, and plug-ins have been bolted on willy-nilly over the years in a race to create the most compelling browsing experience. (Have a look at www.webhistory.org/www.lists/www-talk.1993q1/0182.html to get a perspective on how far we??ve come. In 1993, a pre-Netscape Marc Andreessen tentatively suggested to Tim Berners-Lee and others that HTML might benefit from an image tag.)
A few intrepid souls have been looking at JavaScript as a serious programming language for several years, but on the whole, it is associated with faked-up alert dialogs and ??click the monkey to win?? banners.
Think of Ajax as a rehabilitation center for this misunderstood, ill-behaved child of the browser wars. By providing some guidance and a framework within which to operate, we can turn JavaScript into a helpful model citizen of the Internet, capable of enhancing the real usability of a web application??and without enraging the user or trashing the browser in the process. Mature, well-understood tools are available to help us do this. Design patterns are one such tool that we make frequent use of in our work and will refer to frequently in this book.
Introducing a new technology is a technical and social process. Once the technology is there, people need to figure out what to do with it, and a first step is often to use it as if it were something older and more familiar. Hence, early bicycles were referred to as ??hobbyhorses?? or ??dandy horses?? and were ridden by pushing one??s feet along the ground. As the technology was exposed to a wider audience, a second wave of innovators would discover new ways of using the technology, adding improvements such as pedals, brakes, gears, and pneumatic tires. With each incremental improvement, the bicycle became less horse-like (figure 1.10).


Figure 1.10 Development of the modern bicycle


The same processes are at work in web development today. The technologies behind Ajax have the ability to transform web pages into something radically new. Early attempts to use the Ajax technologies resembled the traditional web page document and have that neither-one-thing-nor-the-other flavor of the hobbyhorse. To grasp the potential of Ajax, we must let go of the concept of the web page and, in doing so, unlearn a lot of the assumptions that we have been making for the last few years. In the short few months since Ajax was christened, a lot of unlearning has been taking place.