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Professional Ajax
by Nicholas C. Zakas, Jeremy McPeak, Joe Fawcett
March 2007, Paperback


Excerpt from Professional Ajax

Ajax Submission Throttling

By Nicholas C. Zakas

Since Ajax emphasizes avoiding page refreshes, the question of when to send user data is important. In a traditional web site or web application, each click makes a request back to the server so that the server is always aware of what the client is doing. In the Ajax model, the user interacts with the site or application without additional requests being generated for each click.

One solution would be to send data back to the server every time a user action occurs, similar to that of a traditional web solution. Thus, when the user types a letter, that letter is sent to the server immediately. The process is then repeated for each letter typed. The problem with this approach is that it has the possibility to create a large number of requests in a short amount of time, which may not only cause problems for the server but may cause the user interface to slow down as each request is being made and processed. The Submission Throttling design pattern is an alternative approach to this problematic issue.

Using Submission Throttling, you buffer the data to be sent to the server on the client and then send the data at predetermined times. The venerable Google Suggest feature does this brilliantly. It doesn't send a request after each character is typed. Instead, it waits for a certain amount of time and sends all the text currently in the text box. The delay from typing to sending has been fine-tuned to the point that it doesn't seem like much of a delay at all. Submission Throttling, in part, gives Google Suggest its speed.

Submission Throttling typically begins either when the web site or application first loads or because of a specific user action. Then, a client-side function is called to begin the buffering of data. Every so often, the user's status is checked to see if he or she is idle (doing so prevents any interference with the user interface). If the user is still active, data continues to be collected. When the user is idle, which is to say he or she is not performing an action, it's time to decide whether to send the data. This determination varies depending on your use case; you may want to send data only when it reaches a certain size, or you may want to send it every time the user is idle. After the data is sent, the application typically continues to gather data until either a server response or some user action signals to stop the data collection. Figure 1 outlines this process.

Figure 1
Figure 1
The Submission Throttling pattern should never be used for mission-critical data. If information must be posted to the server within a specific amount of time, you are better off using a traditional form to ensure the correct and timely delivery of the information.

Incremental Form Validation Example

As mentioned previously, Submission Throttling can be achieved through various user interactions. When using forms, it's sometimes useful to upload data incrementally as the form is being filled out. The most common usage is to validate data as the user is filling in the form instead of waiting until the end to determine any errors. In this case, you would most likely use the onchange event handler of each form element to determine when to upload the data.

The change event fires for a <select/> element whenever a different option is selected; it fires for other controls when its value has changed and it has lost focus. For example, if you typed a couple of letters into a text box and then clicked elsewhere on the screen (causing the text box to lose focus), the change event fires and the onchange event handler is called. If you click in the text box again, and then click elsewhere (or press the Tab key), the text box will lose focus but the change event will not fire because no changes have been made. Using this event handler for Submission Throttling can prevent extraneous requests.

Normally, the form validation is simply a precursor to submission. The form's submit button starts out disabled, becoming enabled only when all fields in the form have been validated by the server. For example, suppose you are running a web site where users must sign up to gain access to certain features. This may be a shopping site that requires sign-in to purchase items or a site that requires membership to access the message board. The items you'll want to be sure of when creating this new account are:

  • The user name must not be taken.
  • The e-mail address must be valid.
  • The birthday must be a valid date.

Of course, the type of data required will differ depending on your usage, but these items provide a good starting point for most applications.