Design a KV store with QPS API

Design a key-value (KV) store that supports basic read/write operations and also exposes an API to report QPS (queries per second).

Functional requirements

• Support:
  • Put(key, value)
  • Get(key) -> value | not_found
• Provide a QPS query API, e.g.:
  • GetQPS(windowSeconds) -> qps
  • (Clarify if QPS is system-wide , per-node , or per-key ; and whether it counts all requests or only Get / only successful requests.)

Requirements to clarify (ask the interviewer)

1. Scope of QPS : cluster-wide vs single instance; overall vs per-key.
2. Time window semantics : last N seconds rolling window (sliding) vs fixed wall-clock buckets.
3. Accuracy : exact vs approximate; acceptable error bounds.
4. Latency/SLA : expected p99 for Get / Put and for GetQPS .
5. Durability : must values survive restarts? required RPO/RTO.
6. Consistency : strong vs eventual for reads after writes.
7. Scale : key cardinality, value sizes, expected QPS, hot keys.
8. Multi-tenancy : separate QPS per tenant/app?

Non-functional requirements

• High availability and horizontal scalability (if cluster scope).
• QPS API should be fast and not significantly impact Get/Put performance.