Consistency & CAP Study Portal

Key takeaways

• Replicas disagree during replication lag; consistency models define what readers may see then.
• CAP is a choice during partitions only; PACELC adds the everyday latency-versus-consistency cost.
• Linearizable for locks, uniqueness and money; causal or session guarantees for most user-facing data; eventual for counters and caches.
• Async failover loses acknowledged writes; fencing tokens prevent split-brain.
• Last-writer-wins silently drops writes; use version vectors, CRDTs or domain merges where it matters.
• Multi-region: single write region, geo-partitioned home regions, or active-active — chosen by what must be strongly consistent.