The classic web paradigm (known as "pull") is synchronous; it has the client (browser) solicit data from the server in a synchronous manner. This means that every time the client needs a data update, it has to ask the server expressly to find out if the data has changed and obtain the new value. In other words, for every request from a client there is a corresponding reply from a server. When a web page is visualized, the data contained within it is static on the user's browser and is not updated until a page refresh is made (manual or automatic). There are, however, a growing number of applications that necessitate the visualization of real-time data. Current examples are stock prices from online trading sites, betting odds from gambling portals, sports results, and messages exchanged through online communities. These are just a few cases of systems which, in order to offer the maximum in usability and quality of user experience, require continual updates of the visualized data in the browser page.
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