SparkLend Conduit

Github repo

Overview

The SparkLend Conduit is a conduit contract designed to be used within the Maker Allocation System. It implements the IAllocatorConduit interface, so it will be able to work within the constraints on the Allocation System design. There are two contracts in this repo:

1. SparkConduit: Facilitates the movement of funds between the Maker Allocation System and the SparkLend protocol.

2. DaiInterestRateStrategy: Calculates the interest rate that is to be paid by all borrowers of DAI through the SparkLend protocol.

In later iterations of this code's development, it is expected for other SparkLend Conduits to be developed to support multichain deployments.

Roles/Permissions

1. wards: The wards mapping tracks the administrative permissions on this contract. Admin can upgrade the contract and set key storage values.

2. operator: The operator performs actions on behalf of a given ilk (SubDAO identifier in the Maker Allocation System). The operator can deposit, withdraw, request funds, and cancel fund requests in the Conduit. Onboarding and offboarding of operator actors is done by Maker admin in the core system.

Admin Configuration

1. roles: The roles contract to perform operator authentication.

2. registry: Returns the buffer contract for a given ilk (source of funds).

3. subsidySpread: The delta between the Base Rate and the Subsidy Rate. [RAY]

Functionality

deposit

The deposit function is used to move funds from a given ilk's buffer into the Conduit. From the Conduit, the funds are used to supply in the SparkLend Pool. The result is that:

1. Funds are moved from the buffer to SparkLend's aToken for that asset.

2. aTokens are minted and sent to the Conduit.

3. The Conduit state to track the ilk's portion of the aTokens in the Conduit is increased.

withdraw

The withdraw function is used to withdraw funds from the SparkLend Pool into the Conduit. From the Conduit, the funds are sent to the ilk's buffer. The result is that:

1. Funds are moved from SparkLend's aToken for that asset to the buffer.

2. The Conduit's aTokens corresponding to the underlying asset withdrawn are burned.

3. The Conduit state to track the ilk's portion of the aTokens in the Conduit is reduced.

requestFunds

The requestFunds function is used to signal that a given ilk would like to withdraw funds from the Conduit. This is only possible when there is zero liquidity in SparkLend for the desired asset.

The result is that:

• The Conduit state to track the ilk's requested funds in the Conduit is increased. Importantly, totalRequestedShares[asset] is increased.

When recompute() is called in DaiInterestRateStrategy, it calls getInterestData on the SparkConduit. The targetDebt that is returned will be lower because it is totalShares - totalRequestedShares. This means that the debtRatio that is saved to storage will be greater than one, which means that the conditional logic to raise the interest rates (outlined in the DaiInterestRateStrategy section) is put into effect.

totalRequestedShares only reduces when an ilk either:

1. Cancels a withdrawal request.

2. Withdraws funds from the Conduit.

cancelRequest

The cancelRequest function is used to signal that a given ilk would like to cancel a request to withdraw funds from the Conduit. This is only possible when there is an active request to withdraw funds from the Conduit.

The result is that:

• The Conduit state to track the ilk's requested funds in the Conduit is decreased. Importantly, totalRequestedShares[asset] is decreased.

Invariants

$totalShares[asset] = \sum_{n=0}^{numIlks}{shares[asset][ilk]}$

$totalRequestedShares[asset] = \sum_{n=0}^{numIlks}{requestedShares[asset][ilk]}$

$getTotalDeposits(asset) = \sum_{n=0}^{numIlks}{getDeposits(asset, ilk)}$

$getTotalRequestedFunds(asset) = \sum_{n=0}^{numIlks}{getRequestedFunds(asset, ilk)}$

$totalRequestedShares[asset] = \sum_{n=0}^{numIlks}{requestedShares[asset][ilk]}$

$totalShares[asset] \le aToken.scaledBalanceOf(conduit)$

$getTotalDeposits(asset) \le aToken.balanceOf(conduit)$

NOTE: The last two invariants are not strict equalities because of the potential for a permissionless transfer of the aToken into the conduit. For this reason alone, they are expressed as inequalities.

Upgradeability

Since the SparkLend Conduit will likely require maintenance as its desired usage evolves, it will be an upgradeable contract, using upgradeable-proxy for upgradeable logic. This is a non-transparent proxy contract that gives upgrade rights to the PauseProxy.

DaiInterestRateStrategy

The DaiInterestRateStrategy contract is used to calculate the interest rate that is to be paid by all borrowers of DAI through the SparkLend protocol. It implements the IInterestRateStrategy interface, which is standard in SparkLend for all interest strategies. It is an auxiliary contract to SparkConduit that allows SubDAOs to influence interest rates if they require liquidity and it is not available.

To clarify interest rate-related naming in the contracts:

• subsidyRate (SparkConduit): Annualized dsr from Maker Core

• subsidySpread (SparkConduit): Spread set by Maker to make lending to decentralized collateral protocols advantageous. Borrow rate for DAI for the subDAOs.

• baseRate (SparkConduit): Base rate that is used for borrowing by all subDAOs.

• spread (DaiInterestRateStrategy): Spread above the subsidyRate that borrowers pay and SubDAOs earn.

The DaiInterestRateStrategy implements two important functions:

calculateInterestRates()

This function is called by SparkLend. The important distinction between this contract and the standard implementation is that there are two paths to determine interest:

debtRatio == 1

When debtRatio == 1, the interest rate used to charge to DAI borrowers is the baseRate. This value is determined by Maker core governance.

debtRatio > 1

When debtRatio > 1, the interest rate is dynamically calculated based on the following function:

$borrowRate = maxRate - \frac{maxRate - baseRate}{\frac{currentDebt}{targetDebt}}$

Below is an illustrative example of the above formula, with the following configuration:

1. maxRate = 75%

2. baseRate = 5%

3. currentDebt = 100

Each of the lines demonstrates a different scenario, where the amount of requested funds (and therefore the targetDebt is different). In the functions below, r is defined as the resulting interest rate, and a as the amount that has been returned after the original change in the target debt. The domains of each of these functions are limited from debtRatio > 1. It can be seen that the minimum rate returns back to the baseBorrowRate in all scenarios once all the requested liquidity has been repaid.

It is important to note that Maker will penalize SubDAOs that do not perform withdrawals after the funds are returned by users. This is to prevent gamification occurring where SubDAOs can profit by artificially requesting funds to spike interest rates. This results in a very bad UX for SparkLend borrowers, so it is the intention that this functionality be used very rarely, and when it is done that the SubDAOs are financially incentivized to withdraw the returned liquidity immediately.

Technical Assumptions

1. As with most MakerDAO contracts, non standard token implementations are assumed to not be supported. As examples, this includes tokens that:

   • Do not have a decimals field or have more than 18 decimals.

   • Do not revert and instead rely on a return value.

   • Implement fee on transfer.

   • Include rebasing logic.

   • Implement callbacks/hooks.

2. The penalization from Maker for not withdrawing funds after a fund request is sufficient to prevent gamification of the interest rate strategy.

Testing

To run the tests, do the following:

forge test