Blockchain

This is a blockchain

Git is also a blockchain

• Each commit has a unique hash, based on its content
• Each commit has a parent commit
• Each repo has its own copy of all the commits

Ethereum

You can have regular addresses, used by people

These addresses have a balance in ether that can transfer to any other address

Addresses

There's also contract addresses, created when you deploy a new contract

These addresses also have a balance, but they cannot initiate a tx by their own

They also contain code and memory to persist data

Transactions

Each Ethereum block contains transactions

A transactions is made basically by:

• from
• to
• value
• data

Transfer

• from the sender
• to the receiver
• value ether
• data empty

Contract Creation

• from the sender
• to empty
• value ether (if any)
• data contract code

Contract Invocation

• from the sender
• to the contract
• value ether (if any)
• data method with params

Gas

Each operation costs gas

Each transaction defines how much ether is willing to pay per gas consumed

Higher the price, the tx will be mined faster

Lower the price, the tx will be mined slower, or not mined at all

Smart Contract

• Can be written in any language, as long as it compiles to Ethereum bytecode
• Most popular languages are Solidity and Serpent (python)
• Contracts are the analog to classes in OOP
• Have their own space in memory to store their instance properties

• Have no access to the outside word, no network requests, no file system. Only data in blockchain can be consumed
• Contracts cannot initiate actions by their own
• The code cannot be modified
• Cannot be destroyed from the outside
• Have limited execution stack

Solidity

import { Company } from './company.sol';

contract Employee {
  enum Roles { frontend, backend, scrum }

  struct Skill {
    string name;
    int8 level;
  }

  uint public salary;
  uint8 private age;
  string name;
  boolean active;
  Company company;
  address account;
  Employee[] bosses;
  Skills[] skills;
  Roles role;

  function Employee(address companyAddress, address employeeAddress) public {
    salary = 100;
    age = 26;
    name = 'Diego';
    active = true;
    company = Company(companyAddress);
    account = employeeAddress;
    role = Roles.frontend;
  }
}

contract Employee {
  string public name;
  string lastName;
  uint8 age;
  boolean updatedOnce = false;

  function getFullName() constant public returns (string _name, string _lastName) {
    _name = name;
    _lastName = lastName;
  }

  function multiply(uint a, uint b) pure public returns (uint) {
    return a * b;
  }

  modifier updateOnce() {
    if (!updatedOnce) {
      _;
    }

    updatedOnce = true;
  }

  modifier adult(uint8 _age) {
    if (_age > 18) {
      _;
    }
  }

  function setAge(uint8 _age) adult(_age) updateOnce public {
    age = _age;
  }
}

Truffle & Ganache

npm install truffle ganache-cli

npx truffle unbox metacoin
# or
./node_modules/.bin/truffle unbox metacoin

# Start the dev node
npx ganache

# Build the contracts
npx truffle build

# Run the unit tests
npx truffle test

# Run the console of the dev node
npx truffle console