How do I approach System Design interview questions?

System Design questions require understanding of core concepts and practice. PracHub provides solutions with explanations to help you master system design interviews.

What difficulty level is this interview question?

This is a medium difficulty System Design question, commonly asked during Onsite rounds at Capital One.

What role is this question designed for?

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

Design a highly reliable account balance system | Capital One Interview Question

Quick Overview

This question evaluates a candidate's ability to architect a highly reliable transactional account-balance service, testing competencies in data modeling and storage choice, API design for idempotency, concurrency and consistency mechanisms, replication/failover, partitioning for scale, and auditability.

Scenario

You are designing a backend service that stores and updates user account balances (like a wallet/bank ledger). The system must be highly reliable and correct.

Requirements

Support operations:
- Deposit funds
- Withdraw funds (must reject if insufficient funds)
- Transfer funds between two accounts (atomic: both sides succeed or neither does)
- GetBalance(accountId)
Correctness is critical :
- No lost updates, no double-spend, no negative balance (unless explicitly allowed).
- Operations may be retried by clients; must handle idempotency .
High availability with clear consistency guarantees.
Handle high write throughput (many concurrent transactions).
Provide auditability: ability to explain how a balance was derived.

Deliverables

Describe:

Data model and storage choice
API design (including idempotency)
Concurrency/consistency approach
Replication/failover strategy
How you would scale (sharding/partitioning)
Key metrics and failure modes

Design a highly reliable account balance system

Quick Overview

Scenario

Requirements

Deliverables

Solution

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