[throwdbaaway]

Based on the comment from pushrax, it looks like this is just circular async replication between the old writer and the new writer. For some reason, the old implementation had to send both read and write traffic to the old writer, while the new implementation can do proper read-write split, by reading from dedicated read replicas hanging off the new writer (again, via async replication).

Due to power law, ecommerce generally benefits a lot from things like caching and read-write split. Reading between the lines, it feels like shopify may not yet have sufficient experience in dealing with async replication, and all the potential issues caused by replication lag. Fun time ahead.