Choose strong consistency when:
- Financial transactions (double-spending is unacceptable)
- Inventory management (overselling hurts business)
- User authentication (security-critical)
- Anything with regulatory requirements
Choose eventual consistency when:
- Social feeds (stale content is acceptable)
- Analytics and metrics (slight delays are fine)
- Caches (stale data is better than no data)
- Any read-heavy workload where staleness is tolerable
Ask: "What's the cost of showing stale data?" If the answer is "annoyed users," eventual consistency is fine. If it's "lost money" or "security breach," choose strong.