  We study model checking of client - server systems, where the servers offer
several types of services that may depend, at any time, on how many clients of
specific types are active at that time. Since there are unboundedly many
clients, the state space of such systems is infinite, rendering specification
and verification hard. This problem can be circumvented by using a
specification language which has monadic first-order (MFO) sentences closed
with standard temporal modalities. The MFO sentences throw up a bound which
can, in turn, be used to bound the state space of the input client - server
system, thereby making the verification problem decidable. This scheme is
implemented using the NuSMV tool.
