CAN YOU DESIGN A LEDGER?

Can You Design a Ledger?

10 Advanced Questions on Financial Ledger Architecture

1 / 10

You are reviewing a proposed ledger architecture. The team says, “We’ll choose PostgreSQL vs. NoSQL first and define the ledger semantics afterward.” What should you challenge first?

A. Whether the database supports horizontal scaling

B. Whether the database has a distributed cache

C. Whether the financial invariants, posting semantics, concurrency model, and failure boundaries have been explicitly defined

D. Whether the database is supported by the preferred cloud provider

2 / 10

A ledger entry has both a business-effective date and a database insertion timestamp. Why might the distinction matter?

A. Only the database timestamp is financially meaningful

B. Financial processing, reporting, and ordering may depend on different notions of time

C. It eliminates the need for transaction IDs

D. It guarantees distributed transactions are atomic

3 / 10

Your internal ledger shows $10,000 of settled payments, while an external settlement file shows $9,800. What should happen?

A. Change the ledger to match the external file

B. Change the external file to match the ledger

C. Treat the difference as a reconciliation exception and investigate the underlying transactions

D. Ignore the difference if the percentage is small

4 / 10

A balance projection becomes corrupted after a software deployment. What should the architecture ideally allow?

A. Manually edit the affected balances

B. Restore the entire production database

C. Rebuild the projection from the authoritative ledger

D. Ignore the corruption until the next reconciliation cycle

5 / 10

Your ledger contains 20 billion entries. The system needs to return an account’s current balance in milliseconds. Which architecture is strongest?

A. Scan all entries belonging to the account on every request

B. Store only the latest balance and discard historical entries

C. Preserve the ledger as the authoritative record and maintain a recoverable balance projection

D. Cache balances without retaining the underlying transaction history

6 / 10

A posted $1,000 ledger transaction is discovered to be incorrect. What is generally the safest correction model?

A. Update the original entries in place

B. Delete the original transaction and recreate it

C. Append compensating/reversal entries and, if necessary, a corrected transaction

D. Modify only the account balance

7 / 10

A client retries the same transfer after a timeout. The first request may already have committed, but the client doesn’t know. What is the strongest design?

A. Generate a new transaction ID for every attempt

B. Let both attempts execute and reconcile later

C. Use a business-level idempotency key whose successful result can be safely replayed

D. Deduplicate requests using the HTTP connection ID

8 / 10

Two withdrawals of $80 arrive concurrently against an account with $100 available. Both requests independently observe the $100 balance. What must the ledger architecture guarantee?

A. Both withdrawals should succeed and the balance can be repaired later

B. At least one withdrawal must fail or be prevented from consuming the same funds

C. Both should succeed if the requests have different IDs

D. The read model should eventually correct the balance

9 / 10

A transfer of $500 moves value from Account A to Account B. Which invariant must hold for the ledger transaction to be valid?

A. Both account balances must be updated within one second

B. Account A must have a lower balance after posting

C. The total debits and credits for the transaction must balance

D. Both accounts must belong to the same database partition

10 / 10

What should be the authoritative representation of a posted financial transaction?

A. The account's current balance

B. A mutable transaction status record

C. Immutable ledger entries that represent the financial event

D. A cached account summary

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