Complete section: The Blockchain Network - Flask API and Pub/Sub

Author: 15Dkatz

README.md

@@ -23,3 +23,11 @@ Make sure to activate the virtual environment.
 ```
 python3 -m backend.app
 ```
+
+**Run a peer instance**
+
+Make sure to activate the virtual environment.
+
+```
+export PEER=True && python3 -m backend.app
+```

backend/app/__init__.py

@@ -1,9 +1,15 @@
+import os
+import requests
+import random
+
 from flask import Flask, jsonify
 
 from backend.blockchain.blockchain import Blockchain
+from backend.pubsub import PubSub
 
 app = Flask(__name__)
 blockchain = Blockchain()
+pubsub = PubSub(blockchain)
 
 @app.route('/')
 def route_default():
@@ -19,6 +25,24 @@ def route_blockchain_mine():
 
     blockchain.add_block(transaction_data)
 
-    return jsonify(blockchain.chain[-1].to_json())
+    block = blockchain.chain[-1]
+    pubsub.broadcast_block(block)
+
+    return jsonify(block.to_json())
+
+ROOT_PORT = 5000
+PORT = ROOT_PORT
+
+if os.environ.get('PEER') == 'True':
+    PORT = random.randint(5001, 6000)
+
+    result = requests.get(f'http://localhost:{ROOT_PORT}/blockchain')
+    result_blockchain = Blockchain.from_json(result.json())
+
+    try:
+        blockchain.replace_chain(result_blockchain.chain)
+        print('\n -- Successfully synchronized the local chain')
+    except Exception as e:
+        print(f'\n -- Error synchronizing: {e}')
 
-app.run()
+app.run(port=PORT)

backend/blockchain/block.py

@@ -73,6 +73,13 @@ def genesis():
         """
         return Block(**GENESIS_DATA)
 
+    @staticmethod
+    def from_json(block_json):
+        """
+        Deserialize a block's json representation back into a block instance.
+        """
+        return Block(**block_json)
+
     @staticmethod
     def adjust_difficulty(last_block, new_timestamp):
         """

backend/blockchain/blockchain.py

@@ -36,6 +36,19 @@ def to_json(self):
         """
         return list(map(lambda block: block.to_json(), self.chain))
 
+    @staticmethod
+    def from_json(chain_json):
+        """
+        Deserialize a list of serialized blocks into a Blokchain instance.
+        The result will contain a chain list of Block instances.
+        """
+        blockchain = Blockchain()
+        blockchain.chain = list(
+            map(lambda block_json: Block.from_json(block_json), chain_json)
+        )
+
+        return blockchain
+
     @staticmethod
     def is_valid_chain(chain):
         """

backend/pubsub.py

@@ -4,26 +4,63 @@
 from pubnub.pnconfiguration import PNConfiguration
 from pubnub.callbacks import SubscribeCallback
 
+from backend.blockchain.block import Block
+
 pnconfig = PNConfiguration()
 pnconfig.subscribe_key = 'sub-c-666638f6-ec63-11e9-b715-9abbdb5d0da2'
 pnconfig.publish_key = 'pub-c-82bf7695-ce5e-4bc5-9cd9-a8f8147dc2a8'
-pubnub = PubNub(pnconfig)
-
-TEST_CHANNEL = 'TEST_CHANNEL'
 
-pubnub.subscribe().channels([TEST_CHANNEL]).execute()
+CHANNELS = {
+    'TEST': 'TEST',
+    'BLOCK': 'BLOCK'
+}
 
 class Listener(SubscribeCallback):
+    def __init__(self, blockchain):
+        self.blockchain = blockchain
+
     def message(self, pubnub, message_object):
-        print(f'\n-- Incoming message_object: {message_object}')
+        print(f'\n-- Channel: {message_object.channel} | Message: {message_object.message}')
+
+        if message_object.channel == CHANNELS['BLOCK']:
+            block = Block.from_json(message_object.message)
+            potential_chain = self.blockchain.chain[:]
+            potential_chain.append(block)
 
+            try:
+                self.blockchain.replace_chain(potential_chain)
+                print('\n -- Successfully replaced the local chain')
+            except Exception as e:
+                print(f'\n -- Did not replace chain: {e}')
 
-pubnub.add_listener(Listener())
+class PubSub():
+    """
+    Handles the publish/subscribe layer of the application.
+    Provides communication between the nodes of the blockchain network.
+    """
+    def __init__(self, blockchain):
+        self.pubnub = PubNub(pnconfig)
+        self.pubnub.subscribe().channels(CHANNELS.values()).execute()
+        self.pubnub.add_listener(Listener(blockchain))
+
+    def publish(self, channel, message):
+        """
+        Publish the message object to the channel.
+        """
+        self.pubnub.publish().channel(channel).message(message).sync()
+
+    def broadcast_block(self, block):
+        """
+        Broadcast a block object to all nodes.
+        """
+        self.publish(CHANNELS['BLOCK'], block.to_json())
 
 def main():
+    pubsub = PubSub()
+
     time.sleep(1)
 
-    pubnub.publish().channel(TEST_CHANNEL).message({ 'foo': 'bar' }).sync()
+    pubsub.publish(CHANNELS['TEST'], { 'foo': 'bar' })
 
 if __name__ == '__main__':
     main()

requirements.txt

@@ -1,3 +1,4 @@
 pytest==5.1.2
 Flask==1.1.1
-pubnub==4.1.6
+pubnub==4.1.6
+requests==2.22.0