[karmakaze]

> If a rollback were needed, some features would have to be implemented in both the monolith and Billy. This was time-consuming, so moving fast to remove that code from the monolith and rely 100% on Billy was essential.

This is the hardest bit, where if the monolith is relying on a shared db transaction between the client and service the network boundary makes them separate. Even without explicit rollbacks, at any high scale/load there can be timeouts/failures leaving an inconsistent data state.

The article suggests migrating a less critical client first, then developing such consistency mechanisms before migrating the more critical clients.