A service (say bar.service) is dependent on another service (say foo.service), like below
bar's service file:
[Unit]
After=foo.service
Requires=foo.service
...
If foo.service is restarted (either manually or due to a bug), how can bar.service be automatically restarted as well?
I think the needed option is BindsTo, it handles the misbehaviours too.
[Unit]
Requires=postgresql.service
After=postgresql.service
BindsTo=postgresql.service
BindsTo=
Configures requirement dependencies, very similar in style to Requires=. However, this dependency type is stronger: in addition to the effect of Requires= it declares that if the unit bound to is stopped, this unit will be stopped too. This means a unit bound to another unit that suddenly enters inactive state will be stopped too. Units can suddenly, unexpectedly enter inactive state for different reasons: the main process of a service unit might terminate on its own choice, the backing device of a device unit might be unplugged or the mount point of a mount unit might be unmounted without involvement of the system and service manager.
When used in conjunction with After= on the same unit the behaviour of BindsTo= is even stronger. In this case, the unit bound to strictly has to be in active state for this unit to also be in active state. This not only means a unit bound to another unit that suddenly enters inactive state, but also one that is bound to another unit that gets skipped due to a failed condition check (such as ConditionPathExists=, ConditionPathIsSymbolicLink=, … — see below) will be stopped, should it be running. Hence, in many cases it is best to combine BindsTo= with After=.