Cannot connect to MySQL from python on docker-compose

Question

I'm trying to build a docker-compose for my flask-app running with mysql. The problem is: my flask cannot access to mysql (both in docker-compose) and keeps returning this problem: pymysql.err.OperationalError: (2003, "Can't connect to MySQL server on '127.0.0.1' ([Errno 111] Connection refused)") .

So I decided to write a simple flask app and copy the connection to mysql over. The Flask app on docker-compose works fine alone and mysql also works fine because I can write query in dbeaver. But it will raise error when ever I tried to connect to mysql in the script: connection = pymysql.connect(host='127.0.0.1') with the similar error I mentioned above

The code will break immediately when it hits host=127.0.0.1 .

Here is my app.py:

from flask import Flask
import pymysql

app = Flask(__name__)
    
@app.route('/')
def index():
    return '<h1>Login</h1>'
    
if __name__ == '__main__':
    connection = pymysql.connect(host='127.0.0.1',
                                 user='root',
                                 password='password',
                                 port=33061,
                                 cursorclass=pymysql.cursors.DictCursor)
    cur = connection.cursor()
    cur.execute(''' SELECT * FROM data_warehouse.ofactory_results''')
    order_no = cur.fetchone()
    print(order_no)
    
    print('STARTTTTTTT!')
    app.run(debug=True)

Here is my docker-compose:

version: "3.9"
    
services:
  app:
    build:
      context: .
    ports:
      - 5000:5000
    
  mysql_db:
    container_name: mysql_db
    image: mariadb:latest
    restart: always
    ports:
      - "0.0.0.0:33061:3306"
    environment: 
      - MYSQL_ROOT_PASSWORD=password
      - MYSQL_DATABASE=data_warehouse
      - MYSQL_USER=debezium
          - MYSQL_PASSWORD=debezium
     volumes:
       - ./mysql:/var/lib/mysql
       - ./mysql/my.cnf:/etc/mysql/my.cnf

Here is my dockerfile:

FROM python:3.9
COPY requirements.txt .     
RUN pip install -r requirements.txt
COPY . .
CMD ["python", "app.py"]

Here is the error:

Traceback (most recent call last):
prep-app-1  |   File "/usr/local/lib/python3.9/site-packages/pymysql/connections.py", line 613, in connect
prep-app-1  |     sock = socket.create_connection(
prep-app-1  |   File "/usr/local/lib/python3.9/socket.py", line 844, in create_connection
prep-app-1  |     raise err
prep-app-1  |   File "/usr/local/lib/python3.9/socket.py", line 832, in create_connection
prep-app-1  |     sock.connect(sa)
prep-app-1  | ConnectionRefusedError: [Errno 111] Connection refused
prep-app-1  | 
prep-app-1  | During handling of the above exception, another exception occurred:
prep-app-1  | 
prep-app-1  | Traceback (most recent call last):
prep-app-1  |   File "//app.py", line 10, in <module>
prep-app-1  |     connection = pymysql.connect(host='127.0.0.1',
prep-app-1  |   File "/usr/local/lib/python3.9/site-packages/pymysql/connections.py", line 353, in __init__
prep-app-1  |     self.connect()
prep-app-1  |   File "/usr/local/lib/python3.9/site-packages/pymysql/connections.py", line 664, in connect
prep-app-1  |     raise exc
prep-app-1  | pymysql.err.OperationalError: (2003, "Can't connect to MySQL server on '127.0.0.1' ([Errno 111] Connection refused)")
prep-app-1 exited with code 1

Answer 1

localhost inside your docker container != localhost of your host machine.

You may use:

connection = pymysql.connect(host='mysql_db',
                             user='root',
                             password='password',
                             port=3306,
                             cursorclass=pymysql.cursors.DictCursor)

Note mysql_db which is name of your db service. Docker, internally will resolve that name into container ip address in docker network.