Cheap reads — at the cost of staleness.
Read Replicas and Replica Lag: cheap reads
shape data so reads and writes stay honest
More replicas → more read capacity, more lag risk.
Read replicas multiply read capacity by streaming the primary's writes to many readers. They lag — usually milliseconds, sometimes much more under load — which is the operational cost.
Lag spikes: long writes, network blips, vacuum churn.
Read-your-writes: route the user's read to the primary or a replica that's caught up.
Don't run reports on the primary; that's why replicas exist.
App suddenly slow for reads.