Implement Your Own Blockchain

A Python implementation of Blockchain. You will need some certain amount of basic knowledge about Blockchain in order to understand the source code. To get the app running, you also need Python3.6 knowledge, Flask (A Python framework) and some basic REST API experiences.

Goal

The goal of this project, is to implement the basics of a Blockchain into a Python web application. We will define a Blockchain Python class, this class contains three properties:

• current_transactions - hold all the new transactions for new block
• chain - A list contains all the existing blocks
• nodes - A set contains all the node address in the Blockchain network

Flask-0.12.2+ will be used as API backend to serve REST API calls. When you run this Python application, you should be able to do the following REST API calls:

• Mining - To tell the server to mine a new block
• Transaction - To create a new transaction to be added to a block
• Chain - To check the full Blockchain
• Register - To accept a list of new nodes
• Resolve - To use our own Consensus algorithm to resolve conflicts.

Prerequisite

• OS: Ubuntu 17.04
• git
• Python3.6+
• Flask0.12.2+
• requests2.18.4+
• CURL or Postman

Blockchain Basics:

• Block: A block in Blockchain is a data structure (represented by files) that holds data permanently. A block represents the present and contains information about its past and future. Each time a block is completed, it gives way to the next block in the blockchain. The completed block is a permanent record of transactions in the past the the new transactions are recorded in the current one. In our code, an example block looks like:

block = {
    'index': 'integer - len(blockchain) + 1',
    'timestamp': 'float - the time this block is completed',
    'transaction': 'list of dicts - all the transactions recorded',
    'proof': 'integer - the number that been used to solve the calcualtion problem, will be used for chain validation',
    'previous_hash': 'string - the hash string of previous block',
}

• Proof of Work: PoW is basically how new blocks are mined on the blockchain. In our implementation, the goal of PoW is to discover a number which solves a problem. We define the mathmatical problem as: The hash string of the last proof (from last block) and the new proof contains 4 leading 0s. sha256(last_proof+proof)[:4] == 0000. This new proof number must be difficult to calculate but easy to verify.
• Transaction: A transaction is a piece of information that contains at least:
  • An input/sender address: Which address was used to send the coins.
  • An amount: The amount of bitcoins that will be sent.
  • An output/receiver address: The receiver's address.

# Sample transaction
{
 "sender": "d4ee26eee15148eefdsafdsafdsaf",
 "recipient": "fdsaf87687dsafdsaf6876fdsa",
 "amount": 3
}

• Mining: Mining is the process of collecting new transactions, add new transactions into a new block, append the new block into the existing chain:

  • Calculate the PoW
  • Reward the miner (In our case 4 coins)
  • Create the new block and add current transactions
  • Append the new block to the existing block chain

• Consensus: Since Blockchain is decentralized, how do we ensure that every node refelcts the same chain? This is called "The problem of Consensus". This problem occurs when one node has different chain to another node. In our code, we use the rule of "the longest valid chain is authoritative". The node that contains the longest chain on the blockchain network is the valid one.