Anthropic·Software Engineer·Technical Phone Screen·Senior

Senior

May 2026

Summary

Coding round for a Software Engineer role at Anthropic. The problem was an in-memory key-value store with nested transaction support, which sounds manageable until you actually think through rollback semantics across multiple transaction layers.

Questions Asked (1)

Implement an in-memory key-value store supporting SET, GET, DELETE, and COUNT operations, plus nested transactions with COMMIT and ROLLBACK semantics.

Algorithms & Data StructuresSystem DesignTechnical Trade-offs

Author's notes

The basic CRUD part is trivial.

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AI HintsAI Generated

Suggested Approach

Start by clarifying requirements and constraints, then design a data structure that supports nested transactions efficiently. Use a stack of transaction layers, each recording changes for rollback, and discuss trade-offs between memory and performance.

Pro tip: Mention that you would use a stack of transaction layers where each layer records the previous value of modified keys, enabling O(1) rollback per operation. This shows you understand both correctness and efficiency.

1. Clarify Requirements and Constraints

Ask about expected data size, concurrency, persistence, and whether transactions can be nested arbitrarily. Confirm that operations outside transactions should work as usual.

2. Design Core Data Structure

Propose a hash map for the main store and a stack for transaction layers. Each layer records changes (key, old value, existence) to enable rollback.

3. Implement Operations with Transaction Awareness

For SET, GET, DELETE, and COUNT, check if inside a transaction and log changes accordingly. For COMMIT, merge the top layer into the parent or main store; for ROLLBACK, revert changes using the log.

4. Analyze Complexity and Trade-offs

Discuss time and space complexity: O(1) for basic operations, O(k) for commit/rollback where k is number of changes in the layer. Mention alternative approaches like copy-on-write or persistent data structures.

5. Test with Edge Cases

Walk through scenarios: nested transactions, rollback after commit, operations on non-existent keys, and COUNT inside transactions. Ensure correctness.

Key Points to Mention

Use a stack of transaction layers to support nesting.
Each layer logs old values for rollback (undo log).
COMMIT merges changes into parent; ROLLBACK discards layer and reverts.
COUNT must reflect uncommitted changes within the current transaction.
Time complexity: O(1) for SET/GET/DELETE, O(k) for COMMIT/ROLLBACK.
Space complexity: O(total changes) across all active transactions.

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