1. A system for generating a hierarchical probability model generation system used for availability analysis, comprising: a memory storing instructions; and one or more processors configured to execute the instructions to: generate a temporary event tree by analyzing operating conditions of a multi-tier web application system based on availability requirements of the web application system; extract independent sets of configuration elements of the web application system by analyzing dependence between the configuration elements based on configuration information of the web application system, wherein the configuration elements correspond to a plurality of functions of the web application system; determine basic events based on the temporary event tree and the independent sets of the configuration elements, wherein the basic events are inputs of the temporary event tree and include independent and dependent basic events, the independent events being calculable independently of each other, the basic events being associated with operating status of the configuration elements; specify the independent basic events by extracting the basic events corresponding to the independent sets of the configuration elements; generate an event tree based on the independent basic events but not the dependent basic events; generate state transition models used to calculate occurrence probabilities of each of the independent basic events; and evaluate availability of the web application system based on the generated state transition models.