CAP Choices Per Subsystem: Mixing CP and AP

Key Insight
In production systems, you rarely make one global CAP choice. Instead, you pick CP or AP per subsystem based on correctness requirements and latency targets.
SUBSYSTEMS THAT NEED CP
Payments and ordering: Double-charging or overselling costs money and trust. These services use CP with leader-based replication. During partitions, the system rejects orders rather than risk inconsistency. Config: 5 replicas, 3 acknowledgments required, 15-30ms write latency.
Inventory management: Selling the last item twice creates problems. Strong consistency prevents this, even if some orders are rejected during network issues.
SUBSYSTEMS THAT CAN USE AP
Product catalog: Showing a product that went out of stock 2 seconds ago is fine. AP semantics with 10-20ms reads. During partitions, all zones serve, clients may see slightly stale data.
Recommendations: User preferences lagging by seconds is invisible. Speed matters more than perfect freshness.
Tunable Consistency: Modern databases let you choose consistency per operation. Session storage might default to AP (5ms) but use CP for checkout (15ms). Tune based on data importance.
AVAILABILITY MATH
CP at 99.9% = 8 hours downtime per year. AP at 99.99% = 52 minutes per year. The 10x availability difference is the cost of strong consistency. Choose based on what your subsystem can tolerate.

💡 Key Takeaways

✓Production systems make CP versus AP choices per subsystem, not globally. Financial data uses CP, user content feeds use AP.

✓At 200k requests per second peak, even 10ms of added latency from CP consensus costs capacity. You pay for consistency with throughput.

✓Tunable consistency lets you choose per operation: eventually consistent reads at 5ms p99, strongly consistent reads at 15ms p99 with lower availability.

✓Systems like Netflix use AP stores for viewing history (tolerate seconds of lag) but CP for billing (cannot miss or duplicate charges).

📌 Interview Tips

1When designing a complex system, explicitly call out which subsystems are CP vs AP: "Payments need CP for consistency, product catalog can be AP for speed."

2Mention per-operation tuning: "Most reads use eventual consistency for speed, but checkout validation uses strong consistency to prevent overselling."

3Know the availability math: CP systems at 99.9% availability have 8 hours downtime per year. AP at 99.99% has 52 minutes. The trade-off is quantifiable.

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