Openai·Software Engineer·Technical Phone Screen·Senior

Senior

May 2026

Summary

OpenAI SWE technical screen with a distributed systems coding problem. The question was genuinely interesting but also kind of brutal if you've never thought about async message passing as a first-class constraint.

Questions Asked (1)

You have a directed tree where each node is an independent machine. You can't access the tree structure directly. Using only an async sendMessage/receiveMessage API, design a protocol so that the root node can count the total number of nodes in the tree.

System DesignAlgorithms & Data StructuresTechnical Trade-offs

Author's notes

This one messed with me more than I expected.

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AI HintsAI Generated

Suggested Approach

Design a distributed aggregation protocol where each node, upon receiving a request, asynchronously queries all its children, waits for their responses, and replies to its parent with the sum of child counts plus one. Emphasize the use of unique request IDs to match responses and handle asynchronous communication without blocking.

Pro tip: Mention that the protocol should be robust to node failures or delays by incorporating timeouts and retries, and that the root can initiate the process by sending a message to itself or by having a designated start signal.

1. Define message types and protocol flow

Specify the message formats for request and response, including fields like request ID, sender ID, and count. Describe how a node processes a request: it forwards the request to all children and waits for their responses.

2. Handle asynchronous aggregation

Explain how a node tracks outstanding responses from its children, using a counter or promise-based approach, and how it aggregates the counts once all responses are received.

3. Ensure correct termination and response

Detail how a node knows when it has received all child responses (e.g., by knowing its number of children or using a timeout) and then sends the total count to its parent.

4. Address edge cases and robustness

Discuss handling of leaf nodes (immediate response with count 1), node failures (timeouts, retries), and duplicate messages (idempotency via request IDs).

5. Analyze complexity and trade-offs

Mention the time complexity (proportional to tree height) and message complexity (2 messages per edge), and discuss trade-offs between latency and reliability.

Key Points to Mention

Asynchronous message passing with request/response correlation using unique IDs
Tree traversal via recursive aggregation (each node sums child counts + 1)
Handling of leaf nodes and termination condition
Robustness: timeouts, retries, and idempotency to handle failures
Complexity analysis: O(n) messages, O(height) time
Potential optimizations: batching, pipelining, or using a spanning tree if not already a tree

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