創(chuàng)建一個新的Python文件，名為blockchain.py，我們所有的邏輯都在一個文件完成。

我們創(chuàng)建一個BlockChain類，其構(gòu)造器會創(chuàng)建兩個列表，一個存儲區(qū)塊鏈，另一個存儲交易。下面是我們這個類的第一個版本：

class Blockchain(object):
    def __init__(self):
        self.chain = []
        self.current_transactions = []
        
    def new_block(self):
        # Creates a new Block and adds it to the chain
        pass
    
    def new_transaction(self):
        # Adds a new transaction to the list of transactions
        pass
    
    @staticmethod
    def hash(block):
        # Hashes a Block
        pass

    @property
    def last_block(self):
        # Returns the last Block in the chain
        pass

我們的BlockChain類負(fù)責(zé)管理整個區(qū)塊鏈，它會存儲交易并為新增區(qū)塊等操作提供輔助方法。下面，我們來實(shí)現(xiàn)這些方法。

每個區(qū)塊都有一個索引（index），一個時間戳（timestamp），一系列交易，一個工作量證明（稍后詳述）和前置區(qū)塊的哈希值。下面是單個區(qū)塊的一個簡單實(shí)例：

block = {
    "index": 1,
    "timestamp": 1506057125.900785,
    "transactions": [
        {
            "sender": "8527147fe1f5426f9dd545de4b27ee00",
            "recipient": "a77f5cdfa2934df3954a5c7c7da5df1f",
            "amount": 5,
        }
    ],
    "proof": 324984774000,
    "previous_hash": "2cf24dba5fb0a30e26e83b2ac5b9e29e1b161e5c1fa7425e73043362938b9824"
}

顯而易見，所有的區(qū)塊會構(gòu)成一條鏈---因?yàn)槊總€區(qū)塊都保存了前一區(qū)塊的hash值。這就是區(qū)塊鏈不可篡改的重要原因：如果攻擊者損壞了某一區(qū)塊，那么后面所有的區(qū)塊都會作廢。
如果你不明白上面的話，請花一些時間理解，因?yàn)檫@是區(qū)塊鏈的核心思想。

我們需要一個方法來向區(qū)塊中添加交易記錄，這里命名為new_transaction()，代碼寫的十分直白易懂：

class Blockchain(object):
    ...
    
    def new_transaction(self, sender, recipient, amount):
        """
        Creates a new transaction to go into the next mined Block
        :param sender:  Address of the Sender
        :param recipient:  Address of the Recipient
        :param amount:  Amount
        :return:  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

在new_transaction()方法將交易添加進(jìn)區(qū)塊之后，區(qū)塊索引將會被返回，該區(qū)塊將可能被開采為鏈的最新區(qū)塊，這在之后用戶提交交易的時候十分有用。

當(dāng)BlockChain類初始化的時候，我們需要產(chǎn)生一個創(chuàng)世區(qū)塊（genesis block，即沒有前置區(qū)塊的區(qū)塊）作為區(qū)塊鏈的第一個區(qū)塊。我們還需要添加一個proof字段在創(chuàng)世區(qū)塊中作為挖礦的結(jié)果（或者說本次工作量的證明），我們將在后文繼續(xù)討論挖礦。
除了產(chǎn)生創(chuàng)世區(qū)塊，我們還需要完成一些其他輔助方法(new_block(),new_transaction()和hash())：

import hashlib
import json
from time import time


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

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

    def new_block(self, proof, previous_hash=None):
        """
        Create a new Block in the Blockchain
        :param proof:  The proof given by the Proof of Work algorithm
        :param previous_hash: (Optional)  Hash of previous Block
        :return:  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 the current list of transactions
        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:  Address of the Sender
        :param recipient:  Address of the Recipient
        :param amount:  Amount
        :return:  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

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

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

        # We must make sure that the Dictionary is Ordered, or we"ll have inconsistent hashes
        block_string = json.dumps(block, sort_keys=True).encode()
        return hashlib.sha256(block_string).hexdigest()
  

上面的代碼十分直白，我還添加了一些注釋幫助理解。我們幾乎完成了表示一個區(qū)塊鏈的工作。但此時，你應(yīng)該思考下一個區(qū)塊是如何產(chǎn)生或者說被開采出來的。

工作量證明（PoW）算法是用來產(chǎn)生或開采區(qū)塊的一種機(jī)制，PoW的目標(biāo)是找到一個符合要求的數(shù)字，從算力的角度來說這個數(shù)字對任何人來說都很難找到卻十分容易驗(yàn)證（是否符合要求）。這就是PoW算法的核心思想。
我們舉一個非常簡單的例子來幫助理解：
假定我們需要找到一個整數(shù)y，使得他和整數(shù)x的乘積的哈希值以0結(jié)尾，即hash(x*y) = ac23dc...0。如果x=5那么用Python實(shí)現(xiàn)如下：

from hashlib import sha256
x = 5
y = 0  # We don"t know what y should be yet...
while sha256(f"{x*y}".encode()).hexdigest()[-1] != "0":
    y += 1
print(f"The solution is y = {y}")

第一個符合要求的數(shù)是y=21，因?yàn)椋?/p>

hash(5 * 21) = 1253e9373e...5e3600155e860

在比特幣世界中，PoW算法被稱為哈?，F(xiàn)金（Hashcash）,它和我們上面的例子沒有本質(zhì)區(qū)別。在這算法中，礦工們開始了解決問題的競賽，優(yōu)勝者可以產(chǎn)生一個新的區(qū)塊。通常來說，難度取決于限制字符的數(shù)量。礦工將會因?yàn)檎业揭粋€合法的解答收到一些比特幣作為獎勵，整個比特幣網(wǎng)絡(luò)能夠很容易驗(yàn)證礦工挖掘的區(qū)塊是否合法有效。

下面來為我們的區(qū)塊鏈實(shí)現(xiàn)一個類似的算法，我們的規(guī)則將會和上面的例子十分接近：找到一個數(shù)p，使得它與前置區(qū)塊的哈希值由4個0開頭。

import hashlib
import json

from time import time
from uuid import uuid4


class Blockchain(object):
    ...
        
    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: 
        :return: 
        """

        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:  Previous Proof
        :param proof:  Current Proof
        :return:  True if correct, False if not.
        """

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

我們可以通過設(shè)置前導(dǎo)0的個數(shù)調(diào)整算法的難度，但4個足夠了，你會發(fā)現(xiàn)增加一個0會使找到一個答案的時間大大增加。我們的類幾乎完成了，現(xiàn)在我們將通過HTTP請求與區(qū)塊鏈進(jìn)行交互。

我們將使用Flask，它是一個輕量級的框架，可以很容易將一個網(wǎng)絡(luò)節(jié)點(diǎn)映射為Python函數(shù)，這讓我們可以通過HTTP請求與區(qū)塊鏈交互。我們將創(chuàng)建以下方法：

/transactions/new建立一個新的區(qū)塊。

/mine告訴服務(wù)器開采一個新的區(qū)塊

/chain返回整個區(qū)塊鏈

我們的每個服務(wù)器將對應(yīng)區(qū)塊鏈網(wǎng)絡(luò)中的一個單一節(jié)點(diǎn)。下面是樣板代碼：

import hashlib
import json
from textwrap import dedent
from time import time
from uuid import uuid4

from flask import Flask


class Blockchain(object):
    ...


# Instantiate our Node
app = Flask(__name__)

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

# Instantiate the Blockchain
blockchain = Blockchain()


@app.route("/mine", methods=["GET"])
def mine():
    return "We"ll mine a new Block"
  
@app.route("/transactions/new", methods=["POST"])
def new_transaction():
    return "We"ll add a new transaction"

@app.route("/chain", methods=["GET"])
def full_chain():
    response = {
        "chain": blockchain.chain,
        "length": len(blockchain.chain),
    }
    return jsonify(response), 200

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

下面是簡單的解釋：

15行：實(shí)例化節(jié)點(diǎn)，關(guān)于Flask點(diǎn)擊Quick Start

18行：為節(jié)點(diǎn)創(chuàng)建一個隨機(jī)名字

21行：實(shí)例化BlockChain類

24-26行：創(chuàng)建/mine節(jié)點(diǎn)，這是一個GET請求。

28-30行：創(chuàng)建/transactions/new節(jié)點(diǎn)，因?yàn)樾枰l(fā)送數(shù)據(jù)，所以是POST請求。

32-38行：創(chuàng)建/chain節(jié)點(diǎn)，返回整個區(qū)塊鏈

40-41行：運(yùn)行服務(wù)器5000端口

用戶會想服務(wù)器發(fā)送交易請求，格式類似下面這樣：

{
 "sender": "my address",
 "recipient": "someone else"s address",
 "amount": 5
}

因?yàn)槲覀円呀?jīng)實(shí)現(xiàn)了將交易加入?yún)^(qū)塊的方法，所以剩余部分十分容易：

import hashlib
import json
from textwrap import dedent
from time import time
from uuid import uuid4

from flask import Flask, jsonify, request

...

@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"]
    if not all(k in values for k in required):
        return "Missing values", 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), 201

挖礦節(jié)點(diǎn)很簡單但也很神奇，他需要完成以下任務(wù)：

計算執(zhí)行PoW算法

通過添加一筆交易獎勵礦工1比特幣

產(chǎn)生新的區(qū)塊并添加入鏈

import hashlib
import json

from time import time
from uuid import uuid4

from flask import Flask, jsonify, request

...

@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 receive 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,
    )

    # Forge 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

需要注意的是接受被開采區(qū)塊的地址就是我們的節(jié)點(diǎn)，并且我們的大部分工作就是和BlockChain類的方法交互。我們已經(jīng)完成了這部分，現(xiàn)在可以開始和區(qū)塊鏈交互了。

你可以使用簡單古老的cURL或者Postman來和這些網(wǎng)絡(luò)中的API交互，首先啟動服務(wù)器：

$ python blockchain.py
* Running on http://127.0.0.1:5000/ (Press CTRL+C to quit)

讓我們發(fā)送一個GET請求來開采一個區(qū)塊：

http://localhost:5000/mine

再向http://localhost:5000/transactions/new發(fā)送一個POST請求，參數(shù)是JSON格式的交易數(shù)據(jù)：

如果不想用Postman，cURL也可以做到：

$ curl -X POST -H "Content-Type: application/json" -d "{
 "sender": "d4ee26eee15148ee92c6cd394edd974e",
 "recipient": "someone-other-address",
 "amount": 5
}" "http://localhost:5000/transactions/new"

我重啟了服務(wù)器并開采了兩個區(qū)塊，所以現(xiàn)在總共有3個了，通過http://localhost:5000/chain節(jié)點(diǎn)可以獲取整個區(qū)塊：

{
  "chain": [
    {
      "index": 1,
      "previous_hash": 1,
      "proof": 100,
      "timestamp": 1506280650.770839,
      "transactions": []
    },
    {
      "index": 2,
      "previous_hash": "c099bc...bfb7",
      "proof": 35293,
      "timestamp": 1506280664.717925,
      "transactions": [
        {
          "amount": 1,
          "recipient": "8bbcb347e0634905b0cac7955bae152b",
          "sender": "0"
        }
      ]
    },
    {
      "index": 3,
      "previous_hash": "eff91a...10f2",
      "proof": 35089,
      "timestamp": 1506280666.1086972,
      "transactions": [
        {
          "amount": 1,
          "recipient": "8bbcb347e0634905b0cac7955bae152b",
          "sender": "0"
        }
      ]
    }
  ],
  "length": 3
}

我們已經(jīng)擁有了一個能接收交易的初級區(qū)塊鏈，并且能夠開采出新的區(qū)塊。但整個區(qū)塊鏈最核心的是去中心化，如果去中心了，我們又如何保證所有節(jié)點(diǎn)對應(yīng)的是統(tǒng)一區(qū)塊鏈呢？這就是共識問題，如果我們希望網(wǎng)絡(luò)中有不止一個節(jié)點(diǎn)，就必須實(shí)現(xiàn)共識算法。

在實(shí)現(xiàn)共識算法之前，我們需要讓節(jié)點(diǎn)知道有其他節(jié)點(diǎn)加入了網(wǎng)絡(luò)。網(wǎng)絡(luò)中的每一個節(jié)點(diǎn)應(yīng)該存留其他全部節(jié)點(diǎn)的注冊表，因此我們需要一些其他的服務(wù)器節(jié)點(diǎn)：

/nodes/register用來從URL中接收一系列節(jié)點(diǎn)

/nodes/resolve實(shí)現(xiàn)共識算法，并解決沖突以保證節(jié)點(diǎn)擁有正確的鏈

我們需要修改BlockChain類的構(gòu)造器并提供一個方法來注冊節(jié)點(diǎn)：

...
from urllib.parse import urlparse
...


class Blockchain(object):
    def __init__(self):
        ...
        self.nodes = set()
        ...

    def register_node(self, address):
        """
        Add a new node to the list of nodes
        :param address:  Address of node. Eg. "http://192.168.0.5:5000"
        :return: None
        """

        parsed_url = urlparse(address)
        self.nodes.add(parsed_url.netloc)

現(xiàn)在可以使用set()來存儲節(jié)點(diǎn)列表。這保證了節(jié)點(diǎn)的添加是冪等的，即一個節(jié)點(diǎn)無論添加多少次只會出現(xiàn)一次。

當(dāng)一個節(jié)點(diǎn)的區(qū)塊鏈和另一節(jié)點(diǎn)的區(qū)塊鏈不同時，沖突就發(fā)生了。為了解決這個問題，我們需要制定規(guī)則：最長有效鏈最有權(quán)威性，即網(wǎng)絡(luò)中最長的那條鏈?zhǔn)钦嬲膮^(qū)塊鏈。使用這個算法，我們能夠達(dá)成大多數(shù)節(jié)點(diǎn)的一致。

...
import requests


class Blockchain(object)
    ...
    
    def valid_chain(self, chain):
        """
        Determine if a given blockchain is valid
        :param chain:  A blockchain
        :return:  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("
-----------
")
            # 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:  True if our chain was replaced, False if not
        """

        neighbours = self.nodes
        new_chain = None

        # We"re only looking for chains longer than ours
        max_length = len(self.chain)

        # Grab and verify the chains from all the nodes in our 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

        # Replace our chain if we discovered a new, valid chain longer than ours
        if new_chain:
            self.chain = new_chain
            return True

        return False

valid_chain()通過遍歷每一區(qū)塊并檢查proof和hash的正確與否判斷鏈的有效性。resolve_conflicts()遍歷所有節(jié)點(diǎn)，并通過上面的方法驗(yàn)證其有效性。如果一個有效鏈的長度大于當(dāng)前鏈，那么當(dāng)前鏈將會被替換?，F(xiàn)在添加兩個API端口，一個用來添加節(jié)點(diǎn)，一個用來解決沖突：

@app.route("/nodes/register", methods=["POST"])
def register_nodes():
    values = request.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

現(xiàn)在你可以用不同的計算機(jī)來構(gòu)建網(wǎng)絡(luò)中的這些節(jié)點(diǎn)，當(dāng)然也可以用同一機(jī)器的不同端口。例如，將5001端口也注冊進(jìn)區(qū)塊鏈網(wǎng)絡(luò)：

現(xiàn)在，如果我在第二個節(jié)點(diǎn)開采一個新的區(qū)塊，當(dāng)我在節(jié)點(diǎn)1調(diào)用GET /nodes/resolve的時候，共識算法會保證鏈被更新到現(xiàn)有網(wǎng)絡(luò)中的最長鏈：

現(xiàn)在你可以找一些朋友來和你一起測試這個區(qū)塊鏈了。

我希望這篇文章能夠激發(fā)你的靈感，畢竟我對加密貨幣十分狂熱，我相信他會改變我們對金融、政府和記錄存儲的思考方式。

Update：我計劃寫這個話題的第二部分，我將進(jìn)一步拓展這個區(qū)塊鏈并支持交易驗(yàn)證（ Transaction Validation Mechanism），同時也將討論如何將你的區(qū)塊鏈產(chǎn)業(yè)化。