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Introduction

When we take a look at the blockchain industry, we observe a segregated ecosystem lacking cohesion, interoperability and teamwork. The vision lead to the Blockchain Revolution, knows as “Web3” — a new era of the internet that is user-centered, emphasizing data ownership and decentralized trust.

Sovereign Chains will dismantle the barriers between isolated blockchain networks by allowing smart contracts to seamlessly interact across different Sovereign Chains and the main MultiversX chain. This cross-chain interoperability is crucial for fostering an environment where decentralized apps (dApps) can utilize functionalities or assets from across the ecosystem.

What is Cross-Chain Execution?​

Cross-Chain execution is the ability of a smart contracts or a decentralized applications on one blockchain to invoke actions on another blockchain. This feature allows for smart contract execution or transfer of funds from one chain to another, enabling developers to build applications that are chain agnostic.

Cross-Chain Execution within Sovereign Chains​

Since a Sovereign Chain is a separate blockchain with a different rule-set from the MultiversX blockchain, there has to be a way of communication between them. The interaction is being done by Smart Contracts, the Sovereign Bridge Service and Nodes.

The MultiversX blockchain will be referred as the MultiversX Mainchain in the further sections.

To Sovereign

When a transaction starts from the MultiversX Mainchain, either from a wallet or a smart contract, it goes through the Mvx-ESDT-Safe smart contract. The Observer nodes monitor the events that the deposit transaction emits and then the Sovereign Nodes notarize the state changes inside the Sovereign Chain. This notarization means the end of a Cross-Chain transfer.

From Sovereign

When the transaction is generated from the Sovereign Chain, the Sov-ESDT-Safe smart contract must be used. This smart contract will generate events and from that point, the sovereign nodes, at the end of the round, will read all the outgoing operations (plus all the unconfirmed operations), then these are signed by all the validators, then send to the bridge service, which will make the transaction on main chain

This is a high-level description of the whole process, the smart contracts that take place in it are far more detailed and have a lot of specific scenarios and behaviours. The current Sovereign Chain suite consists of four main contracts, here is the high-level description for some of the cross chain smart contracts:

Mvx-ESDT-Safe & Sov-ESDT-Safe​

The two contracts have the same role: to facilitate a cross-chain execution depending on what side the process starts. The reason for the prefix of Sov and Mvx is to show where the smart contract is deployed, Sov means that the contract is deployed on a Sovereign Chain and Mvx that is deployed on the MultiversX Mainchain. There will be an in-depth description of each smart contract in the upcoming modules (Mvx-ESDT-Safe and Sov-ESDT-Safe). The description will consist of flows for the cross-chain interactions, important modules and endpoints.

Cross-chain transfers imply sending funds through the smart contracts mentioned above. There are two types of bridging mechanism available: Lock&Send and Burn&Mint.

Lock and Send​

Lock & Send: a custodial bridging mechanism in which the source-chain tokens are locked inside the bridge contract, while an equal amount is minted on the destination chain. The locked balance on the source chain backs the circulating supply on the sovereign chain 1-for-1 until the tokens are returned and unlocked.

Burn and Mint​

Mint & Burn: a non-custodial bridging mechanism where the source-chain tokens are burned (permanently removed from supply) and an identical amount is minted on the destination chain, so the total supply moves between chains without any tokens being held in escrow.

Fee-Market​

Since every Sovereign Chain will have a customizable fee logic, it was paramount that this configuration had to be separated into a different contract. The rules set inside this contract are:

  • fee per transferred token
  • fee per gas unit
  • users whitelist to bypass the fee

This contract is also present in the MultiversX Mainchain and in any Sovereign Chain.

Header-Verifier​

Any cross-chain transaction that happens inside a Sovereign Chain is called and operation. The main role of this contract is to verify operations. They have to be signed by the validators of the Sovereign Chain. If the operation is successfully verified it will be registered and then can be executed by the Mvx-ESDT-Safe smart contract. All the BLS keys of the validators will be stored inside this contract. The in-depth description of how those operations are registered can be found in the Header-Verifier module.

note

The source for the smart contracts can be found at the official MultiversX Sovereign Chain SCs repository.

Sovereign Bridge Service​

This feature facilitates the execution of outgoing operations. This service is an application that receives Sovereign operations. After that, it will call the execute_operation endpoint from the Mvx-ESDT-Safe smart contract. The registration and execution of operations looks like this:

  • For N operations there is only one register transaction inside the Header-Verifier smart contract.
  • N transactions for the execution of N operations inside the ESDT-Safe smart contract, one execution transaction per operation.

There can be one or more services deployed in the network at the same time.