IAM roles.
Said less snarky, it should be trivial to define and restrict the dependencies of services (Although there are many ways to do that). If its not trivial, that's a different problem.
Since you called the problem “trivial,” we can now all depend on you to resolve these problems for us at little cost, correct?
I don't mean that. I mean that eventually the business is going to need some feature that requires breaking the acyclic rule.