Implement an in-memory key-field-value DB with TTL

In-Memory DB with TTL + Scan + Backup/Restore

Implement an in-memory database storing records by (key, field) -> value with optional TTL.

Data model / rules

• key and field are strings.
• value is a string (or integer; choose one and be consistent).
• Each stored (key, field) has an expiration time expire_at .
  • If a value is set without TTL, expire_at = +∞ .
  • A value is visible at query time t iff t < expire_at .

API

1. set(key, field, value, timestamp) -> void
   • Store (value, +∞) .
2. set_with_ttl(key, field, value, timestamp, ttl) -> void
   • Store (value, timestamp + ttl) .
3. get(key, field, timestamp) -> value | null
   • Return null if missing or expired at timestamp .
4. delete(key, field, timestamp) -> bool
   • Delete the field (regardless of TTL).
   • Return false if (key, field) does not exist; otherwise true .
5. scan(key, timestamp) -> list<(field, value)>
   • Return all non-expired fields for key at timestamp .
   • Sort results by field ascending.
6. scan_with_prefix(key, prefix, timestamp) -> list<(field, value)>
   • Like scan , but only include field s that start with prefix .
7. backup(timestamp) -> void
   • Persist a full snapshot of the DB state at timestamp .
8. restore(timestamp, restore_to_time) -> void
   • Restore DB state from the most recent backup with backup_time <= restore_to_time .
   • If no such backup exists, restore to an empty DB.

Notes

• Assume timestamps are integers.
• Assume up to ~1e5 operations; aim for efficient scans and correct TTL handling.