Software reliability engineering for client-server systems

Too often, when doing software reliability modeling and prediction, the assumption is made that the software involves either a single module or a single node. The reality in today's increasing use of multi-node client-server systems is that there are multiple software entities that execute on multiple nodes that must be modeled in a system context, if realistic reliability predictions and assessments are to be made. For example, if there are N/sub c/ clients and N/sub x/ servers in a client-server system, it is not necessarily the case that a software failure in any of the N/sub c/ clients or N/sub x/ servers will cause the system to fail. Thus, if such a system were to be modeled as a single entity, the predicted reliability would be much lower than the true reliability, because the prediction would not account for criticality and redundancy. The first factor accounts for the possibility that the survivability of some clients and servers will be more critical to continued system operation than others, while the second factor accounts for the possibility of using redundant nodes to allow for system recovery should a critical node fail. To address this problem, we must identify which nodes-clients and servers-are critical and which are not critical, as defined by whether these nodes are used for critical or non-critical functions, respectively.