This is the second part of the Undo Made Easy with Ajax series. If you are jumping in late, make sure to check out part 1.

Last time we covered the event queue method, an entirely client-side way of implementing a light-weight, multi-level undo. I mentioned a couple of caveats, including that it does not work in multi-user collaborations, and that it doesn’t work for time-sensitive actions like sending emails. I missed a significant caveat that one of my readers, Alexander Botero-Lowry, pointed out: That two tabs, pointed to the same page, could get out of sync. I wrote about how to solve that issue with cookies.

This time, let’s take a look at solving Undo for time-sensitive actions. In the event queue method, we could wait until the “onunload” event to sync the user’s actions from client-side to server-side. For time-sensitive actions like sending emails, we don’t have that luxury. Worse, email is a push technology. Once an email has been released into the wild tubes of the Internet, it cannot be unsent.* For the unfortunate who hits “send” only to realize that they’ve CC’ed their boss on a steamy love letter, the only things left to hope for are power outages and spam filters. Given how often spam slips past my filters, the outlook is not so good. (more…)