CToken: Exchange Rate and Interest Accrual

Key takeaway: Compound V2 accrues interest in O(1) per market via a global borrowIndex plus a per-account snapshot of the index when the user borrowed. To compute a borrower's current debt, the protocol reads their snapshot, reads the current index, and divides; no iteration over borrowers is ever needed. This is what lets Compound V2 scale to hundreds of thousands of borrowers without gas-griefing on every accrual call. The snapshot pattern repeats in any protocol that needs fair interest distribution at scale.

What You Are Building

This section adds the two most fundamental functions in the CToken: exchangeRateStoredInternal() and accrueInterest(). Every other operation in the protocol (mint, redeem, borrow, repay, liquidate) depends on these being correct. They are the heartbeat of the lending market.

You will also build borrowBalanceStoredInternal(), which uses the BorrowSnapshot pattern to compute a borrower's current debt, and getAccountSnapshot(), which the Comptroller uses for liquidity calculations.