In a perfect intercommunication environment, queues would never occur because work flow would be evenly distributed over time, and there would be enough intersystem sessions available to handle the maximum number of requests arriving at any one time. However, in the real world this is not the case, and, with peaks and troughs in the workload, queues do occur: queues come and go in response to the workload. The situation to avoid is an unacceptably high level of queuing that causes a bottleneck in the work flow between interconnected CICS® regions, and which leads to performance problems for the terminal end-user as throughput slows down or stops. This abnormal and unexpected queuing should be prevented, or dealt with when it occurs: a "normal" or optimized level of queuing can be tolerated.
For example, function shipping requests between CICS application-owning regions and connected file-owning regions can be queued in the issuing region while waiting for free sessions. Provided a file-owning region deals with requests in a responsive manner, and outstanding requests are removed from the queue at an acceptable rate, then all is well. But if a file-owning region is unresponsive, the queue can become so long and occupy so much storage that the performance of connected application-owning regions is severely impaired. Further, the impaired performance of the application-owning region can spread to other regions. This condition is sometimes referred to as "sympathy sickness", although it should more properly be described simply as intersystem queuing, which, if not controlled, can lead to performance degradation across more than one region.