Ajax Application Design
This approach had many problems, but the two biggest ones were scalability and security. If our site becomes especially popular, we will have many registered users, so sending a complete list of user names will consume increasing amounts of CPU and bandwidth.
In addition, it is a large security risk to send all of the user names on a site to anyone who requests it. The odds are good that at least one of those users has chosen a poor password, which would make it easy to assume that person's identity. The implications of this security breach depend on your users, your application and your country. Some countries' legal systems might even see this as a prosecutable violation of database privacy laws.
So, for technical and security reasons alike, we need to find a better solution. An obvious candidate, and one we examine this month, involves sending the proposed user name to the server via an Ajax request. The server's response will thus be a short “yes” or “no”, indicating whether the browser should allow or prevent registration.
An Ajax application consists of several parts:
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