[paulmd]

what I'm saying is, use the relational DB for OLTP, but export in a JSON document format to NoSQL in whatever document shapes are efficient for various services. And that can be multiple different shapes generated from the same set of relational "ground truth", if various services need different "views" to run efficiently.

The idea is you always have a relational "source of truth" and optimize that for OLTP, but also get the scalability benefits of documents/microservices/etc by having data already pre-coalesced/pre-digested into your correct format(s), so you're not doing complex analytical/window/aggregation queries on the RDBMS for every request. You run the analytical queries once, convert the result to json, and store that in the NoSQL.

Of course you still potentially have some "sync time" between the OLTP and the final commit to all the various nosql collections... unless you hold OLTP locks until everything is synced, which would be excessive. But this goes back to CAP and there's no magic wand for that - you can either put everything inside the RDBMS and take the performance hit, or you can have external nosql read replicas and accept the inconsistency due to the sync time, or you can hold locks until both systems are consistent at the cost of "availability" (updatability).