[Show abstract][Hide abstract] ABSTRACT: Flash crowd has generally been agreed to be one of the main threats to the availability of web services, and application replication is a promising approach to address such problem. In this paper, we first model the workload of flash crowds according to the characteristics of their request distribution. Then a new mechanism to trigger replication service based on the gradient of cumulative request pattern is proposed. This mechanism, called hybrid adaptive selection algorithm (HASA), targets to minimize the number of discarded requests (NDR) and the spare capacity (NSC) together. Detail simulation of this mechanism with respect to interval size and thresholds of NDR and NSC shows that significant performance improvement can be obtained over the previously proposed single-targeted adaptive selection algorithms.