Proj 15: Making an Ethereum Contract with Truffle (10 pts.)

What You Need

Purpose

We'll use TestRPC, which makes it fast and easy to develop contracts on a simulated test blockchain.

Source

The Hitchhiker's Guide to Smart Contracts in Ethereum

Mac or Ubuntu?

1. Installing Node.js and npm on a Mac

https://nodejs.org/en/download/

Download the installer for your OS, as shown below.

Install it. as shown below.

To update it, in a Terminal window, execute this command:

sudo npm install npm@latest -g

1. Installing Node.js and npm on Ubuntu 16.04

sudo apt install -y curl

In the same Terminal window, execute these commands:

curl -sL https://deb.nodesource.com/setup_8.x | sudo -E bash -
sudo apt-get install -y nodejs

In a Terminal window, execute these commands:

node -v
npm -v

(When I re-did the project on Apr 15, 2019, the versions were v8.15.1 and 6.4.1.)

In a Terminal window, execute this command:

sudo npm install npm@latest -g

2. Installing Truffle

To install it, open a new Terminal window and execute this command:

sudo npm install -g truffle

4. Set Up a Project

(On Ubuntu, you need to prefix the last command with "sudo".)

cd
mkdir solidity-experiments
cd solidity-experiments/
truffle init

5. Starting the Development Testchain

In the Terminal window, execute this command.

(On Ubuntu, you need to prefix this command with "sudo".)

truffle develop

6. Compiling and Deploying Sample Contracts

You should be able to compile the example contracts by running truffle compile. Then, to deploy the contracts to the simulated network using the testrpc node we have running, you need to run truffle migrate.

In the Terminal window, at the truffle(develop)> prompt, execute these commands.

networks --clean
compile
migrate

7. Creating a Smart Contract

In the Terminal window, at the truffle(develop)> prompt, execute these commands.

create contract ProofOfExistence1

8. Adding Solidity Code

cd
cd solidity-experiments
nano contracts/ProofOfExistence1.sol

Remove the existing code and paste in this code:

pragma solidity ^0.5.0;
// Proof of Existence contract, version 1
contract ProofOfExistence1 {
  // state
  bytes32 public proof;
  // calculate and store the proof for a document
  // *transactional function*
  function notarize(string memory document) public {
    proof = proofFor(document);
  }
  // helper function to get a document's sha256
  // *read-only function*
  function proofFor(string memory document) public view returns (bytes32) {
    return sha256(bytes(document));
  }
}

Explanation

• The keyword contract is used to define a "contract", which is like a class in other programming languages.
• Contracts have state and functions.
• There are two types of functions:
  • Read-only (constant) functions: functions that don't perform any state changes. They only read state, perform computations, and return values. As these functions can be resolved locally by each node, they cost no gas. Marked with the keyword constant.
  • Transactional functions: functions that perform a state change in the contract or move funds. As these changes need to be reflected in the blockchain, transactional function execution requires sending a transaction to the network and spending gas.

In the contract above, notarize is a transactional function and proofFor is a constant function.

Saving the ProofOfExistence1.sol File

9. Deploying the Contract

nano migrations/2_deploy_contracts.js

var ProofOfExistence1 = artifacts.require("./ProofOfExistence1.sol");
module.exports = function(deployer) {
  deployer.deploy(ProofOfExistence1);
};

Press Ctrl+X, Y, Enter to save the file. To run the migration again, In your original Terminal window, at the truffle(develop)> prompt, execute this command.

migrate --reset

10. Finding the Smart Contract's Address

In your original Terminal window, at the truffle(develop)> prompt, execute these commands.

let poe = await ProofOfExistence1.at(ProofOfExistence1.address)
poe.address

11. Calculating a Hash

An amazing idea

The SHA256 hash of this message is shown below, and it starts with a328.

Let's use our smart contract to calculate that hash value.

In your original Terminal window, at the truffle(develop)> prompt, execute this command.

poe.proofFor('An amazing idea')

12. Registering a Document

poe.notarize('An amazing idea')
poe.proof()

Notice that the transaction cost some gas, as shown above.

13. Registering Your Name

poe.notarize('YOURNAME')
poe.proof()

Saving a Screen Image

• The command notarize('YOURNAME'), containing your name
• A nonzero hash proving that your name has been placed on the blockchain

Capture a full-screen image.

YOU MUST SUBMIT A FULL-SCREEN IMAGE FOR FULL CREDIT!

Save the image with the filename "YOUR NAME Proj 15", replacing "YOUR NAME" with your real name.

Turning in your Project

Sources

Thanks!

Posted 11-14-17 by Sam Bowne
Updated 8:30 pm 11-14-17
Updated 4-15-19