Intermediate · 30 min · 4 steps
A reward coin ledger that never goes below zero
Meera Nambiar, Financial Ledger Engineer at PerkPoint
Meera Nambiar is an engineer who builds money records at PerkPoint, an app that gives Indian credit card holders reward coins for paying their bills on time. It is 11:30 AM on a festive Monday. PerkPoint has just started its big 'Super-Spin Cashback' offer: users can swap reward coins for electronic gift cards.
The question
How do we build a coin ledger where records are never changed, so users cannot redeem twice and no balance goes below zero?
- Double-entry bookkeeping
- Immutable ledgers
- Event sourcing
- Concurrency invariants
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