Set 'mode' property to 'remote' to make the combogrid be able to filter data on remote server. At the remote filter mode, the text users typed will be sent to server as a parameter named 'q'. If the 'queryParams' of grid is defined, it will also be sent to server. Write some code to retrieve the 'q' parameter and then return some new data set to the browser:
String q = request.getParameter("q");
// do some query from database
// return data set