How do you implement read/write splitting for database scaling?

Read/write splitting directs writes to primary and reads to replicas. Implementation: application-level routing, proxy (ProxySQL, PgBouncer), framework support. Considerations: replication lag (writes may not be immediately readable), session consistency (route user's reads to same replica or primary after write), query classification. Patterns: read-your-writes (read from primary after write), monotonic reads (sticky sessions). Benefits: scale reads independently, reduce primary load. Common for read-heavy workloads.