Continuous review (s, Q) inventory system at a service facility with positive order lead times

In this study, a continuous review (s,Q) inventory system with a service facility is examined. There is only one server and a limited number of customers waiting rooms in this facility. The demands arrive to the queueing-inventory system according to the Poisson process. Every customer needs a single product with a service period that is unpredictable and distributed arbitrarily. An external supplier replenishes the inventory, and the lead time for the reorder is predicated on an independent exponential distribution. Demands that arise during a stock out period must wait in the waiting area, and when the ordered items arrive, they are served using the first-come-first-serve queueing discipline. With the help of the imbedded Markov chain technique, we are able to compute the joint probability distribution of the number of customers in the system and the number of items in inventory at post-departure epoch. With the remaining service time of a customer in service as the supplementary variable, we are able to relate the system length distributions at post-departure and random epochs in order to determine the joint probability distribution at random epoch. The analysis of waiting time of an accepted customer in the queue is also examined. Several stationary system performance measures are computed and the total expected cost is determined under an appropriate cost structure to determine the optimal values for waiting space (N), reorder level (s), and order quantity (Q). In order to explain the important performance indicators of the system, some numerical findings are given for a variety of model parameters.