Server-Side "Conversion"

I'm starting to use the new data tables, but am having trouble figuring out server-side processing. The documentation seems almost too simple. This is what I did before. How do I do this in the new API?

'fnServerData': function(sSource, aoData, fnCallback, oSettings) {
     $.getJSON(sSource, aoData, function(data) {
        fnCallback(data);
        // Do something with the data
    });
}