Dockerized flask server is not respnsive from within the container, but is responsive when is run on host

Question

I'm trying to setup a Flask <-> MongoDB <-> mongo_express application. I have 3 containers defined in my docker-compose.yml, and I start them successfully. However, while the Mongo part is OK (I can access the DB via the express api at localhost:8081), Flask can't access the DB.

What I'm looking for:

1. I want to be able to send requests from host machine (or any other in the network) to Flask (running on 0.0.0.0:5000), from Flask to DB (running on localhost:27017, accessed using pymongo wrapper).
2. Also, I want to be able to have access to mongo_express (on localhost:8081) and from it to the DB [This part is already working!]

---

In order to debug it, I removed the Flask container from the docker-compose.yml, restarted, and run it locally, and voila! everything works (meaning my pytest runs are ok - I have a test where I send a request to the Flask server, which in turn, uses the pymongo wrapper to access the DB, and return data from it to the client).

I guess my network configuration is flawed, but I don't understand where.

Here is my docker-compose.yml (Flask is commented out, since it is currently running locally):

version: "3.8"
services:
    # mgmt_server:
    #     build: ./server # Dockerfile just copies py files, installs pip requirements, and runs "python server.py"
    #     container_name: mgmt_server
    #     restart: always
    #     networks:
    #         - backend   # Connect to flask
    #     ports:
    #         - "5000:5000"
    mongo:
        image: mongo:latest
        container_name: mongodb
        restart: always
        environment:
            - MONGO_INITDB_ROOT_USERNAME=root
            - MONGO_INITDB_ROOT_PASSWORD=pass
        volumes:
            - /data/db:/data/db
        networks:
            - backend   # Connect to flask
            - frontend  # Connect to mongo_express
        ports:
            - "27017:27017"
    mongo-express:
        image: mongo-express:latest
        container_name: mongo_express
        restart: always
        environment:
            - ME_CONFIG_MONGODB_SERVER=mongo
            - ME_CONFIG_MONGODB_PORT=27017
            - ME_CONFIG_MONGODB_ENABLE_ADMIN=true
            - ME_CONFIG_MONGODB_AUTH_DATABASE=admin
            - ME_CONFIG_MONGODB_ADMINUSERNAME=root
            - ME_CONFIG_MONGODB_ADMINPASSWORD=pass
            # Uncomment if a secure login via browser is required
            # - ME_CONFIG_BASICAUTH_USERNAME=root
            # - ME_CONFIG_BASICAUTH_PASSWORD=pass
        links:
            - mongo
        networks:
            - frontend  # Connect to mongo_express
        ports:
            - 8081:8081
networks:
    backend:
        driver: bridge
    frontend: 
        driver: bridge

---

$ docker network ls # When mgmt_server is run locally

NETWORK ID          NAME                 DRIVER              SCOPE
10577560a149        bridge               bridge              local
a037a11e12bb        host                 host                local
b153eea9db12        node_mgmt_backend    bridge              local
c8e1b58ffb44        node_mgmt_frontend   bridge              local
4f7b75b5695a        none                 null                local

Answer 1

The problem was that Flask tried to access Mongo on localhost. When running Flask on the host, this is OK, but when Flask is containerized, it gets its own ip, and so does Mongo, and localhost just doesn't point to the right place.

Editing the docker-compose.yml networks, and redirecting pymongo client to the updated Mongo IP fixed the issue:

version: "3.8"
services:
    mgmt_server:
        build: ./server
        container_name: mgmt_server
        restart: always
        ports:
            - "5000:5000"
        networks:
            app_net:
                ipv4_address: 172.16.238.2
    mongo:
        image: mongo:latest
        container_name: mongodb
        restart: always
        environment:
            - MONGO_INITDB_ROOT_USERNAME=root
            - MONGO_INITDB_ROOT_PASSWORD=pass
        volumes:
            - /data/db:/data/db
        ports:
            - "27017:27017"
        networks:
            app_net:
                ipv4_address: 172.16.238.3
    mongo-express:
        image: mongo-express:latest
        container_name: mongo_express
        restart: always
        environment:
            - ME_CONFIG_MONGODB_SERVER=mongo
            - ME_CONFIG_MONGODB_PORT=27017
            - ME_CONFIG_MONGODB_ENABLE_ADMIN=true
            - ME_CONFIG_MONGODB_AUTH_DATABASE=admin
            - ME_CONFIG_MONGODB_ADMINUSERNAME=root
            - ME_CONFIG_MONGODB_ADMINPASSWORD=pass
            # Uncomment if a secure login via browser is required
            # - ME_CONFIG_BASICAUTH_USERNAME=root
            # - ME_CONFIG_BASICAUTH_PASSWORD=pass
        links:
            - mongo
        ports:
            - 8081:8081
        networks:
            app_net:
                ipv4_address: 172.16.238.4
networks:
    app_net:
        ipam:
            driver: default
            config:
            - subnet: "172.16.238.0/24"

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This was helpful, but eventually I found this to be more informative.