Scaling
Know when to stop scaling vertically and when horizontal distribution is justified.
CAP
Use consistency for operations that cannot tolerate stale data, and availability where a stale response is still acceptable.
Locking
Be explicit about granularity, duration, and whether optimistic concurrency is enough.
Indexing and Protocols
Index around query patterns and choose the protocol that matches the communication shape.
Security and Monitoring
Say auth, authz, encryption, secrets, and observability without being prompted.
Final Shape
The clean interview answer is: explain the system in terms of tradeoffs, not technologies first.