If a channel is capable of master-mode operations (as specified with the master keyword), such operations may be initiated either by a periodic service job or on demand as delivery is needed:
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immediate, which is the default, specifies that jobs should run on demand for messages of appropriate urgency. |
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periodic inhibits initiation of delivery jobs on demand for the channel it is associated with, regardless of priority. |
| What appropriate urgency means is controlled by the keywords: |
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immurgent enables immediate delivery processing on messages with a priority setting of urgent. Messages with a lower priority must wait for periodic processing. |
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immnormal enables immediate delivery for messages with normal or urgent priority (immnormal is the default keyword with immediate). |
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immnonurgent enables immediate delivery for urgent, normal, and nonurgent messages. |
Thus the default behavior (immediate immnormal) enables immediate processing for all but nonurgent or lower priority messages.
Delivery via periodic service jobs is always possible unless the channel is marked with the slave keyword. Channels capable of master-mode operation are periodically checked for pending messages by periodic service jobs. These jobs run at fixed intervals, usually every four hours, though you can change this interval. On UNIX systems, the interval is determined in the crontab entry for the post job.
Not all channels need service at the same intervals. For example, a channel might see little traffic and be expensive to service. Servicing such a channel at longer intervals than that of a single period between periodic jobs can lower the cost of operation without significantly affecting the quality of service.
In another case, one particular channel may see very heavy traffic and may require frequent service, while other channels need servicing much less often. In this situation it may be appropriate to service the heavily used channel more often than any other.
The period keyword can be used to control how often a channel is serviced. This keyword must be followed by an integer value N. The channel is then serviced by every Nth service job. The default value of the period keyword is 1, which means that every periodic service job checks the channel for pending messages.