Preface

This special issue of the IBM Systems Journal focuses on responsive systems, a term in which we include real-time and event-based systems. A realtime system is a computing system subject to constraints on the interval from the occurrence of an external event until the system response. A real-time constraint can be hard, in which case failure to complete the required computation before the deadline amounts to a system failure, or it can be soft, in which case delayed responses are tolerated but viewed as lowering the quality of the service provided by the system. An event-based system is a computing system whose components operate in response to events. The events processed by a component can be either external to the system or they can be generated by another component.