Ways of approaching the problem and recommendations

The queue limit mechanism can be used to control the number of tasks waiting for the use of an intersystems link.

You should use the control to ensure that even at its maximum length the queue does not use too many of the MXT slots in the system. You can also use the MAXACTIVE setting of a TRANCLASS definition if you can separate your transactions into classes that correspond to the remote regions they require.

To ensure free availability during normal running, provide a sufficient number of intersystems sessions. Session definitions do not occupy excessive storage, and the occupancy of transaction storage probably outweighs the extra storage for the session. The number of sessions should correspond to the peak number of transactions in the system which are likely to use the connection—you can see the maximum number of sessions being used from the terminal statistics for the connection. If all sessions are used, the connections statistics show the number of times allocates were queued compared with the total number of requests.

Even in a system that has no problems, there are significant variations in the numbers of transactions that are active at any time, and the actual peak number might be larger than the average over a few minutes at the peak time for your system. You should use the average rather than the actual peak; the queuing mechanism is intended to cope with short-term variations, and the existence of a queue for a short time is not a cause for concern.

The start of a queue is used by the queue limiting mechanism as a signal to start monitoring the response rate of the connection. If queues never form until there is a large problem, the detection mechanism is insensitive. If there are always queues in the system, you might experience false diagnosis.

You should set the queue limit to a number that is roughly the same size as the number of sessions—within the limits imposed by MXT if there are many connections whose cumulative queue capacity would reach MXT. In this latter case, design your own method—using ZXIQUE—of controlling queue lengths so that the allocation of queue slots to connections is more dynamic.

The MAXQTIME parameter can be set to reflect the maximum wait time expected of users for responses in case of potential problems. The MAXQTIME parameter should not be set at a low value in combination with a queue limit that is low, because this leads to a sensitive detection criterion.