That design is poor. There should be an actor modeling the seats (passed a customer and as a non-blocking operation either seats them or ejects them) and an act

That design is poor. There should be an actor modeling the seats (passed a customer and as a non-blocking operation either seats them or ejects them) and an actor modeling the barber (loops doing a blocking poll of the seats for a customer and doing the haircut). Mailboxes are not queues!