The module dojox.rpc.Client was introduced in Dojo Toolkit version 1.2, providing extra request headers on XMLHttpRequests to facilitate implementing deterministic request handling on servers. Browsers typically use two or more TCP connections to send HTTP requests to a server, while subsequent requests are often routed on separate TCP connections. This means there is no guarantee that the request that is sent first will arrive at the server first. With sophisticated web applications, non-determinism can lead to bugs that are very difficult to track down.
If a web application sends multiple requests to a server and assumes that the server will handle them in the order they were sent, this non-determinism can be problematic. Furthermore, messages that arrive out of order can be rare, and therefore problems can be hard to reproduce. On old modem connections, packets were sequenced over a slow PPP connection that made out of order messages almost non-existent. With modern broadband connections, the race becomes more interesting. HTTP pipelining adds another mechanism that can lead to messages arriving out of order and with browsers beginning to support and use pipelining, messages arriving out of order will become even more likely in the future.
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