pezkuwi-subxt/bridges/README.md

Parity Bridges Common

This is a collection of components for building bridges.

These components include runtime modules to help you construct your bridge's runtime, as well as bridge relays for cross-chain communication.

A bridge node is also available. The node can be used to run a test network which has support for bridging Ethereum PoA chains to Substrate. We're working on expanding this functionality in the future.

🚧 The bridges are currently under construction - a hardhat is recommended beyond this point 🚧

Contents

• Installation
• Project Layout
• Rialto Runtime
• Ethereum Node
• Bridge Relay
• Running the Bridge

Installation

To get up and running you need both stable and nightly Rust. Rust nightly is used to build the Web Assembly (WASM) runtime for the node. You can configure the WASM support as so:

rustup install nightly
rustup target add wasm32-unknown-unknown --toolchain nightly

Once this is configured you can build and test the repo as follows:

git clone https://github.com/paritytech/parity-bridges-common.git
cd parity-bridges-common
cargo build --all
cargo test --all

If you need more information about setting up your development environment Substrate's Getting Started page is a good resource.

Project Layout

Here's an overview of how the project is laid out. The main bits are the node, which is the actual "blockchain", the modules which are used to build the blockchain's logic (a.k.a the runtime) and the relays which are used to pass messages between chains.

├── bin
│  └── node              // Bridge ready chain implementation
├── modules              // Runtime Modules
│  ├── ethereum          // Manage Ethereum PoA chain info
│  ├── ethereum-contract // Ethereum built-in for validating Substrate block info
│  ├── currency-exchange // Cross-chain fund transfers
│  └── substrate         // Manage Substrate chain info
├── primitives           // Shared runtime and node code
│  └── ethereum-poa      // Helpers for Ethereum PoA
├── relays               // Cross-chain communication
│  ├── ethereum          // Sync and communicate between Ethereum PoA + Substrate chains
│  └── substrate         // 🚧 WIP 🚧

Rialto Runtime

The node runtime consists of several runtime modules, however not all of them are used at the same time. When running an Ethereum PoA to Substrate bridge the modules required are the Ethereum module and the currency exchange module. When running a Substrate to Substrate bridge the Substrate and currency exchange modules are required.

Below is a brief description of each of the runtime modules.

Ethereum Bridge Runtime Module

The main job of this runtime module is to keep track of useful information an Ethereum PoA chain which has been submitted by a bridge relayer. This includes:

• Ethereum headers and their status (e.g are they the best header, are they finalized, etc.)
• Current validator set, and upcoming validator sets

This runtime module has more responsibilties than simply storing headers and validator sets. It is able to perform checks on the incoming headers to verify their general integrity, as well as whether or not they've been finalized by the authorities on the PoA chain.

This module is laid out as so:

├── ethereum
│  └── src
│     ├── error.rs        // Runtime error handling
│     ├── finality.rs     // Manage finality operations
│     ├── import.rs       // Import new Ethereum headers
│     ├── lib.rs          // Store headers and validator set info
│     ├── validators.rs   // Track current and future PoA validator sets
│     └── verification.rs // Verify validity of incoming Ethereum headers

Currency Exchange Runtime Module

The currency exchange module is used to faciliate cross-chain funds transfers. It works by accepting a transaction which proves that funds were locked on one chain, and releases a corresponding amount of funds on the recieving chain.

For example: Alice would like to send funds from chain A to chain B. What she would do is send a transaction to chain A indicating that she would like to send funds to an address on chain B. This transaction would contain the amount of funds she would like to send, as well as the address of the recipient on chain B. These funds would now be locked on chain A. Once the block containing this "locked-funds" transaction is finalized it can be relayed to chain B. Chain B will verify that this transaction was included in a finalized block on chain A, and if successful deposit funds into the recipient account on chain B.

Chain B would need a way to convert from a foreign currency to its local currency. How this is done is left to the runtime developer for chain B.

This module is one example of how an on-chain light client can be used to prove a particular action was taken on a foreign chain. In particular it enables transfers of the foreign chain's native currency, but more sophisticated modules such as ERC20 token transfers or arbitrary message transfers are being worked on as well.

Substrate Bridge Runtime Module

👷 Under Construction 👷‍♀️

Ethereum Node

On the Ethereum side of things, we require two things. First, a Solidity smart contract to track the Substrate headers which have been submitted to the bridge (by the relay), and a built-in contract to be able to verify that headers have been finalized by the Grandpa finality gadget. Together this allows the Ethereum PoA chain to verify the integrity and finality of incoming Substrate headers.

The Solidity smart contract is not part of this repo, but can be found here if you're curious. We have the contract ABI in the ethereum/relays/res directory.

Bridge Relay

The bridge relay is responsible for syncing the chains which are being bridged, and passing messages between them. The current implementation of the relay supportings syncing and interacting with Ethereum PoA and Substrate chains.

The folder structure of the bridge relay is as follows:

├── relays
│  ├── ethereum
│  │  ├── res
│  │  │  └── ...
│  │  └── src
│  │     ├── ethereum_client.rs          // Interface for Ethereum RPC
│  │     ├── ethereum_deploy_contract.rs // Utility for deploying bridge contract to Ethereum
│  │     ├── ethereum_exchange.rs        // Relay proof of PoA -> Substrate exchange transactions
│  │     ├── ethereum_sync_loop.rs       // Sync headers from Ethereum, submit to Substrate
│  │     ├── ethereum_types.rs           // Useful Ethereum types
│  │     ├── exchange.rs                 // Relay proof of exchange transactions
│  │     ├── headers.rs                  // Track synced and incoming block headers
│  │     ├── main.rs                     // Entry point to binary
│  │     ├── substrate_client.rs         // Interface for Substrate RPC
│  │     ├── substrate_sync_loop.rs      // Sync headers from Substrate, submit to Ethereum
│  │     ├── substrate_types.rs          // Useful Ethereum types
│  │     ├── sync.rs                     // Sync configuration and helpers
│  │     ├── sync_loop.rs                // Header synchronization between source and target chains
│  │     ├── sync_types.rs               // Useful sync types
│  │     └── utils.rs                    // General utilities

Running the Bridge

To run the Bridge you need to be able to connect the bridge relay node to the RPC interface of nodes on each side of the bridge (home & foreign chain). An easy way to build all the required nodes is through Docker.

Local Development Build

Building

First you'll need to build the bridge node and relay. This can be done as follows:

# In `parity-bridges-common` folder
cargo build -p rialto-bridge-node
cargo build -p ethereum-poa-relay

Next you'll need to clone the following fork of OpenEthereum. If you're doing development which only involves the Ethereum to Substrate side of the bridge you may use the master branch. Otherwise you'll need to checkout the substrate-builtins-stubs branch.

# Should be at the same level as `parity-bridges-common` folder
git clone https://github.com/paritytech/openethereum.git openethereum
git fetch
git checkout substrate-builtins-stubs

If you've checked out the substrate-builtins-stubs branch make sure you've cloned the OpenEthereum repo at the same level as parity-bridges-common since it references the repo.

Next you'll need to build the Ethereum node:

# In `openethereum` folder
cargo build

Running

To run a simple dev network you'll can use the scripts located in the scripts folder. Since the relay connects to both the Substrate and Ethereum chains it must be run last.

# In `parity-bridges-common` folder
./scripts/run-openethereum-node.sh
./scripts/run-rialto-bridge-node.sh
./scripts/run-eth2sub-relay.sh

At this point you should see the relayer submitting blocks from the Ethereum chain to the Substrate chain.

Local Docker Build

If you want to make a Docker container using your local source files you can run the following command at the top level of the repository:

docker build . -t bridge-relay-dev

You can also build and run the Substrate based node as follows:

docker build . -t bridge-node-dev --build-arg PROJECT=rialto-bridge-node

To run the Substrate node you can do the following:

docker run -it bridge-node-dev --dev --tmp

Notice that the docker run command will accept all the normal Substrate flags. For local development you should at minimum run with the --dev flag or else no blocks will be produced.

Full Network Docker Setup

See Deployments README to learn more about how to run a more sophisticated test network using docker-compose setup.