Start by clarifying requirements and constraints, then propose a data model that uses a nested hash map (key -> field -> value) with auxiliary structures for TTL and versioning. Outline the core operations (get, set, delete, scan, history) and discuss how to handle expiration and historical lookups efficiently. Finally, dive into implementation details, trade-offs, and potential optimizations.
Pro tip: Demonstrate awareness of real-world constraints by discussing memory management and concurrency, and suggest using a min-heap or timing wheel for efficient TTL expiration rather than scanning all keys.
Ask about expected scale, read/write ratio, consistency requirements, and whether TTL is per key-field pair or per key. Clarify the granularity of historical lookups (e.g., versioned values or time-series).
Propose a nested map: outer map keyed by 'key', inner map keyed by 'field' storing value and metadata (TTL, version). For history, consider maintaining a list of (timestamp, value) per field or a separate versioned store.
Describe how to implement get, set (with conditional update), delete (with condition), scan by prefix, and historical lookup. Explain how TTL expiration is checked lazily or via background cleanup.
Discuss strategies for TTL: lazy expiration on access, active expiration using a min-heap or timing wheel, and how to handle expired entries during scans and history lookups.
Talk about memory overhead, concurrency control (e.g., sharding, locks), and potential improvements like using a trie for prefix scans or a time-series database for history.
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