Causal Read-Your-Own-Writes in Java Using Postgres LSN Tokens Instead of Primary Routing

The article argues that sending reads to the primary database right after a write is a costly shortcut that limits write capacity at scale. It recommends capturing the Postgres log sequence number (LSN) during the mutation transaction and returning it to the client, for example as an HTTP header. Subsequent reads carry this token, and a routing layer sends them to a replica only if the replica has replayed past that position. A fallback to the primary applies when replication lag exceeds a set threshold, such as 25ms. The author presents this as a lighter alternative to distributed locks or consensus protocols when read-your-own-writes consistency is the only requirement.