quorum-web3j-starter

Starter blockchain application built using Spring Boot-Web3j-Quorum

This repository is a very basic example of how you can use Web3j to build a Quorum based distributed application in Java Spring Boot. Key things to understand:

Web3j

Web3j is a lightweight Java library that provides an interface to connect with nodes in an Ethereum based Blockchain, such as Quorum. It allows to work with smart contracts as well.

Quorum

Quorum is a permissioned implementation of Ethereum, which is a Blockchain based distributed ledger protocol.

Very simply put:

Ethereum:

• Network of nodes that can together maintain a common data pool or ledger.
• Accounts are clients of a node and are defined by public private key pairs. Accounts communicate with each other using their public keys
• Contracts define certain methods and rules for interacting with the data pool. They can also contain local data.
• Transactions are how accounts perform operations in a Blockchain network - be it sending data to other accounts or adding contracts
• Blocks are how transactions move in a Blockchain network. Blocks are cryptographically secured and chained to each other, so each block contains the header of the next block.

Quorum:

• JP Morgan built Quorum on top of the Go implementation of Ethereum called go-ethereum
• Only permissioned nodes can be a part of the Quorum network.
• It provides a transaction manager to create encrypted transactions and contracts to enable private communication between accounts
• Supports public state accessible by all accounts to enable the distributed ledger provision of a Blockchain based network
• Private contracts and transactions cannot modify public state of the Quorum

web3j-quorum

web3j-quorum is an extension to Web3j to connect a Java application to a Quorum network

web3j-maven-plugin

In Ethereum, smart contracts are written in Solidity. To harness the power of smart contracts, the web3j-maven-plugin generates Java wrapper classes that can be used in our application as actual contracts.

What does this application actually do?

We have implemented a very basic messaging functionality between accounts in a Quorum network. A message thread can exist between 2 or more accounts. Every time a new thread is started, a new contract is generated which is private to the participating accounts. Thereafter, every message sent to this thread by any participant, updates the existing contract. Since it is a private contract, messages sent to it are only visible to the thread participants.

Run this application:

• Step 1: Set up a Quorum network

A sample Quorum network of 7 nodes (having 1 account each already created) can be setup as instructed in this example. For running this application, we used the Docker version of the setup.

• Step 2: Download this repository (Java Spring Boot application)

This application was created in IntelliJ using the Spring Initializr. Its based on maven and includes Spring Web and Thymeleaf dependancies.

• Step 3: Build and run the application

Run the Spring Boot application and access it at http://localhpst:8080/threads in the browser

• Step 4: Extending the application for other nodes

This application is distributed over the 7 nodes, which means we can run upto 7 instances of the application. Each instance can connect to a node using its IP address and the account created on the node. This instance in this repository connects to Node1 of the Quorum. Configurations To run an instance for another node - for example - node7:

1. Download the repository into a separate folder

2. Change the following parameters in QuorumConnection.java

node = "node7";
nodeKey = "ROAZBWtSacxXQrOe3FGAqJDyJjFePR5ce4TSIzmJ0Bc=";
nodeAddress = "0xcc71c7546429a13796cf1bf9228bff213e7ae9cc"
admin = Admin.build(new HttpService("http://localhost:22006"));
quorum = Quorum.build(new HttpService("http://localhost:22006"));

3. Specify a separate server port in src/resources/application.properties to run the application on

server.port=8081

4. Build and run the application

5. Access the node7 instance of the application on the browser at http://localhost:8081/threads

Docker issues

1. Fixes for common Docker issues can be found in the Troubleshooting Docker section here

2. If timeouts occur frequently, check the status of docker containers using docker ps and ensure they are all healthy. Else, tear the Quorum network down using docker-compose down -v (Its a good idea to remove volumes as well). Spin it up back again using docker-compose up -d