How to send data from one user to other without storing data in database

[SystemFiles]

i got an assignment to make something similar to http://typewith.me .The problem i am facing is that i have to send xml through AJAX from one user to server(im using PHP) and the other user will receive it when he makes request through AJAX. But storing XML in database in the intermediate time will make it slow. So where else can i store the XML. Remember there can be multiple users and it all should be synchronized. Please Help. Thanks.