An activity monitor for a modular software system, the system having a first software component for use in configuring operation of a second software component via a proxy and stub interface, the activity monitor comprising; an input arranged to monitor a first event associated with the first software component and a second event associated with the proxy; a processor arranged to compare the first and second events to a schedule of events to assess compliance with the schedule of events; and an output arranged to generate an error message in response to non-compliance with the schedule of events, the error message being for use in treating the error. A method of generating an activity monitor for monitoring operation of a modular software system.