Loan Contract

Zoe v0.24.0. Last updated August 25, 2022.

View the code on Github (Last updated: Nov 23, 2021)

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The basic loan contract has two parties, a lender and a borrower. It lets the borrower add collateral of a particular brand and get a loan of another brand. The collateral (also known as margin) must be a certain percentage of the loan value (the default is 150%). The exact percentage required is defined by the Maintenance Margin Requirement (mmr) in the terms of the contract.

The loan does not have a distinct end time. Rather, if the value of the collateral changes such that insufficient margin is provided, the collateral is liquidated, and the loan is closed. At any time, the borrower can add collateral or repay the loan with interest, closing the loan.

Note that all collateral must be of the same brand and all of the loaned amount and interest must be of the same (separate) brand.

Terms

• mmr (default = 150) - the Maintenance Margin Requirement, in percent. The default is 150, meaning that collateral should be worth at least 150% of the loan. If the value of the collateral drops below mmr, liquidation can occur.
• priceAuthority - used for getting the current value of collateral and setting liquidation triggers.
• autoswapInstance - The running contract instance for AMM installation. The publicFacet of the instance is used to make an invitation to sell the collateral on liquidation.
• periodNotifier - the notifier used for notifications that a period has passed, on which compound interest will be calculated using the interestRate.
• interestRate - the rate in basis points that will be multiplied with the debt on every period to compound interest.
• interestPeriod - the period at which interest will be compounded.

IssuerKeywordRecord

All keyword records use the following, regardless of their role in the contract:

• Keyword: Collateral - The issuer/payment for the digital assets to be escrowed as collateral.
• Keyword: Loan - The issuer/payment for the digital assets to be loaned out.

The Lender

The lender puts up the amount to be loaned to the borrower, but has no further actions. The loan is ongoing until it is paid back entirely or liquidated, at which point the lender receives a payout. This means the lender's payout will be in Loan-branded digital assets, not Collateral-brand. (The only exception is if the scheduling of liquidation triggers with the priceAuthority results in a error. In that case, we must give the collateral to the lender. The borrower has already exited with their loan.)

The lender will want the loan interest in addition to either their refund or the liquidation results. If the collateral price drops before liquidation, the total payout could be zero. Therefore, the lender cannot want anything in their proposal.

The lender must be able to exit on demand until borrowing occurs. If the exit rule was waived, a borrower could hold on to their invitation and effectively lock up the lender's escrowed loan amount forever.

const terms = {
  mmr: makeRatio(150n, loanBrand),
  autoswapInstance,
  priceAuthority,
  periodNotifier,
  interestRate: 5,
  interestPeriod: 5,
};

const { creatorInvitation: lendInvitation } = await E(zoe).startInstance(
  installation,
  issuerKeywordRecord,
  terms,
);

const maxLoan = AmountMath.make(loanBrand, 1000n);

const proposal = harden({
  give: { Loan: maxLoan },
});

const payments = harden({
  Loan: loanPayment,
});

const lenderSeatPromise = E(zoe).offer(lendInvitation, proposal, payments);

// E() can operate on a promise for an object. This enables promise pipelining.
const borrowInvitationPromise = E(lenderSeatPromise).getOfferResult();

The Borrower

The borrower receives an invitation to join, makes an offer with Zoe escrowing their collateral, and receives their loan. The collateral is moved to an internal seat called the collateralSeat, and the borrower seat is exited at this point so the borrower gets the payout of their loan. The borrower also gets an object (borrowFacet) as their offerResult that lets them continue interacting with the contract.

const borrowerProposal = harden({
  want: { Loan: maxLoan },
  give: { Collateral: allCollateralAmount },
});

const borrowerPayments = {
  Collateral: collateralPayment,
};
const borrowSeatPromise = E(zoe).offer(
  borrowInvitationPromise,
  borrowerProposal,
  borrowerPayments,
);

const borrowFacetPromise = E(borrowSeatPromise).getOfferResult();

Once the loan starts, the borrower can repay the loan in its entirety at any time (at which point the lender receives the loan amount back plus interest, and the contract closes), or add more collateral to prevent liquidation.

const closeLoanInvitationPromise = E(
  borrowFacetPromise,
).makeCloseLoanInvitation();

const addCollateralInvitationPromise = E(
  borrowFacetPromise,
).makeAddCollateralInvitation();

Contract Shutdown

The contract shuts down under any one of 3 conditions:

1. The loan (plus interest) is repaid.
   • The lender gets the repayment and the borrower gets their collateral back.
2. The value of the collateral drops and the collateral must be liquidated.
   • The lender gets the outcome of the collateral sale, and the borrower keeps their loan.
3. An error occurs when trying to use the priceAuthority.
   • The lender gets the collateral, and the borrower keeps their loan.

Debt and Interest Calculation

Interest is calculated and compounded when the periodNotifier pushes a new value. The interest rate per period is defined by the interestRate parameter.

The borrowFacet has methods to get the current amount owed (E(borrowFacet).getRecentCollateralAmount()), or get a notifier that will be updated when the debt is recalculated. The contract also reveals the last timestamp at which debt was recalculated: (E(borrowFacet).getLastCalculationTimestamp()).

Scheduling Liquidation

Liquidation is scheduled using the priceAuthority parameter. Specifically, the contract gets a promise resolved when the value of the collateral falls below a trigger value defined by the mmr parameter:

const internalLiquidationPromise = E(priceAuthority).quoteWhenLT(
  allCollateralAmount,
  liquidationTriggerValue,
);
internalLiquidationPromise.then(liquidate);

The borrower can self-forewarn about a potential liquidation by setting up their own margin calls. They do this by getting the priceAuthority from the terms and calling:

E(priceAuthority)
  .quoteWhenLT(allCollateralAmount, myWarningLevel)
  .then(priceQuote => doAddCollateral(priceQuote));

where myWarningLevel is the value of the collateral in the Loan brand at which they want a reminder to add collateral.

Liquidating

Actual liquidation is done through an AMM regardless of its current price. Even if the price is worse or better than what our priceAuthority quoted, we still liquidate.