Consider a service that generates postfix lookup tables with
postmap(1), like Mailman. It needs the Postfix configuration file to
exist, but Postfix qmgr needs all the lookup tables its configured
with to exist before it starts. So the service that runs postmap
needs to run after the Postfix configuration and directory structure
is generated, but before Postfix itself is started. To enable this,
we split Postfix into two units: a oneshot unit that sets up the
configuration, and a longrun unit that supervises the Postfix
daemons. The postmap services can then be inserted in between these
two units.
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