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Implement a thread-safe money transfer

RevolutSoftware Engineer · Technical Screen
MediumSoftware EngineerTechnical ScreenCoding & Algorithms

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This question evaluates a candidate's mastery of concurrent programming and data-integrity principles, focusing on thread safety, atomic state updates, and deadlock avoidance when transferring funds between shared accounts.

Problem: Implement a concurrent-safe transfer function

You are given an in-memory store of bank accounts:

  • Each account has an accountId and a balance (integer cents).
  • Multiple threads may call transfer(fromId, toId, amount) concurrently.

Task

Implement transfer(fromId, toId, amount) such that it is:

  • Correct under concurrency (no lost updates).
  • Atomic: either the money moves from fromId to toId entirely, or nothing changes.
  • Deadlock-free under concurrent transfers between overlapping accounts.

Rules

  • Validate inputs:
  • amount > 0
  • accounts must exist
  • reject/return failure on insufficient funds
  • The total sum of money across all accounts must remain unchanged after any successful transfer.

Assumptions

  • Single process / single JVM (no distributed transactions).
  • You may use locks (e.g., synchronized/ReentrantLock/ReadWriteLock) or other concurrency primitives.

Deliverable

Provide the API design and implementation approach, including how you prevent deadlocks and ensure correctness.