Flash Loans

Flash Loans are special uncollateralised loans that allow the borrowing of an asset, as long as the borrowed amount (and a fee) is returned before the end of the transaction. There is no real world analogy to Flash Loans, so it requires some basic understanding of how state is managed within blocks in blockchains.

Flash Loans are an advanced concept aimed at developers. You must have a good understanding of Ethereum, programming, and smart contracts to take advantage of them.

Overview

For developers, a helpful mental model to consider when developing your solution:

  1. Your contract calls the LendingPool contract, requesting a Flash Loan of a certain amounts of reserves using flashLoan().

  2. After some sanity checks, the LendingPool transfers the requested amounts of the reserves to your contract, then calls executeOperation() on your contract (or another contract that you specify as the _receiver).

  3. Your contract, now holding the flash loaned amounts, executes any arbitrary operation in its code.

    • If you are performing a 'traditional' flash loan, then when your code has finished, you transfer the flash loaned amounts of reserves back to the LendingPool.

      • The LendingPool contract updates the relevant details of the reserves and pulls the flash loaned amount + fee.

        • This is different from v1 flash loans, where the flash loaned amount needed to be pushed back to the LendingPool contract.

      • If the amount owing is not available (due to a lack of balance or approval), then the transaction is reverted.

    • If you are performing a flash loan to incur debt (see the mode parameter in the flashLoan() function), then a debt will be incurred.

  4. All of the above happens in 1 transaction (hence in a single ethereum block).

Applications of Flash Loans

Aave Flash Loans are already used extensively with Aave v2 for swapping and/or migrating positions. Other examples in the wild include:

  • Arbitrage between assets, without needing to have the principal amount to execute the arbitrage. Example: ArbitrageDAO.

  • Swapping collateral of loan positions, without having to repay the debt of the loan positions. Example: Collateral Swap, DeFiSaver.

  • Other examples and ideas are listed on our blog here and here.

Example Code

The examples listed below are code examples from the community or from the Genesis team, which can be used as a starter or a guide for your own project.

Flash loans example (as detailed in the Step by step)

Flash loans using Scaffold-ETH by David K

Flash loans using Remix by Fiona K

Step by step

TL;DR: A reference example flash loan contract can be found on Kovan etherscan and Github code examples repo.

1. Setting up

Your contract that receives the flash loaned amounts must conform to the IFlashLoanReceiver interface by implementing the relevant executeOperation() function. In the example below, we inherit from FlashLoanReceiverBase, which conforms to the IFlashLoanReceiver.

Also note that since the owed amounts will be pulled from your contract, your contract must give allowance to the LendingPool to pull those funds to pay back the flash loan debts + premiums.

pragma solidity 0.6.12;
import { FlashLoanReceiverBase } from "./FlashLoanReceiverBase.sol";
import { ILendingPool } from "./ILendingPool.sol";
import { ILendingPoolAddressesProvider } from "./ILendingPoolAddressesProvider.sol";
import { IERC20 } from "./IERC20.sol";
/**
!!!
Never keep funds permanently on your FlashLoanReceiverBase contract as they could be
exposed to a 'griefing' attack, where the stored funds are used by an attacker.
!!!
*/
contract MyV2FlashLoan is FlashLoanReceiverBase {
/**
This function is called after your contract has received the flash loaned amount
*/
function executeOperation(
address[] calldata assets,
uint256[] calldata amounts,
uint256[] calldata premiums,
address initiator,
bytes calldata params
)
external
override
returns (bool)
{
//
// This contract now has the funds requested.
// Your logic goes here.
//
// At the end of your logic above, this contract owes
// the flashloaned amounts + premiums.
// Therefore ensure your contract has enough to repay
// these amounts.
// Approve the LendingPool contract allowance to *pull* the owed amount
for (uint i = 0; i < assets.length; i++) {
uint amountOwing = amounts[i].add(premiums[i]);
IERC20(assets[i]).approve(address(LENDING_POOL), amountOwing);
}
return true;
}
}

2. Calling flashLoan()

To call flashloan() on the LendingPool, we need to pass in the relevant parameters. There are 3 ways you can do this.

From an EOA ('normal' ethereum account)

To use an EOA, send a transaction to the relevant LendingPool calling the flashLoan() function. See the flashLoan() function documentation for parameter details, ensuring you use your contract address from step 1 for the receiverAddress.

From a different contract

Similar to sending a transaction from an EOA as above, ensure the receiverAddress is your contract address from step 1.

From the same contract

If you want to use the same contract as in step 1, use address(this) for the receiverAddress parameter in the flashLoan function.

The example below shows this third case, where the executeOperation() is in the same contract calling flashLoan() on the LendingPool.

Never keep funds permanently on your FlashLoanReceiverBase contract as they could be exposed to a 'griefing' attack, where the stored funds are used by an attacker.

pragma solidity 0.6.12;
import { FlashLoanReceiverBase } from "./FlashLoanReceiverBase.sol";
import { ILendingPool } from "./ILendingPool.sol";
import { ILendingPoolAddressesProvider } from "./ILendingPoolAddressesProvider.sol";
import { IERC20 } from "./IERC20.sol";
/**
!!!
Never keep funds permanently on your FlashLoanReceiverBase contract as they could be
exposed to a 'griefing' attack, where the stored funds are used by an attacker.
!!!
*/
contract MyV2FlashLoan is FlashLoanReceiverBase {
/**
This function is called after your contract has received the flash loaned amount
*/
function executeOperation(
address[] calldata assets,
uint256[] calldata amounts,
uint256[] calldata premiums,
address initiator,
bytes calldata params
)
external
override
returns (bool)
{
//
// This contract now has the funds requested.
// Your logic goes here.
//
// At the end of your logic above, this contract owes
// the flashloaned amounts + premiums.
// Therefore ensure your contract has enough to repay
// these amounts.
// Approve the LendingPool contract allowance to *pull* the owed amount
for (uint i = 0; i < assets.length; i++) {
uint amountOwing = amounts[i].add(premiums[i]);
IERC20(assets[i]).approve(address(LENDING_POOL), amountOwing);
}
return true;
}
function myFlashLoanCall() public {
address receiverAddress = address(this);
address[] memory assets = new address[](2);
assets[0] = address(INSERT_ASSET_ONE_ADDRESS);
assets[1] = address(INSERT_ASSET_TWO_ADDRESS);
uint256[] memory amounts = new uint256[](2);
amounts[0] = INSERT_ASSET_ONE_AMOUNT;
amounts[1] = INSERT_ASSET_TWO_AMOUNT;
// 0 = no debt, 1 = stable, 2 = variable
uint256[] memory modes = new uint256[](2);
modes[0] = INSERT_ASSET_ONE_MODE;
modes[1] = INSERT_ASSET_TWO_MODE;
address onBehalfOf = address(this);
bytes memory params = "";
uint16 referralCode = 0;
LENDING_POOL.flashLoan(
receiverAddress,
assets,
amounts,
modes,
onBehalfOf,
params,
referralCode
);
}
}

3. Completing the flash loan

Once you have performed your logic with the flash loaned assets (in your executeOperation() function), you will need to pay back the flash loaned amounts if you used mode=0 for any of the assets in the modes parameter.

Paying back a flash loaned asset

Ensure your contract has the relevant amount + premium to payback the loaned asset. You can calculate this by taking the sum of the relevant entry in the amounts and premiums array passed into the executeOperation() function.

You do not need to transfer the owed amount back to the LendingPool. The funds will be automatically pulled at the conclusion of your operation.

Incurring a debt (i.e. not immediately paying back)

If you initially used a mode=1 or mode=2 for any of the assets in the modes parameter, then the address passed in for onBehalfOf will incur the debt if the onBehalfOf address has previously approved the msg.sender to incur debts on their behalf.

This means that you can have some assets that are paid back immediately, while other assets incur a debt.

Encoding and Decoding Parameters

If you would like to pass parameters into your flash loan function, you will first need to encode them, then decode them in your executeOperation().

Encoding

If you're encoding in solidity, you can use the in-built abi.encode():

// Encoding an address and a uint
bytes memory params = abi.encode(address(this), 1234);

If you're encoding off-chain, then you can use a package like web3.js which has an abi.encodeParameters():

const params = web3.eth.abi.encodeParameters(
["bytes32"],
[
web3.utils.utf8ToHex("some_value")
]
)

Decoding

When decoding in your executeOperation(), you will need to use the in-build abi.decode():

(bytes32 someValue) = abi.decode(params, (bytes32));

For more, see the official Solidity docs.