Design a highly reliable account balance system
Company: Capital One
Role: Software Engineer
Category: System Design
Difficulty: medium
Interview Round: Onsite
## 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
Quick Answer: 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.
