# Modules

Modules are smart contracts that inherit the [`AbstractModuleV2`](https://github.com/Consensys/linea-attestation-registry/blob/dev/contracts/src/abstracts/AbstractModuleV2.sol)contract and are registered in the registry. They allow for attestation creators to run custom logic to do things like:

* verify that attestations conform to some business logic
* verify a signature or a zk-snark
* perform other actions like transferring some tokens or minting an NFT
* recursively create another attestation

Modules are specified in a [portal](/verax-documentation/core-concepts/portals.md) and all attestations created by that portal are routed through the specified modules. Modules can also be chained together into discrete pieces of specific functionality.

Each module exposes a public function called `run`:

```solidity
function run(
  AttestationPayload calldata attestationPayload,
  bytes calldata validationPayload,
  address initialCaller,
  uint256 value,
  address attester,
  address portal,
  OperationType operationType
) public virtual;
```

The function executes whatever logic it needs to, and reverts if the incoming transaction doesn't conform to the required logic. The parameters are:

* `attestationPayload`: The raw data of the incoming attestation
* `validationPayload`: Any qualifying data required for verification, but that doesn't make it into the on-chain attestation (e.g. a snark proof, merkle proof or signature)
* `initialCaller`: The address of the initial transaction sender
* `value`: The amount of ETH paid with the attestation transaction
* `attester`: The address defined by the Portal as the attester for this payload
* `portal`: The address of the Portal issuing the attestation
* `operationType`: The type of operation being performed (see below)

### OperationType Enum

The `OperationType` parameter indicates what operation is being performed:

* `Attest`: A new attestation is being created
* `BulkAttest`: Multiple attestations are being created in a single transaction
* `Replace`: An existing attestation is being replaced
* `BulkReplace`: Multiple attestations are being replaced in a single transaction

This allows modules to apply different validation logic depending on the operation type.

As well as implementing the `Module` interface, a module must also implement [ERC-165](https://eips.ethereum.org/EIPS/eip-165) to ensure that it can be verified properly when being registered.

{% hint style="warning" %}
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 %}

## Module Metadata

Once the module smart contract is deployed, it can be registered with the following metadata:

<table><thead><tr><th width="171.32064128256513">Field</th><th width="120">Type</th><th>Description</th></tr></thead><tbody><tr><td>moduleAddress</td><td>address</td><td>(required) The address of the module smart contract</td></tr><tr><td>name</td><td>string</td><td>(required) A descriptive name for the module</td></tr><tr><td>description</td><td>string (URI)</td><td>(optional) A link to documentation about the module, its intended use, etc.</td></tr></tbody></table>

The metadata above is intended to help to discover modules that can be reused once created. Modules are chained together and executed in [portals](/verax-documentation/core-concepts/portals.md). The next section dives into portals, what they are, and how to create them.


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