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.

What difficulty level is this interview question?

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

What role is this question designed for?

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

Implement a thread-safe money transfer | Revolut Interview Question

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.

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.

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.

Implement a thread-safe money transfer

Quick Overview

Problem: Implement a concurrent-safe transfer function

Task

Rules

Assumptions

Deliverable

Comments (0)

Implement a thread-safe money transfer

Quick Overview

Problem: Implement a concurrent-safe transfer function

Task

Rules

Assumptions

Deliverable

Comments (0)