tp2.py

custom_lock = "SELECT pg_advisory_xact_lock(%s);"

def lock(conn, lock_key):
    cur = conn.cursor()
    try:
        # Acquire the custom lock outside of the transaction
        cur.execute(custom_lock, (lock_key,))
        conn.commit()
    except Exception as e:
        # Handle exceptions
        conn.rollback()
        print(f"Error acquiring lock: {e}")
    finally:
        # Close the cursor
        cur.close()


custom_unlock = "SELECT pg_advisory_unlock(%s);"


def unlock(conn, lock_key):
    cur = conn.cursor()
    try:
        # Release the custom lock outside of the transaction
        cur.execute(custom_unlock, (lock_key,))
        conn.commit()
    except Exception as e:
        # Handle exceptions
        conn.rollback()
        print(f"Error releasing lock: {e}")
    finally:
        # Close the cursor
        cur.close()


"""lock_key: C'est un entier qui peut être l'ID de la transaction par exemple qu'on peut obtenir comme suit:"""


def get_transaction_ID(cur):
    # Execute the txid_current() function to obtain the current transaction ID
    cur.execute("SELECT txid_current();")
    # Fetch the result
    transaction_id = cur.fetchone()[0]
    print(f"Current Transaction ID: {transaction_id}")

def get_account_id(conn, sender):
    cur = conn.cursor()
    try:
        select_sender_account_query = sql.SQL("""
            SELECT account_id
            FROM account AS a
            JOIN customer AS c ON c.customer_id = a.customer_id
            WHERE c.first_name = %s;
        """)
        cur.execute(select_sender_account_query, (sender,))
        sender_account_id = cur.fetchone()[0]
        return sender_account_id
    except Exception as e:
        print(f"Error getting account id: {e}")
    finally:
        cur.close()


def pessimistic_method(conn, sender, recipient, amount, transfert_name="0"):
    cur = conn.cursor()
    conn.autocommit = False
    cur.execute("START TRANSACTION;")
     
    try:
        sender_account_id = get_account_id(conn, sender)
        recipient_account_id = get_account_id(conn, recipient)
    
        # todo deadlock ?
        print(f"{transfert_name}: Waiting for lock {sender} and {recipient}")
        lock(conn, sender_account_id)
        print(f"{transfert_name}: Lock {sender}({sender_account_id}) acquired")
        time.sleep(3)
        lock(conn, recipient_account_id)
        print(f"{transfert_name}: Lock {recipient}({recipient_account_id}) acquired")
        perform_transfer(conn, sender, recipient, amount)
        print(f"{transfert_name}: Transfer performed : {sender} -> {recipient} ({amount})")
        
       # unlock(conn, sender_account_id)
       # unlock(conn, recipient_account_id)
        
        conn.commit()
        print(f'Transfer completed: {amount} from {sender} to {recipient}')
    except psycopg2.Error as e:
        # Handle exceptions
        conn.rollback()
        print(f"Error: {e}")

    finally:
        # Close the cursor
        cur.close()

# test méthode pessimistes
Host = "localhost"
Port = 5432
User = "postgres"
Password = "postgres"
Database = "postgres"

conn1 = psycopg2.connect(dbname=Database,
                         host=Host,
                         user=User,
                         password=Password,
                         port=Port)

conn2 = psycopg2.connect(dbname=Database,
                         host=Host,
                         user=User,
                         password=Password,
                         port=Port)

try:
    process1 = multiprocessing.Process(target=pessimistic_method, args=(conn1, 'Brandy', 'Crystal', 7336, "1"))
    process2 = multiprocessing.Process(target=pessimistic_method, args=(conn2, 'Brandy', 'George', 7336, "2"))

    process1.start()
    process2.start()

    process1.join()
    process2.join()

finally:
    # Retrieve the message after COMMIT using the connection
    print("Connection status:", conn1.status)
    print("Connection status:", conn2.status)
    # Close the connection
    conn1.close()
    conn2.close()