> ## Documentation Index
> Fetch the complete documentation index at: https://docs.cow.bleu.builders/llms.txt
> Use this file to discover all available pages before exploring further.

# IBorrower

> Interface for borrower adapters that bridge the Flash-Loan Router with various lending protocols

## Overview

The `IBorrower` interface defines the contract API for borrower adapters that serve as intermediaries between CoW Protocol's Flash-Loan Router and various lending protocols. Since different flash-loan protocols have varying callback mechanisms, each requires a dedicated borrower implementation.

## Contract Details

* **Source**: `src/interfaces/IBorrower.sol`
* **License**: GPL-3.0-or-later
* **Solidity**: ^0.8.28

## Functions

### flashLoanAndCallBack

```solidity theme={null}
function flashLoanAndCallBack(
    address lender,
    IERC20 token,
    uint256 amount,
    bytes calldata callBackData
) external;
```

Initiates a flash loan request with a specific lender. The borrower contacts the lender, receives funds via a protocol-specific callback, then notifies the router through `borrowerCallBack()`.

**Parameters:**

* `lender`: Address of the flash loan provider
* `token`: ERC-20 token to borrow
* `amount`: Amount of tokens to borrow
* `callBackData`: Data to pass back to the router unchanged

**Requirements:**

* Only callable by the registered Flash Loan Router

### approve

```solidity theme={null}
function approve(
    IERC20 token,
    address target,
    uint256 amount
) external;
```

Grants spending permissions on borrowed tokens. Used by the settlement contract to authorize lender repayment.

**Parameters:**

* `token`: ERC-20 token to approve
* `target`: Address to approve for spending (typically the lender)
* `amount`: Amount to approve

**Requirements:**

* Only callable by the settlement contract

<Note>
  Approvals persist beyond individual transaction execution. Setting unlimited approvals once can save gas across multiple settlements.
</Note>

### settlementContract

```solidity theme={null}
function settlementContract() external view returns (ICowSettlement);
```

Returns the CoW Protocol settlement contract associated with this borrower.

### router

```solidity theme={null}
function router() external view returns (IFlashLoanRouter);
```

Returns the Flash Loan Router contract that manages this borrower.

## Protocol-Specific Implementations

Different lending protocols require different callback mechanisms:

| Protocol   | Flash Loan Function | Callback Function          |
| ---------- | ------------------- | -------------------------- |
| Aave V3    | `flashLoan()`       | `executeOperation()`       |
| ERC-3156   | `flashLoan()`       | `onFlashLoan()`            |
| Uniswap V2 | `swap()`            | `uniswapV2Call()`          |
| Uniswap V3 | `flash()`           | `uniswapV3FlashCallback()` |
| Balancer   | `flashLoan()`       | `receiveFlashLoan()`       |

Each protocol requires a separate borrower implementation adapting these mechanisms to the standard `IBorrower` interface.

## Implementation Requirements

When implementing a borrower contract:

1. **Restrict access**: Only the router should be able to trigger flash loans via `flashLoanAndCallBack()`
2. **Pass data unchanged**: The `callBackData` must be forwarded to the router exactly as received
3. **No fund retention**: The borrower should not hold funds between transactions
4. **Careful approvals**: Manage token approvals carefully to prevent unauthorized transfers
5. **Implement provider callback**: Handle the provider-specific callback and forward to `router.borrowerCallBack()`

## Security Considerations

* `flashLoanAndCallBack()` must only be callable by the authorized router
* `approve()` must only be callable by the settlement contract
* Callback data integrity must be maintained throughout the execution flow
* Token approvals should be managed with care to prevent exploitation

## Next Steps

<CardGroup cols={2}>
  <Card title="IFlashLoanRouter" icon="route" href="/flash-loan-router/contracts/interfaces/iflash-loan-router">
    Router interface reference
  </Card>

  <Card title="Borrower Base" icon="code" href="/flash-loan-router/contracts/base/borrower">
    Abstract base implementation
  </Card>
</CardGroup>