POA Bridge - NodeJS Oracle

Bridge Overview

The POA Bridge allows users to transfer assets between two chains in the Ethereum ecosystem. It is composed of several elements which are located in different POA Network repositories:

Bridge Elements

1. An oracle written in NodeJS, contained in this repository.
2. Solidity smart contracts. Used to manage bridge validators, collect signatures, and confirm asset relay and disposal.
3. Bridge UI Application. A DApp interface to transfer tokens and coins between chains.
4. Bridge Monitor. A tool for checking balances and unprocessed events in bridged networks.
5. Bridge Deployment Playbooks. Manages configuration instructions for remote deployments.

The bridge oracle is deployed on specified validator nodes (only nodes whose private keys correspond to addresses specified in the smart contracts) in the network. The oracle connects to two chains via a Remote Procedure Call (RPC). It is responsible for:

• listening to events related to bridge contracts
• sending transactions to authorize asset transfers

Following is an overview of the NodeJS bridge oracle and instructions for getting started with the POA Bridge.

Interoperability

Interoperability is the ability to share resources between networks. The POA Bridge is an interoperability protocol where users can transfer value (ERC20 compatible tokens and network coins) between chains in the Ethereum ecosystem. This creates opportunities to use different chains for different purposes. For example, smart contracts can allocate resource intensive operations to a sidechain where transactions are fast and inexpensive.

Network Processes

Network Definitions

Bridging occurs between two networks.

• Home - or Native - is a network with fast and inexpensive operations. All bridge operations to collect validator confirmations are performed on this side of the bridge.

• Foreign can be any chain, but generally refers to the Ethereum mainnet.

Operational Modes

The POA bridge currently provides two operational modes, with a 3rd mode in development.

• Native-to-ERC20 Coins on a Home network can be converted to ERC20-compatible tokens on a Foreign network. Coins are locked on the Home side and the corresponding amount of ERC20 tokens are minted on the Foreign side. When the operation is reversed, tokens are burnt on the Foreign side and unlocked in the Home network.
• ERC20-to-ERC20 ERC20-compatible tokens on the Foreign network are locked and minted as ERC20-compatible tokens (ERC677 tokens) on the Home network. When transferred from Home to Foreign, they are burnt on the Home side and unlocked in the Foreign network. This can be considered a form of atomic swap when a user swaps the token "X" in network "A" to the token "Y" in network "B".
• ERC20-to-Native: Currently in development. Pre-existing tokens in the Foreign network are locked and coins are minted in the Home network. The Home network consensus engine in this case should support invocation of Parity's Block Reward contract (https://wiki.parity.io/Block-Reward-Contract.html) to mint coins as per the bridge contract request.

Architecture

Native-to-ERC20

ERC20-to-ERC20

Watcher

A watcher listens for a certain event and creates proper jobs in the queue. These jobs contain the transaction data (without the nonce) and the transaction hash for the related event. The watcher runs on a given frequency, keeping track of the last processed block.

If the watcher observes that the transaction data cannot be prepared, which generally means that the corresponding method of the bridge contract cannot be invoked, it inspects the contract state to identify the potential reason for failure and records this in the logs.

There are three Watchers:

• Signature Request Watcher: Listens to UserRequestForSignature events on the Home network.
• Collected Signatures Watcher: Listens to CollectedSignatures events on the Home network.
• Affirmation Request Watcher: Depends on the bridge mode.
  • Native-to-ERC20: Listens to UserRequestForAffirmation raised by the bridge contract.
  • ERC20-to-ERC20 and ERC20-to-Native: Listens to Transfer events raised by the token contract.

Sender

A sender subscribes to the queue and keeps track of the nonce. It takes jobs from the queue, extracts transaction data, adds the proper nonce, and sends it to the network.

There are two Senders:

• Home Sender: Sends transaction to the Home network.
• Foreign Sender: Sends transaction to the Foreign network.

RabbitMQ

RabbitMQ is used to send jobs from watchers to senders.

Redis DB

Raddis is used to store number of blocks that were already inspected by watchers, and the NOnce which was used by the sender last time to send a transaction.

For more information on the Redis/RabbitMQ requirements, see #90

How to Use

Installation and Deployment

Deploy the Bridge Contracts

1. Deploy the bridge contracts

2. Open bridgeDeploymentResults.json generated by the bridge contract deployment process.

   For Native-to-ERC20 mode it looks like:

   {
       "homeBridge": {
           "address": "0xc60daff55ec5b5ce5c3d2105a77e287ff638c35e",
           "deployedBlockNumber": 123321
       },
       "foreignBridge": {
           "address": "0x3f5ce5fbfe3e9af3971dd833d26ba9b5c936f0be",
           "deployedBlockNumber": 456654,
           "erc677": {
               "address": "0x41a29780309dc2582f080f6af89953be3435679a"
           }
       }
   }

   For ERC20-to-ERC20 mode it looks like:

   {
       "homeBridge": {
           "address": "0x765a0d90e5a5773deacbd94b2dc941cbb163bdab",
           "deployedBlockNumber": 789987,
           "erc677": {
               "address": "0x269f57f5ae5421d084686f9e353f5b7ee6af54c2"
           }
       },
       "foreignBridge": {
           "address": "0x7ae703ea88b0545eef1f0bf8f91d5276e39be2f7",
           "deployedBlockNumber": 567765
       }
   }

Configuration

1. Create a .env file: cp .env.example .env

2. Fill in the required information using the output data from bridgeDeploymentResults.json. Check the tables with the set of parameters below to see their explanation.

Run the Processes

There are two options to run the nodejs oracle:

1. By using docker containers. This requires Docker and Docker Compose installed. If you are on Linux, it's also recommended that you create a docker group and add your user to it, so that you can use the CLI without sudo.
2. By using NodeJs Package Manager.

NPM

• redis-server starts Redis. redis-cli ping will return a pong if Redis is running.
• rabbitmq-server starts RabbitMQ. Use rabbitmqctl status to check if RabbitMQ is running.
• npm run watcher:signature-request
• npm run watcher:collected-signatures
• npm run watcher:affirmation-request
• npm run sender:home
• npm run sender:foreign

Docker

• Start RabbitMQ and Redis: docker-compose up -d
• docker-compose run bridge npm run watcher:signature-request
• docker-compose run bridge npm run watcher:collected-signatures
• docker-compose run bridge npm run watcher:affirmation-request
• docker-compose run bridge npm run sender:home
• docker-compose run bridge npm run sender:foreign

Bridge UI

See the Bridge UI installation instructions to configure and use the optional Bridge UI.

Rollback the Last Processed Block in Redis

If the bridge does not handle an event properly (i.e. a transaction stalls due to a low gas price), the Redis DB can be rolled back. You must identify which watcher needs to re-run. For example, if the validator signatures were collected but the transaction with signatures was not sent to the Foreign network, the collected-signatures watcher must look at the block where the corresponding CollectedSignatures event was raised.

Execute this command in the bridge root directory:

bash ./reset-lastBlock.sh <watcher> <block num>

or

docker-compose run bridge bash ./reset-lastBlock.sh <watcher> <block num>

where the watcher could be one of:

• signature-request
• collected-signatures
• affirmation-request

Configuration parameters

Variable	Description	Values
BRIDGE_MODE	The bridge mode. The bridge starts listening to a different set of events based on this parameter.	NATIVE_TO_ERC / ERC_TO_ERC
HOME_RPC_URL	The HTTPS URL(s) used to communicate to the RPC nodes in the Home network. Several URLs can be specified, delimited by spaces. If the connection to one of these nodes is lost the next URL is used for connection.	url(s)
HOME_BRIDGE_ADDRESS	The address of the bridge contract address in the Home network. It is used to listen to events from and send validators' transactions to the Home network.	hexidecimal beginning with "0x"
HOME_POLLING_INTERVAL	The interval in milliseconds used to request the RPC node in the Home network for new blocks. The interval should match the average production time for a new block.	integer
FOREIGN_RPC_URL	The HTTPS URL(s) used to communicate to the RPC nodes in the Foreign network. Several URLs can be specified, delimited by spaces. If the connection to one of these nodes is lost the next URL is used for connection.	url(s)
FOREIGN_BRIDGE_ADDRESS	The address of the bridge contract address in the Foreign network. It is used to listen to events from and send validators' transactions to the Foreign network.	hexidecimal beginning with "0x"
ERC20_TOKEN_ADDRESS	Used with the ERC_TO_ERC bridge mode, this parameter specifies the ERC20-compatible token contract address. The token contract address is used to identify transactions that transfer tokens to the Foreign Bridge account address. Omit this parameter with other bridge modes.	hexidecimal beginning with "0x"
FOREIGN_POLLING_INTERVAL	The interval in milliseconds used to request the RPC node in the Foreign network for new blocks. The interval should match the average production time for a new block.	integer
HOME_GAS_PRICE_ORACLE_URL	The URL used to get a JSON response from the gas price prediction oracle for the Home network. The gas price provided by the oracle is used to send the validator's transactions to the RPC node. Since it is assumed that the Home network has a predefined gas price (e.g. the gas price in the Core of POA.Network is 1 GWei), the gas price oracle parameter can be omitted for such networks.	url
HOME_GAS_PRICE_SPEED_TYPE	Assuming the gas price oracle responds with the following JSON structure: {"fast": 20.0, "block_time": 12.834, "health": true, "standard": 6.0, "block_number": 6470469, "instant": 71.0, "slow": 1.889}, this parameter specifies the desirable transaction speed. The speed type can be omitted when HOME_GAS_PRICE_ORACLE_URL is not used.	instant / fast / standard / slow
HOME_GAS_PRICE_FALLBACK	The gas price (in GWei) that is used if both the oracle and the fall back gas price specified in the Home Bridge contract are not available.	integer
HOME_GAS_PRICE_UPDATE_INTERVAL	An interval in milliseconds used to get the updated gas price value either from the oracle or from the Home Bridge contract.	integer
FOREIGN_GAS_PRICE_ORACLE_URL	The URL used to get a JSON response from the gas price prediction oracle for the Foreign network. The provided gas price is used to send the validator's transactions to the RPC node. If the Foreign network is Ethereum Foundation mainnet, the oracle URL can be: https://gasprice.poa.network. Otherwise this parameter can be omitted.	url
FOREIGN_GAS_PRICE_SPEED_TYPE	Assuming the gas price oracle responds with the following JSON structure: {"fast": 20.0, "block_time": 12.834, "health": true, "standard": 6.0, "block_number": 6470469, "instant": 71.0, "slow": 1.889}, this parameter specifies the desirable transaction speed. The speed type can be omitted when FOREIGN_GAS_PRICE_ORACLE_URLis not used.	instant / fast / standard / slow
FOREIGN_GAS_PRICE_FALLBACK	The gas price (in GWei) used if both the oracle and fall back gas price specified in the Foreign Bridge contract are not available.	integer
FOREIGN_GAS_PRICE_UPDATE_INTERVAL	The interval in milliseconds used to get the updated gas price value either from the oracle or from the Foreign Bridge contract.	integer
VALIDATOR_ADDRESS_PRIVATE_KEY	The private key of the bridge validator used to sign confirmations before sending transactions to the bridge contracts. The validator account is calculated automatically from the private key. Every bridge instance (set of watchers and senders) must have its own unique private key. The specified private key is used to sign transactions on both sides of the bridge.	hexidecimal without "0x"
HOME_START_BLOCK	The block number in the Home network used to start watching for events when the bridge instance is run for the first time. Usually this is the same block where the Home Bridge contract is deployed. If a new validator instance is being deployed for an existing set of validators, the block number could be the latest block in the chain.	integer
FOREIGN_START_BLOCK	The block number in the Foreign network used to start watching for events when the bridge instance runs for the first time. Usually this is the same block where the Foreign Bridge contract was deployed to. If a new validator instance is being deployed for an existing set of validators, the block number could be the latest block in the chain.	integer
QUEUE_URL	RabbitMQ url used by watchers and senders to communicate to the message queue. Typically set to: amqp://127.0.0.1.	local url
REDIS_URL	Redis DB url used by watchers and senders to communicate to the database. Typically set to: redis://127.0.0.1:6379.	local url
REDIS_LOCK_TTL	Threshold in milliseconds for locking a resource in the Redis DB. Until the threshold is exceeded, the resource is unlocked. Usually it is 1000.	integer
ALLOW_HTTP	Only use in test environments - must be omitted in production environments.. If this parameter is specified and set to yes, RPC urls can be specified in form of HTTP links. A warning that the connection is insecure will be written to the logs.	yes / no

Useful Commands for Development

RabbitMQ

Command	Description
rabbitmqctl list_queues	List all queues
rabbitmqctl purge_queue home	Remove all messages from home queue
rabbitmqctl status	check if rabbitmq server is currently running
rabbitmq-server	start rabbitMQ server

Redis

Use redis-cli

Command	Description
KEYS *	Returns all keys
SET signature-request:lastProcessedBlock 1234	Set key to hold the string value.
GET signature-request:lastProcessedBlock	Get the key value.
DEL signature-request:lastProcessedBlock	Removes the specified key.
FLUSHALL	Delete all the keys in all existing databases.
redis-cli ping	check if redis is running.
redis-server	start redis server.

Testing

npm test

E2E tests

See the E2E README for instructions.

Native-to-ERC20 Mode Testing

When running the processes, the following commands can be used to test functionality.

• To send deposits to a home contract run node scripts/sendUserTxToHome.js <tx num> (or docker-compose run bridge node scripts/sendUserTxToHome.js <tx num>), where <tx num> is how many tx will be sent out to deposit.

• To send withdrawals to a foreign contract run node scripts/sendUserTxToForeign.js <tx num> (or docker-compose run bridge node scripts/sendUserTxToForeign.js <tx num>), where <tx num> is how many tx will be sent out to withdraw.

ERC20-to-ERC20 Mode Testing

• To deposit from a Foreign to a Home contract run node scripts/sendUserTxToErcForeign.js <tx num>.

• To make withdrawal to Home from a Foreign contract run node scripts/sendUserTxToErcHome.js <tx num>.

Configuration parameters for testing

Variable	Description
HOME_RPC_URL	The HTTPS URL(s) used to communicate to the RPC nodes in the Home network.
FOREIGN_RPC_URL	The HTTPS URL(s) used to communicate to the RPC nodes in the Foreign network.
USER_ADDRESS	An account - the current owner of coins/tokesn
USER_ADDRESS_PRIVATE_KEY	A private key belonging to the account
HOME_BRIDGE_ADDRESS	Address of the bridge in the Home network to send transactions
HOME_MIN_AMOUNT_PER_TX	Value (in eth or tokens) to be sent in one transaction for the Home network
FOREIGN_BRIDGE_ADDRESS	Address of the bridge in the Foreign network to send transactions
FOREIGN_MIN_AMOUNT_PER_TX	Value (in eth or tokens) to be sent in one transaction for the Foreign network
ERC20_TOKEN_ADDRESS	An address of the token deployed on the Home side for ERC20-to-ERC20 mode
BRIDGEABLE_TOKEN_ADDRESS	An address of the token deployed on the Home side for Native-to-ERC20 mode or on the Foreign side for ERC20-to-ERC20

Contributing

See the CONTRIBUTING document for contribution, testing and pull request protocol.

License

This project is licensed under the GNU Lesser General Public License v3.0. See the LICENSE file for details.

References

• POA Bridge FAQ