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, +∞).

1. set_with_ttl(key, field, value, timestamp, ttl) -> void

• Store (value, timestamp + ttl).

1. get(key, field, timestamp) -> value | null

• Return null if missing or expired at timestamp.

1. delete(key, field, timestamp) -> bool

• Delete the field (regardless of TTL).
• Return false if (key, field) does not exist; otherwise true.

1. scan(key, timestamp) -> list<(field, value)>

• Return all non-expired fields for key at timestamp.
• Sort results by field ascending.

1. scan_with_prefix(key, prefix, timestamp) -> list<(field, value)>

• Like scan, but only include fields that start with prefix.

1. backup(timestamp) -> void

• Persist a full snapshot of the DB state at timestamp.

1. 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.