Talking between containers in docker-compose

When using docker-compose, we can define several containers which are connected through a network. How can we speak between containers? More specifically, suppose one container has a MySQL database and the other container has some R/Python/Julia container in which you want to query/append data. Usually we would only refer to the IP of the database when defining the connection but all of these containers sort of share the same IP, right?

I tried this and it could not connect to the localhost because it couldn’t find the database. After browsing a bit, it was easier that I thought. Here’s a docker-compose.yml with a MySQL image:

version: '2.1'
services:
  # Database to append data to
  db:
    hostname: sql_db
    image: mysql:8.0
    restart: always
    ports:
      - 3306:3306
    environment:
      - MYSQL_ROOT_PASSWORD=123456
    volumes:
      - ./sql:/docker-entrypoint-initdb.d
      - ./data:/var/lib/mysql
      - ./sql_conf:/etc/mysql/conf.d

Notice that there’s a hostname argument? That is the IP we’ll use. Assume that the entrypoint for Docker defines a database called test with a table named test_table. We make up some username/password just for this example. In Python parlance, for example, we could define the connection like this and query the contents:

# Make sure you have the package mysql and mysql-connector from pip as well
import pandas as pd
import sqlalchemy

db_username='test_user'
db_password='123'

# Here is the name of the container
db_ip='sql_db'
db_name='test'
db_port=3306
db_connection=sqlalchemy.create_engine('mysql+mysqlconnector://{0}:{1}@{2}:{3}/{4}?auth_plugin=mysql_native_password'.format(db_username, db_password, db_ip, db_port, db_name))
df=pd.read_sql_query("SELECT * FROM test.test_table", db_connection)

Note that this works when the code above is executed from another container inside the network (and not locally). Although this looks like a simple trick it opens a whole new world for talking between containers in a stable manner.