# Design a double-entry financial ledger: system design interview question

> A ledger must preserve accounting rules and account balances through concurrent transfers, repeated requests, and failures.

Source: https://sysround.com/questions/design-double-entry-financial-ledger
Updated: 2026-11-05


Difficulty: hard. Levels: Senior, Staff+.

## How the interviewer opens it

"Design a service that records financial transactions and returns account balances. It must preserve correct balances when requests overlap, clients retry, or servers fail."

## What to clarify first

- Ledger operations
- Accounts and deployment
- Traffic and read:write mix
- Balance and account rules
- Latency
- Consistency and availability
- Repeated requests
- Durability and history
- Identity and authorization
- Exclusions

## Scope of a strong answer

Core requirements:

- Record internal transfers with equal debit and credit totals.
- Return current balances and account history.
- Prevent repeated financial effects for the same request and prevent customer overdrafts.
- Record economic corrections as new balanced transfers.

Usually out of scope:

- External payment systems
- Currency exchange
- Pending funds and credit products
- Active writes across multiple regions
- User identity services
- General accounting reports

## What interviewers probe

### Concurrent transfers and partial failures

Separate transfers can spend the same balance. A server failure must never expose only part of a transfer.

### Repeated requests

Retries must preserve successful and rejected results. Later changes to funds or account status must not change those results.

### Balance discrepancies and history

A balance error can affect the derived value or the economic record. Each case requires a different correction.

### Busy accounts and service failures

One shared account can serialize transfers. Failover must preserve financial effects and every terminal request result.

## Common mistakes

- Using floating-point amounts or checking funds outside the transaction that changes the balance.
- Retaining successful request results but discarding terminal business rejections.
- Checking mutable business rules before returning a stored request result.
- Treating a timeout as failure or promising duplicate prevention across different request keys.
- Changing committed entries to correct an economic error.
- Adding database shards without defining how a transfer commits across accounts.

## The bar by level

- **Senior:** Defines lock order, concurrent request behavior, and recovery from unknown commit results. Returns stored results before mutable business checks, subject to current authorization. Distinguishes derived balance repair from economic correction. Defines consistent history and safe failover.
- **Staff+:** Quantifies shared-account contention and defines overload behavior. Proves failover preserves financial effects and terminal request results. Plans archive verification and seven-year request retention. Explains coordination costs and contract changes when transfers span databases.

Practice this question with an AI interviewer: https://sysround.com/practice/new?problem=financial-ledger


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SysRound (https://sysround.com) is an AI practice tool for system design interviews. It has a voice or typed AI interviewer and a shared whiteboard. At the end, you get feedback on each skill and a model answer. The AI feedback can be wrong.