# FeeModule

## [Link to the code](https://github.com/Consensys/linea-attestation-registry/blob/dev/contracts/src/stdlib/FeeModuleV2.sol)

## When to use this module?

Use this module when you want to:

* Charge a fee for issuing attestations through your portal
* Set different fees for different schemas
* Monetize your attestation service
* Create economic incentives or disincentives for certain types of attestations

The FeeModule is particularly useful for portals that provide value-added services and want to generate revenue or cover operational costs.

## When not to use this module?

Don't use this module if:

* You want attestations to be completely free
* You need complex fee logic (e.g., dynamic pricing, discounts) - in that case, create a custom module
* You want the fees to be automatically transferred to a specific address (this module only checks the fee, it doesn't collect it)

## How to use this module?

### 1. Set fees for your portal and schemas

Only the portal owner can set fees. Call the `setFees` function with the following parameters:

```solidity
function setFees(address portal, bytes32[] calldata schemaIds, uint256[] calldata fees) public onlyPortalOwner(portal);
```

**Parameters:**

* `portal`: The address of your portal
* `schemaIds`: An array of schema IDs for which you want to set fees
* `fees`: An array of fees (in wei) corresponding to each schema ID

**Example:**

```solidity
bytes32[] memory schemas = new bytes32[](2);
schemas[0] = 0x1234...; // Schema ID 1
schemas[1] = 0x5678...; // Schema ID 2

uint256[] memory fees = new uint256[](2);
fees[0] = 0.001 ether; // Fee for schema 1
fees[1] = 0.005 ether; // Fee for schema 2

feeModule.setFees(portalAddress, schemas, fees);
```

### 2. Include the module when registering your portal

When deploying or registering your portal, include the FeeModule address in the modules array.

### 3. Withdraw collected fees

The fees are sent to the portal contract. Use the portal's `withdraw` function to retrieve them:

```solidity
portal.withdraw(payable(recipient), amount);
```

## How to check the fees?

The FeeModule exposes a public mapping to check fees:

```solidity
mapping(address portal => mapping(bytes32 schemaId => uint256 attestationFee)) public attestationFees;
```

You can query this mapping to see what fee is required for a specific portal and schema combination.

## Important notes

{% hint style="warning" %}
\*\*The FeeModule only validates that the fee is paid - it does not collect or transfer the fee.\*\* The fee (sent as \`msg.value\`) remains in the portal contract. Make sure your portal implements a \`withdraw\` function to retrieve collected fees.
{% endhint %}

{% hint style="info" %}
When multiple Modules are used in a workflow, ensure that at most one Module processes \`msg.value\` to avoid accounting issues, as the total \`msg.value\` is forwarded to all Modules.
{% endhint %}


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