Three keywords are used to specify whether a channel is served by a
master program (
master), a slave program
slave), or both (
default, if none of these keywords is specified, is
bidirectional. These keywords determine whether PMDF
bothers to initiate delivery activity when a message is queued to the
channel --- there is no point in doing this on a slave-only channel.
The use of these keywords reflects certain fundamental characteristics of the corresponding channel program or programs. The descriptions of the various channels PMDF supports indicate when and where these keywords should be used.