DUdustingetz·Sep 22, 2017
ACID's C is about the write side: "ACID is a set of properties of database transactions" (wikipedia) which is different than CAP's C: https://en.wikipedia.org/wiki/Consistency_(database_systems)...
Postgresql with read replicas is ACID-Consistent transactions because single writer, and CAP-AP eventually consistent because distributed reads.
It is easy to agree with you here: "PostgreSQL basically stops working well too, once you start mixing transaction processing and business reporting and BI in the same box at scale." It's worse than that - the app has to be coded before it can be scaled so you get fucked by complexity-scale via ORM a lot sooner than bigdata-scale. But as you sort of say, the point of a data warehouse is to be cap-AP (eventually consistent) through read-replicas without breaking ACID-consistent transactions for the syst
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em as a whole.
as to the argument "everything is bad at scale" i think that has merit too, I just gave a talk at ClojureNYC about building real apps with Postgres (tables) vs Neo4J (graph) vs Mongo (documents) vs Datomic (eavt): https://github.com/hyperfiddle/hypercrud.browser/issues/4 (Did I say real "apps"? I meant "balls of mud")