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Xai·Software Engineer·Technical Phone Screen·Senior

SeniorPrefer not to say
Jun 2026

Summary

xAI software engineering interview with a thread safety debugging question. Pretty focused technical session, one meaty problem and a lot of follow-up probing around the edges.

Questions Asked (1)

Q1

You have a BankAccount class used by multiple threads simultaneously. Two deposits are submitted concurrently via a thread pool. Given the deposit implementation reads the balance, sleeps briefly, then writes back, explain why the final balance can be wrong, identify the concurrency bug, propose a thread-safe fix, and discuss any trade-offs or alternative approaches.

System DesignTechnical Trade-offsRoot Cause Analysis
Author's notes

I spotted the race condition pretty fast, the read-modify-write gap with the sleep in between is basically a neon sign.

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AI HintsAI Generated

Suggested Approach

Start by explaining the race condition: two threads read the same initial balance, each adds their deposit, and the last write wins, losing the other deposit. Then propose a fix using synchronization (e.g., synchronized methods or locks) and discuss trade-offs like contention and alternatives such as atomic variables or optimistic locking.

Pro tip: Mention that the sleep in the deposit method widens the race window, making the bug more likely; also note that thread-safety must be considered for all methods that access shared state, not just deposit.

1. Explain the race condition

Describe how two threads interleave: both read the same balance, then both write back their own computed balance, causing one deposit to be lost.

2. Identify the concurrency bug

Point out the lack of atomicity in the read-modify-write sequence and the absence of synchronization, leading to a lost update.

3. Propose a thread-safe fix

Suggest using synchronization (e.g., synchronized keyword, ReentrantLock) to make the deposit operation atomic, or use atomic variables like AtomicLong with compareAndSet.

4. Discuss trade-offs

Compare approaches: synchronization is simple but can cause contention and reduce throughput; atomic variables are lock-free but may require retries; optimistic locking can improve concurrency but adds complexity.

5. Consider alternatives and broader implications

Mention alternative designs like using a concurrent data structure, immutable objects, or message passing; also note that thread-safety should be ensured for all shared state.

Key Points to Mention

  • Race condition and lost update
  • Read-modify-write non-atomicity
  • Synchronization (synchronized, locks)
  • Atomic variables (AtomicLong, compareAndSet)
  • Trade-offs: contention, throughput, complexity
  • Alternative approaches: optimistic locking, immutable objects, actor model

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