Control different relation sequence

[Mike Sam]

Imagine a unit needs to be added to an existing service like service A.
Service A is already in relations with other services like Service B and
Service C on different "requires".

For the new unit on Service A to work, it needs to first process the
relation_joined and relation_changed with the units of service B before it
could process  relation_joined and relation_changed with the units of
service C.

Is there a way to enforce such desired sequence relationship establishment
at the charm level? In other words, I do not think we can control the hook
execution sequence of different relationships officially but then I am
wondering how can we do a situation like above nicely?

Thanks,
Mike
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