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I'm trying to run Docker + Flask + MySQL, and here is my docker-compose.yaml:

 services: dockerflaskexample: image: dockerflaskexample build: context: . dockerfile: ./Dockerfile ports: - 5002:5002 volumes: - app_code:/app depends_on: - mysql container_name: dockerflaskexample mysql: image: mysql:latest restart: always environment: MYSQL_ROOT_PASSWORD: webapp-db-root-password MYSQL_DATABASE: webapp-db MYSQL_USER: webapp-db-user MYSQL_PASSWORD: webapp-db-password volumes: - mysql_data:/var/lib/mysql ports: - "3306:3306" volumes: mysql_data: app_code:

... and in __init__.py:

# Establishing a connection to the MySQL database connection = pymysql.connect( host='mysql', database='webapp-db', user='webapp-db-user', password='webapp-db-password', port=3306, charset='utf8mb4', # adjust charset if necessary )

After I docker-compose up, the dockerflaskexample container always fails with the following error:

2024-04-02 16:25:05 pymysql.err.OperationalError: (2003, "Can't connect to MySQL server on 'mysql' ([Errno 111] Connection refused)")

However, if I simply run the Exited (3) container again, it works.

May I know why this is so? Is it because mysql isn't ready when the webapp started?

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  • It sounds like a timing issue. MySQL needs some time to get ready to accept connections, so if you try to connect immediately, it'll refuse the connection. depends_on only waits for the database container to be started. Not for it to be ready. Commented Apr 2, 2024 at 8:17
  • This might help solve it: stackoverflow.com/questions/42567475/… Commented Apr 2, 2024 at 8:18
  • you can add timeout in your container, moreover, you can add a health check to your database container also. adding health check to your db coriander makes sense. Commented Apr 2, 2024 at 10:25
  • Docker Compose wait for container X before starting Y also discusses this startup-order problem. Commented Apr 2, 2024 at 10:30

1 Answer 1

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Thank you all for suggestions!

I changed my docker-compose like this and it worked:

version: '3.4' services: dockerflaskexample: image: dockerflaskexample build: context: . dockerfile: ./Dockerfile ports: - 5002:5002 volumes: - app_code:/app depends_on: mysql: condition: service_healthy container_name: dockerflaskexample mysql: image: mysql:latest restart: always environment: MYSQL_ROOT_PASSWORD: webapp-db-root-password MYSQL_DATABASE: webapp-db MYSQL_USER: webapp-db-user MYSQL_PASSWORD: webapp-db-password volumes: - mysql_data:/var/lib/mysql ports: - "3306:3306" healthcheck: test: ["CMD", "mysqladmin" ,"ping", "-h", "localhost"] timeout: 20s retries: 10 volumes: mysql_data: app_code:
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