Implement a thread-safe money transfer

Q: Implement a thread-safe money transfer

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.

Q: How do I approach Coding & Algorithms interview questions?

Coding & Algorithms questions require understanding of core concepts and practice. PracHub provides solutions with explanations to help you master coding & algorithms interviews.

Q: What difficulty level is this interview question?

This is a medium difficulty Coding & Algorithms question, commonly asked during Technical Screen rounds at Revolut.

Q: What role is this question designed for?

This question is commonly asked for Software Engineer candidates at Revolut during technical interviews.

Question

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.