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Design a highly reliable account balance system

Capital OneSoftware Engineer · Onsite
MediumSoftware EngineerOnsiteSystem Design

Last updated

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