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

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Apr 2026

Summary

Meta system design round focused entirely on a versioned key-value store. Pretty deep rabbit hole once the rollback semantics came up, and the conversation about memory tradeoffs went longer than I expected.

Questions Asked (1)

Design an in-memory key-value store that supports get, put, and rollback to any previous state of the entire store. Walk through snapshot creation, restore mechanics, and how you'd handle per-key version history.

System DesignAlgorithms & Data StructuresTechnical Trade-offs

Author's notes

Started with the obvious stuff, a hash map with a snapshot list, but they pushed hard on the tradeoff between storing full copies versus diff logs.

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Suggested Approach

Start by clarifying requirements (e.g., snapshot frequency, memory constraints, concurrency) and then propose a design that balances simplicity and efficiency. Use a versioned key-value store with copy-on-write snapshots and a global version counter to enable rollback to any previous state. Walk through the data structures, snapshot creation, restore mechanics, and per-key version history, discussing trade-offs.

Pro tip: Emphasize that rollback is O(1) by keeping snapshots as immutable references, and use persistent data structures to avoid full copies. Mention that per-key version history can be pruned based on snapshot retention policies to manage memory.

1. Clarify Requirements and Constraints

Ask about expected workload (read/write ratio, snapshot frequency), memory limits, concurrency needs, and whether rollback is to a specific snapshot or arbitrary point. This shapes the design.

2. Design Core Data Structures

Propose a versioned key-value store where each key maps to a list of (version, value) pairs, and a global version counter increments on each put. Snapshots are represented by a version number and a persistent map of key to version at that time.

3. Explain Snapshot Creation

Describe how to create a snapshot efficiently: either copy-on-write (e.g., using a persistent hash map) or by recording the current global version and lazily capturing changes. Discuss O(1) snapshot creation vs. O(n) full copy.

4. Detail Restore Mechanics

Explain that rollback to a snapshot involves setting the current version to the snapshot's version and updating the key-value store to reflect the state at that version. With versioned keys, this is O(1) by just changing the current version pointer.

5. Discuss Per-Key Version History and Trade-offs

Cover how to store per-key versions (e.g., linked list or array), how to handle garbage collection of old versions, and trade-offs between memory usage and rollback granularity. Mention concurrency control if needed.

Key Points to Mention

Use of a global version counter and per-key version lists to enable rollback to any point.
Copy-on-write or persistent data structures (e.g., immutable AVL tree, hash array mapped trie) for efficient snapshots.
O(1) snapshot creation and rollback by storing only version references.
Garbage collection strategy for old versions based on snapshot retention.
Concurrency considerations: locking or MVCC to handle concurrent reads/writes during snapshots.
Trade-offs: memory overhead vs. rollback speed, and simplicity vs. performance.

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