Periodic Refreshing for the Browser-Server Dialogue
The first part of this series of articles dealing with browser-server information flow discussed call tracking. This one covers periodic refreshing. The second of a multi-part series, this article is excerpted from chapter 10 of the book Ajax Design Patterns, written by Michael Mahemoff (O'Reilly, 2006; ISBN: 0596101805).
Devi's coding up a ticket sales web site. For each event, she wants to keep the browser updated with the number of tickets remaining. Thus, she introduces a timer so that every 30 seconds, it calls a web service to pull down the latest sales stats.
How can the application keep users informed of changes occurring on the server?
The state of many web apps is inherently volatile. Changes can come from numerous sources, such as other users, external news and data, results of complex calculations, and triggers based on the current time and date.
HTTP requests can only emerge from the client. When a state change occurs, there's no way for a server to open connections to interested clients.
One way to keep the browser updated is HTTP Streaming (Chapter 6), but, as the "Alternatives" section for that pattern explains, it's not always ideal. In particular, its not very scalable.
The browser periodically issues an XMLHttpRequest Call to gain new information; e.g., one call every five seconds. The solution makes use of the browser's Scheduling (Chapter 7) capabilities to provide a means of keeping the user informed of latest changes.
In its simplest form, a loop can be established to run the refresh indefinitely, by continuously issuing XMLHttpRequest Calls (Chapter 6):
Here, the callServer function will invoke the server, having registered a callback function to get the new information. That callback function will be responsible for updating the DOM according to the server's latest report. Conventional web apps, even most of those using XMLHttpRequest Calls, operate under a paradigm of one-way communication: the client can initiate communication with the server, but not vice versa. Periodic Refresh fakes a back-channel: it approximates a situation where the server pushes data to the client, so the server can effectively receive new information from the browser. Indeed, as some of the examples show, the server can also mediate between users in almost real-time. So Periodic Refresh can be used for peer-to-peer communication too.
But before we get too carried away with Periodic Refresh, it's important to note that it's a serious compromise, for two key reasons:
The period between refreshes would ideally be zero, meaning instant updates. But that's not realistic and the browser will always lag behind. Latency is particularly problematic when the user is interacting with a representation of volatile server-side data. For instance, a user might be editing an object without knowing that another user has already deleted it.
There is a significant cost attached to Periodic Refresh. Each request, no matter how tiny, demands resources at both ends, all the way down to operating-system level. Traffic-wise, each request also entails some bandwidth cost, which can add up if refreshes are occurring once every few seconds.
So a key design objective must be to increase the average refresh period and reduce the content per refresh, while maintaining a happy user experience. One optimization is a timeout: the refreshes cease when the system detects the user is no longer active, according to a Timeout (Chapter 17) mechanism. You also want to make sure each refresh counts; it's wasteful to demand lots of updates if the network isn't capable of delivering them. Thus, the browser script can do some monitoring and dynamically adjust the period so it's at least long enough to cope with all incoming updates. Many of the Performance Optimization patterns can also be applied to Periodic Refresh--see the next section.