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Servers that Break Down II

Another variant of the server breakdown model concerns what happens to the transaction that is in service when the breakdown occurs. In the model in Figure 10.14, even though the server is stopped when it breaks, the transaction in service completes service. The model in Figure 10.15 adds the preemption of the transaction in service, which is routed back into the queue.

Figure 10.15: A Server that Breaks Down

By default, the transaction is placed at the end of the FIFO queue. So, if there were other transactions waiting for service, the preempted transaction would be behind them. Another variant on this model would place the preempted transaction into the front of the queue, even though the queue was a FIFO for nonpreempted transactions. This variant could be accomplished using a priority queue where the transaction priority is the simulation time at the time the transaction arrived to the queue and the queue has decreasing priority (see Figure 3.5). See the preceding example on priority queues.