Traditional major version upgrades in PostgreSQL force a painful trade-off: schedule a maintenance window with extended downtime, or rely on pg_upgrade link mode which still requires shutting down both server instances.
Using logical replication changes this dynamic by converting hours of downtime into a sub-200ms write stall. The primary server publishes transactions to the new target version while production traffic continues uninterrupted.
The critical engineering challenge is managing failback without data loss. By establishing a reverse publication from the target back to the source before the cutover, you ensure that transactions can flow backwards if validation fails. Using marker rows and comparing Log Sequence Numbers allows you to verify zero replication lag before redirecting connection pool traffic.
This turns a terrifying all-or-nothing database migration into a controlled, reversible deployment.









