app.py

import hashlib
import json
import requests
from textwrap import dedent
from time import time
from uuid import uuid4
from urllib.parse import urlparse

from flask import Flask, jsonify, request

class Blockchain(object):
  def __init__(self):
    self.chain = []
    self.current_transactions = []
    self.nodes = set()

    # Create the genesis block
    self.new_block(previous_hash=1, proof=100)

  def register_node(self, address):
    """
    Add a new node to the list of nodes

    :param address: <str> Address of node. eg. 'http://192.168.0.5:5000'
    :return: None
    """
    parsed_url = urlparse(address)
    self.nodes.add(parsed_url.netloc)

  def valid_chain(self, chain):
    """
    Determine if a given blockchain is valid

    :param chain: <list> A blockchain
    :return: <bool> True if valid, False if not
    """

    last_block = chain[0]
    current_index = 1

    while current_index < len(chain):
      block = chain[current_index]
      print(f'{last_block}')
      print(f'{block}')
      print("\n--------------\n")
      # Check that the hash of the block is correct
      if block['previous_hash'] != self.hash(last_block):
        return False

      # Check that the Proof of Work is correct
      if not self.valid_proof(last_block['proof'], block['proof']):
        return False

      last_block = block
      current_index += 1

    return True

  def resolve_conflicts(self):
    """
    This is our Consensus Algorithm, it resolves conflicts
    by replacing our chain with the longest one in the network

    :return: <bool> True if our chain was replaced, False if not
    """

    neighbours = self.nodes
    new_chain = None

    # Only look for a longer chain
    max_length = len(self.chain)

    # Grab and verify the chains from all the nodes in the network
    for node in neighbours:
      response = requests.get(f'http://{node}/chain')

      if response.status_code == 200:
        length = response.json()['length']
        chain= response.json()['chain']

        # Check if the length is longer and the chain is valid
        if length > max_length and self.valid_chain(chain):
          max_length = length
          new_chain = chain

    if new_chain:
      self.chain = new_chain
      return True

    return False


  def new_block(self, proof, previous_hash=None):
    """
    Create a new Block in the Blockchain

    :param proof: <int> The proof given by Proof of Work algorithm
    :param previous_hash: (Optional) <str> Hash of previous block
    :return <dict> New Block
    """

    block = {
      'index': len(self.chain) + 1,
      'timestamp': time(),
      'transactions': self.current_transactions,
      'proof': proof,
      'previous_hash': previous_hash or self.hash(self.chain[-1])
    }

    # Reset current transactions list
    self.current_transactions = []

    self.chain.append(block)
    return block

  def new_transaction(self, sender, recipient, amount):
    """
    Creates a new transaction to go into the next mined Block
    :param sender: <str> Address of the Sender
    :param recipient: <str> Address of the Recipient
    :param amount: <int> Amount
    :return <int> The index of the Block that will hold this transaction
    """

    self.current_transactions.append({
      'sender': sender,
      'recipient': recipient,
      'amount': amount,
    })

    return self.last_block['index'] + 1

  @staticmethod
  def hash(block):
    """
    Creates a SHA-256 hash of a Block

    :param block: <dict> Block
    :return <str>
    """

    block_string = json.dumps(block, sort_keys=True).encode()
    return hashlib.sha256(block_string).hexdigest()

  @property
  def last_block(self):
    return self.chain[-1]

  def proof_of_work(self, last_proof):
    """
    Simple Proof of Work Algorithm
     - Find a number p' such that hash(pp') contains leading 4 zeroes, where p is the previous p'
     - p is the previous proof, and p'is the new proof

    :param last_proof: <int>
    :return: <int>
    """

    proof = 0

    while self.valid_proof(last_proof, proof) is False:
      proof += 1

    return proof

  @staticmethod
  def valid_proof(last_proof, proof):
    """
    Validates the Proof: Does hash(last_proof, proof) contain 4 leading zeroes?

    :param last_proof: <int> Previous Proof
    :param proof: <int> Current Proof
    :return <bool> True if correct, False if not
    """

    guess = f'{last_proof}{proof}'.encode()
    guess_hash = hashlib.sha256(guess).hexdigest()
    return guess_hash[:4] == "0000"


# Api code
app = Flask(__name__)

# Generate a globally unique address for this node
node_identifier = str(uuid4()).replace('-', '')

# Instanciate the blockchain
blockchain = Blockchain()

@app.route('/mine', methods=['GET'])
def mine():
  # We run the proof of work algorithm to get the next proof...
  last_block = blockchain.last_block
  last_proof = last_block['proof']
  proof = blockchain.proof_of_work(last_proof)

  # We must recive a reward for finding the proof
  # The sender is "0" to signify that this node has mined a new coin
  blockchain.new_transaction(
    sender="0",
    recipient=node_identifier,
    amount=1,
  )

  # Forget the new Block by adding it to the chain
  previous_hash = blockchain.hash(last_block)
  block = blockchain.new_block(proof, previous_hash)

  response = {
    'message': "New Block Forged",
    'index': block['index'],
    'transactions': block['transactions'],
    'proof': block['proof'],
    'previous_hash': block['previous_hash']
  }

  return jsonify(response), 200

@app.route('/transactions/new', methods=['POST'])
def new_transaction():
  values = request.get_json()

  # Check that the required fields are in the POST'ed data
  required = ['sender', 'recipient', 'amount']
  try:
    if not all(k in values for k in required):
      return 'Missing Values', 400
  except:
    return 'Failed to parse data. Make sure to send amount, recipient and sender as JSON', 400

  # Create a new Transaction
  index = blockchain.new_transaction(
    values['sender'],
    values['recipient'],
    values['amount'],
  )

  response = { 'message': f'Transaction will be added to Block {index}'}
  return jsonify(response), 200

@app.route('/chain', methods=['GET'])
def full_chain():
  response = {
    'chain': blockchain.chain,
    'chain': len(blockchain.chain)
  }

  return jsonify(response), 200

@app.route('/nodes/register', methods=['POST'])
def register_nodes():
  values = requests.get_json()

  nodes = values.get('nodes')
  if nodes is None:
    return "Error: Please supply a valid list of nodes", 400

  for node in nodes:
    blockchain.register_node(node)

  response = {
    'message': 'New nodes have been added',
    'total_nodes': list(blockchain.nodes),
  }

  return jsonify(response), 201

@app.route('/nodes/resolve', methods=['GET'])
def consensus():
  replaced = blockchain.resolve_conflicts()

  if replaced:
    response = {
      'message': 'Our chain was replaced',
      'new_chain': blockchain.chain
    }
  else:
    response = {
      'message': 'Our chain is authoritative',
      'chain': blockchain.chain
    }

  return jsonify(response), 200

if __name__ == "__main__":
  app.run(host='0.0.0.0', port=5000)