Server Components

The Server holds the transaction while it is served. The service time can be a sample of a random variable (from one of several distributions), a fixed amount, or the value of a variable read from a SAS data set. By default, the service time is an observation of an exponential random variable with parameter 1.

An MServer, or multiple-server, can service multiple transactions simultaneously. The capacity of an MServer is set using the slider labeled "Capacity" on its control panel.

Notice that in the lower-right corner of the server components, there is a small rectangle. This is a Balk node. If a transaction arrives at a Server when it is busy or at an MServer when it is at capacity, the transaction will flow out the Balk node. Consider, for example, a situation where transactions are either serviced upon arrival by server 1 or, if server 1 is not free, wait for service from server 2. This is modeled by the network in Figure 2.4.

When the transaction leaves the Server or MServer, it has an attribute as named in the control panel that contains the time that the transaction spent in the server. This attribute can be used for controlling the simulation logic and for measuring the performance of the simulation by displaying it in one of the chart components or saving it in a SAS data set.

Transaction Arrival

If the server is busy, at capacity, or stopped, the transaction flows out the Balk node; otherwise, service is scheduled. On service completion, a request for transaction message is sent to arcs directed into the server. If a transaction is found, then its flow is initiated. Regardless, the transaction that just finished service flows on each of the arcs directed out of the server.

Request for Transaction

If the server is not busy or stopped, then pass on the request to all arcs directed into the server. The order in which the requests for service are issued is determined by the order of the components in the "Pull from" list box on the Sever Control Panel. Also, if a component is not included in the "Pull from" list box, then the request for transaction message is not propagated on the arc leading to that component.

Are You Busy Message

If the server is not busy and not stopped, then return FALSE; otherwise, return TRUE.

Query Message

Keyword	Meaning
busy	returns TRUE if the Server is busy or the MServer
	is at capacity or either is stopped; else, returns FALSE.
capacityIs	returns the capacity of the MServer
full	returns TRUE if the Server is busy or the MServer
	is at capacity; else, returns FALSE.
id	returns the server's unique identifier.
off	returns TRUE if the Server or MServer is stopped.
sizeIs	returns the number of multiple-server units that are busy.
space	returns TRUE if the Server is free or the MServer
	is not at capacity; else, return FALSE.

Trigger Message

Keyword	Meaning
preempt	removes all the transactions that are being served. They flow
	out of the Balk node.
preemptContinue	removes all the transactions that are being served.
	They flow out of the Balk node. The server requests transactions from
	upstream components.
removeIt	removes the transaction at the Trigger, if it is also being served.
	It flows out of the Balk node.
reset	resets the Sever and MServer destroying all waiting transactions.
seize	attempts to obtain service for the transaction that arrived at
	the Trigger.
setCapacity	sets the MSever capacity from the transaction
	attribute "capacity."
setDistribution	sets the distribution from the transaction attribute "distribution."
setParameter1	sets the first parameter in the distribution from the
	transaction attribute "parameter1."
setParameter2	sets the second parameter in the distribution from the
	transaction attribute "parameter2."
start	starts the Server component.
stop	stops the Server component. Transactions in service have
	normal completion.

Icon	Component	Description
	Server	provides service for a transaction
	MServer	provides service simultaneously for multiple transactions