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Perplexity·Software Engineer·Onsite - System Design / Architecture·Senior

SeniorPrefer not to say
Jul 2026

Summary

Perplexity system design round for a software engineering role. The core problem was building a timestamped key-value store from scratch with time-travel and rollback, which sounds manageable until you actually have to think through all the edge cases live.

Questions Asked (1)

Q1

Design and implement an in-memory key-value store where every operation carries a monotonically increasing timestamp. It must support set, get, delete, a range query over a time interval, and a full-store rollback to any past timestamp. Discuss the data structures you'd use and any assumptions about ties at the same timestamp.

System DesignAlgorithms & Data StructuresTechnical Trade-offs
Author's notes

This is a meaty one.

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AI HintsAI Generated

Suggested Approach

Start by clarifying requirements and assumptions, especially around timestamp ties and rollback semantics. Then propose a design using a versioned map with per-key sorted structures (e.g., balanced BST or skip list) and a global version index for rollback. Discuss trade-offs between memory and time complexity, and mention how to handle range queries efficiently.

Pro tip: Demonstrate awareness of real-world constraints by discussing memory reclamation and concurrency, and explicitly state your tie-breaking rule (e.g., last-write-wins with operation sequence number) to show attention to detail.

1. Clarify requirements and assumptions

Ask about timestamp uniqueness, tie-breaking rules, rollback semantics (does it affect subsequent operations?), and expected read/write patterns. State your assumptions clearly.

2. Design core data structures

Propose a versioned key-value store where each key maps to a sorted list of (timestamp, value) pairs, and maintain a global sorted index of all versions for rollback. Consider using a balanced BST or skip list for efficient range queries.

3. Define operations and algorithms

Explain how set, get, delete, range query, and rollback work with your structures. For get, binary search for the latest version <= timestamp. For rollback, truncate all versions after the target timestamp.

4. Analyze complexity and trade-offs

Discuss time and space complexity for each operation, and compare alternatives (e.g., LSM trees, copy-on-write). Mention how tie-breaking affects correctness.

5. Address scalability and edge cases

Talk about memory management, concurrency control, and persistence if needed. Handle edge cases like rollback to a timestamp with no operations or concurrent writes.

Key Points to Mention

  • Use of per-key version lists with binary search for point queries
  • Global version index (e.g., balanced BST) for efficient rollback and range queries
  • Tie-breaking rule: e.g., last-write-wins with operation sequence number
  • Rollback semantics: truncating versions after target timestamp, and whether it's destructive
  • Time complexity: O(log n) for get/set/delete, O(log n + k) for range query, O(m) for rollback where m is number of versions removed
  • Space complexity: O(total number of versions) and potential need for compaction

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.