Q.Design a ledger that can reconcile against a bank feed with a two-day lag, never double-posts, and survives a partial outage.

Reported by 4 candidates across Ramp's payments loop in the last 60 days. It's the question they use to find out whether you've actually shipped money-movement code, or just read about it.

What Coach listens for

Idempotency model
They're watching for append-only ledgers keyed by idempotency tokens — not whether you can spell ACID.
Reconciliation window
Most candidates forget the 2-day lag clause. Coach flags it in the first minute if you do too.
Partial-outage failure modes
What happens when the bank feed is late but the user's already clicked 'send'? Walk through the state machine.
Retroactive corrections
Banks send corrections. The interviewer pushes here around minute 20. Have an answer.

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