I've worked on ETL integrations in the past where there's an API for service A that's read-only and an API for service B that's write-only, service A is provided by a third-party vendor (so you're not exactly going to get the vendor's code to alter or fork it) and politically the people who work on the integration are different from the people who are responsible for service B; the integrators neither have permission to make alterations to service B nor will they get service B engineers to take backlog items to integrate read functionality or otherwise integrate with the vendor for service A directly. The integrators simply need to build a completely separate integration that lives outside of service A and service B to ETL from service A into service B.

If service A is in one cluster, and service B is in another cluster, where billing is per-cluster, in which cluster do you put the ETL integration? It belongs to either both or neither, depending on how Finance wants to bill it.