Design a highly reliable account balance system

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