1. A servo for realizing a predetermined function on a mobile client device having a user interface and utilizing at least intermittent communication with a server environment, the servo comprising: a series of transformation elements expressed in a predetermined servo description language (SDL) consistent with a predetermined SDL schema; the servo transformation elements including a data schema declaration for defining a data type on which the servo can be executed; and the servo transformation elements further including at least two transformation rules, each rule for transforming selected elements of the data type defined by the data schema from an input document so as to provide output to the client device user interface; and further wherein the servo transformation elements include at least one view declaration for selecting a group of the said transformation rules so as to define at least a part of an output interface realizable on the client device user interface.