I don't know whether it is a strange idea or not, but is there a web app that stores the data on the client side? Basically, the data is stored locally (on the client's computer) and the web app accesses it each time the user opens/uses the respective app. (All the app logic and scripts reside on the web server, not on the client's computer.)
closed as off topic by Barry♦ Feb 14 '12 at 10:18
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I'm not sure I understand your question, but the only way to store data on the client side is in your browser (cache)—XML and JSON being two of the options. However, this method is highly unreliable due to the way a browser works—if you clear the cache, you lose all the data.
It could be also possible to accomplish this by browsing for your file (i.e. data source), and uploading it to the server, each time you access the respective web app and then downloading it back at the end of the session.