# Overview

## Introduction

MAP Omnichain Service (MOS) provides common modules for building cross-chain DApps, lowering the development threshold. MOS supports two types of cross-chain operations:

* **Asset Cross-chain**: Transfer tokens between different blockchains
* **Message Cross-chain**: Call contracts or sync data across chains

## Core Components

### MOS Relay

MOS Relay is the main contract on MAP Relay Chain, responsible for:

* Processing cross-chain transfers from users
* Calling LightNodeManager to verify messages
* Forwarding transactions to other chain's MOS contracts
* Managing token minting/burning and Vault permissions
* Handling fee distribution

### MOS

MOS is the main contract on source/destination chains, responsible for:

* Processing cross-chain transfers from users
* Calling LightNode to verify MAP Relay Chain transactions
* Parsing cross-chain events

### Messenger

Messenger is an independent inter-chain program that:

* Listens to events on source chains
* Builds Merkle proofs on source chain's ledger
* Transmits messages and proofs to destination chain
* Prepays gas fees and earns rewards

**Key Properties:**

* As long as one honest Messenger works, all cross-chain messages can be transferred
* Malicious Messenger attacks only cause verification failure, not asset loss
* Messenger SDK is open to DApp developers

### Vault

Vault is the equity token contract for each cross-chain token:

* Stakes user liquidity, issues VToken
* Records cross-chain fees and distributes to liquidity providers
* Handles liquidity withdrawal and transfer

### Fee

Fee contract manages cross-chain fee collection and distribution:

* Sets fee distribution ratio for Vault, Relay, and Protocol
* Sets fee charging standards

## Proof Verification

Cross-chain data verification flow:

1. Verify Proof in transaction body
2. Prove transaction body can construct ReceiptRoot
3. Prove ReceiptRoot exists in block header
4. Verify header legitimacy against LightNode's stored headers

## Cross-chain Types

| Type                                                                                                   | Description                    | Use Case           |
| ------------------------------------------------------------------------------------------------------ | ------------------------------ | ------------------ |
| [Asset Cross-chain](https://docs.mapprotocol.io/develop/cross-chain-application/asset-cross-chain)     | Transfer tokens between chains | Token bridges, DEX |
| [Message Cross-chain](https://docs.mapprotocol.io/develop/cross-chain-application/message-cross-chain) | Call contracts or sync data    | Omnichain DApps    |

## Contract Addresses

See [v1 Contracts](https://github.com/mapprotocol/docs/blob/master/develop/network/v1-contracts.md) for MOS contract addresses on supported chains.

## Getting Started

* [Asset Cross-chain Guide](https://docs.mapprotocol.io/develop/cross-chain-application/asset-cross-chain)
* [Message Cross-chain Guide](https://docs.mapprotocol.io/develop/cross-chain-application/message-cross-chain)
* [Build OmniApp Tutorial](https://github.com/mapprotocol/docs/blob/master/develop/cross-chain-app/omni-app.md)