Janus is a read/write split proxy for MySQL/ActiveRecord 7.1+. It works by sending any read queries to a replica, and any write queries to the primary database server. If there’s been a write statement within the same thread we stick any future queries to the primary server unless the developer specifically chooses not to.

Other than that Janus should be pretty hands off. We deployed on our infrastructure (migrating from Makara) on the 2024-04-09 and saw no effect on our metrics.

https://tech.olioex.com/ruby/2024/04/16/introducing-janus.html