# Queueing model with two servers

I have a two-server queue with Poisson arrival rate and $\lambda$ exponential services with $\mu$ ( first server service rate) and 2$\mu$ ( 2nd server service rate). Capacity is infinite.

Then why is the number of customers in the queue at time $t$ not a Markov Process?

You need extra information to make the process a Markov process. The Markov property requires that the future depend only on the current state, but suppose you have only a single customer in the system. The future behaviour of the system depends on the history of the process (namely which server that job started service with) not just the current state. (It could experience service at rate either $\mu$ or $2\mu$.)