Design a Key-Value Store with Expiration
Last updated: February 21, 2025
Quick Overview
Implement a key-value store that supports get, put, and automatic expiration of entries after a specified time-to-live (TTL).
Rivian
February 21, 202514
5
1,825 solved
Implement a key-value store that supports get, put, and automatic expiration of entries after a specified time-to-live (TTL).
TTL-based caching is used extensively in Rivian's vehicle software for session tokens, cached API responses, and temporary configuration overrides. This tests your ability to design a practical data structure that handles time-based operations efficiently.
What the Interviewer Expects
- Implement get and put with TTL support using a hash map and expiration tracking
- Handle lazy expiration on access and periodic cleanup
- Achieve O(1) average time for get and put operations
- Handle edge cases like overwriting existing keys with new TTLs
- Discuss the tradeoffs between lazy and eager expiration strategies
Key Topics to Cover
How to Approach This
- Clarify input constraints and edge cases before writing code.
- Walk through your approach verbally and confirm with the interviewer before coding.
- Start with a brute force solution, then optimize. Mention time and space complexity.
- Test your solution with examples, including edge cases like empty input or duplicates.
- Consider common patterns: sliding window, two pointers, hash map, BFS/DFS, dynamic programming.
Possible Follow-up Questions
- How would you implement periodic cleanup without blocking read/write operations?
- What if you needed to support sliding TTL that resets on each access?
- How would you make this distributed across multiple nodes?
- How does this compare to Redis's expiration implementation?
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Practice DSA ProblemsSample Answer
Data Structure Design
Use a hash map where each value is stored alongside its expiration timestamp (current_time + TTL). A secondary data structure (sorted set or min-heap)...
Expiration Strategies
Two strategies work together. Lazy expiration: on every get, check if the entry is expired. If so, delete it and return not found. This is O(1) but me...
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