1. A computerized method for determining a reliability of a composite service having a plurality of service components, where the composite service is capable of failing only where underlying physical resources by which the composite service is effectuated fail, the method comprising: where the composite service is represented as a plurality of continuous-time Markov chains (CTMC's), each CTMC corresponding to one of the underlying physical resources, constructing a product of the CTMC's, by a processor, the product encompassing a plurality of states of the composite service; determining a plurality of steady-state probabilities for the product of the CTMC's, by the processor, each steady-state probability corresponding to a likelihood that a corresponding state of the composite service will be a steady state of the composite service; for each state of the composite service, determining a reward structure of the state of the composite service, by the processor, the reward structure corresponding to a likelihood that the state will successfully use the underlying physical resources without failure, wherein the reward structure is determined for a given state of the composite service based on the steady-state probability corresponding to the given state and based on a plurality of discrete-time Markov chains (DTMC's), each DTMC corresponding to one of the service components of the composite service; wherein determining the reward structure of a given state of the composite service comprises determining a probability of successful completion of the service components within the given state, based on the DTMC's; extending the DTMC's to include a failure state, based on the probability determined; and, determining a probability of the DTMC's as extended to reach successful completion within the given state. wherein determining the probability of successful completion of the service components within the given state, based on the DTMC's, comprises: for each state of the composite service encompassed by the product of the CTMC's, determining whether a particular tuple is located within the states of the DTMC's that use resources, the particular tuple including the state, the DTMC corresponding to the state, and a reliability of the underlying physical resource corresponding to the state; where the particular tuple is located within the states of the DTMC's that invoke external resources, determining a probability of successful usage of the underlying physical resource corresponding to the state, the external resources being resources other than the underlying physical resources by which the composite service is effectuated; where the particular tuple is not located within the states of the DTMC's that invoke external resources, locating a particular pair within the DTMC's, the particular pair including the state and the DTMC corresponding to the state; and, determining a reward structure of the service component corresponding to the DTMC corresponding to the state, the reward structure corresponding to a likelihood that the service component corresponding to the DTMC corresponding to the state will successfully reach the given state without failure; determining the reliability of the composite service based on the reward structure of each state of the composite service, by the processor; and, outputting the reliability of the composite service as determined, by the processor.