Simple example: mint & balanceOf

AToken::balanceOf returns:

super.balanceOf(userAddress) * POOL.getReserveNormalisedIncome(_underlyingAsset)

AToken employs a unit increase model

  • deposit 1 DAI -> user receives 1 aDAI

  • interest is accumulated over time

  • user's aDAI balance increases, reflecting incrementing interest

    • 1 aDAI -> 1.5 aDAI

  • user redeems aTokens 1:1 with underlying

    • can redeem 1.5 DAI

On deposit/supply:

User's deposit is recorded as "scaledBalance":

scaledBalance=amountDepositedcurrentLiquidityIndexscaledBalance = \frac{amountDeposited}{currentLiquidityIndex}

scaledBalance is the actual value stored on-chain, within _userState[userAddress].balance.

All deposits are scaled against the liqudity index and stored as scaled values via _userState[userAddress].balance

Example

mint

The user is minted aTokens according to the scaledBalance, not the deposit value.

  • however, balanceOf is calculated by scaling up the scaledBalance against the incrementing liquidity index.

  • this is why, from the end-user's perspective it looks like they have minted aToken 1:1 to their deposits, and

  • their aToken balance continually increments over time - because the liquidity index increments

AToken balance

aTokenBalance = scaledBalance * currLiquidityIndex

  • LiquidityIndex only increments

  • An account's aTokenBalance can never go down, unless a withdrawal occurs

Simplified illustration:

super.balanceOf(userAddress) * POOL.getReserveNormalisedIncome(_underlyingAsset)

_userState[userAddress].balance * reserve.LiquidityIndex

scaledBalance * currLiquidityIndex

Links

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