Queues and resources

In systems where customers arrive and depart, after being served we need at least three events (or an entity for both the customer and server). These types are in fact general for every queuing application. The difference then comes from the arrival and service pattern, from the service policy, from the resource amount and from the queuing priority of every event. These can be modeled as follows :

• arrival and service pattern - In the arrival event a next arrival is scheduled after the appropriate time. The service event schedules a departure event after the service time.
• service policy - When an arrival occurs it is served according to the service policy. The most common ones are the LIFO (last in first out) and FIFO (first in first out) queues. The first one can be modeled by adding an arrival to the front of the queue with the queue::prepend method, the last one is modeled by adding the arrival event to the back with the queue::append method. In both cases we then take the front of the queue with the queue::removefront method. Additional service policies can easily be modeled as variations of the ones above.
• resource amount - The number of arrivals that can be served depends on the resource amount. When a resource is created it receives its initial amount. This amount is then increased and decreased every time a certain amount is released or acquired, with the resource::release and resource::acquire methods. A resource can be released in the departure event (after being served) and can be acquired in the service event if the asked amount is available, which can be checked with the resource::available method.
• queuing priority - A priority queue can be modeled by giving a queuing priority to an event, when created. When appended or prepended an event that has a higher priority goes in front of an event with a lower priority.