# Message Receiver Adapters

The receiver adapter uses the [ERC-2771 standard](https://eips.ethereum.org/EIPS/eip-2771) to forward the message to `MultiBridgeMessageReceiver` .

### IMessageReceiverAdapter

The `IMessageReceiverAdapter` interface extends the `MessageExecutor` interface defined below. It defines the following additional functions.

#### updateSenderAdapter

Updates the address of the corresponding `IMessageSenderAdapter` contract for this bridge on the source chain. 

```solidity
function updateSenderAdapter(address _senderAdapter) external
```

#### name

This function returns the name of the message bridge.

```solidity
function name() external view returns (string memory);
```

The interface defines the following event:

#### SenderAdapterUpdated

Emitted when the sender adapter address is updated through the `updateSenderAdapter()` function.

```solidity
event SenderAdapterUpdated(address indexed oldSenderAdapter, address indexed newSenderAdapter);
```

### MessageExecutor

The interface defines the following event:

#### MessageIdExecuted

This event is emitted when a message or message batch has been executed.

```solidity
event MessageIdExecuted(uint256 indexed fromChainId, bytes32 indexed messageId);
```

\
The interface defines the following error types.

#### MessageIdAlreadyExecuted

Is thrown if a `messageId` has already been executed.

```solidity
error MessageIdAlreadyExecuted(bytes32 messageId);
```

#### MessageFailure

Is thrown if the execution of an individual message fails. 

```solidity
error MessageFailure(bytes32 messageId, bytes errorData);
```

#### MessageBatchFailure

Is thrown if the execution of a batch of messages fails.  *(not currently used)*

```solidity
error MessageBatchFailure(bytes32 messageId, uint256 messageIndex, bytes errorData);
```

{% hint style="info" %}
**Most importantly**

MessageExecutor MUST append the **ABI-packed (messageId, fromChainId, from)** to the calldata for each message being executed. This allows the receiver of the message to verify the cross-chain sender and the chain that the message is coming from.

* `to.call(abi.encodePacked(data, messageId, fromChainId, from));`
  {% endhint %}

The contract which the executor calls (`to`) MUST be `ExecutorAware`. The `ExecutorAware` contract allows contracts on a receiving chain to execute messages from an origin chain. These messages are sent by the `MessageDispatcher` contract which live on the origin chain. The `MessageExecutor` contract on the receiving chain executes these messages and then forward them to an `ExecutorAware` contract on the receiving chain.