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're likely encountering this issue because you're running **Docker in Docker (DinD)** or using **Docker Compose inside a container**, but the Docker daemon inside your container cannot resolve the service names properly — especially when you're trying to connect to a service *from another container* in the same Docker network. Here’s a breakdown of the issue and how to fix it: --- ## ✅ Root Cause > **You're probably running `docker-compose up` inside a Docker container (e.g., a CI runner like GitLab CI, or a custom dev container), and trying to access another container (e.g., `mysql`) by its service name (like `mysql`) — but you're using `127.0.0.1` or `localhost` instead of the correct service name.** In Docker Compose, **service names are DNS names** within the custom network created by Compose. So if your `docker-compose.yml` has: ```yaml services: mysql: image: mysql:8.0 ports: - "3306:3306" environment: MYSQL_ROOT_PASSWORD: root ``` Then, **from another service in the same Compose network**, you connect to `mysql:3306`, **NOT** `localhost:3306` or `127.0.0.1:3306`. But **if you're running your application inside a container and trying to connect to `localhost:3306`**, it’s trying to connect to *itself*, not the `mysql` service. --- ## ✅ Solution: Use the correct hostname ### ✔️ Fix: Use the service name as the host If you're connecting from a container in the same Docker Compose network (e.g., your app container), **use the service name**: ```python # In your app (Python, Node.js, etc.) host = "mysql" # <-- NOT "localhost" or "127.0.0.1" port = 3306 ``` Example in Python with `pymysql`: ```python import pymysql connection = pymysql.connect( host='mysql', # <-- This is the service name from docker-compose.yml user='root', password='root', database='mydb' ) ``` If you're using `docker-compose up` **on your host machine**, and run your app **on the host**, then use `localhost:3306`. But if you're running your app **inside a container**, then **you must use `mysql`** (service name), because `localhost` inside the container ≠ the `mysql` container. --- ## ✅ Bonus: Verify Your Network Setup Check the network created by Compose: ```bash docker network ls docker network inspect # e.g., myapp_default ``` Look for the containers and their IP addresses and hostnames. You should see `mysql` listed as a container with the same network, and service name resolution should work. You can also test connectivity inside your app container: ```bash # Enter your app container docker exec -it your_app_container sh # Test if you can reach mysql service ping mysql # or telnet mysql 3306 # or nc -vz mysql 3306 ``` If this fails → network not connected or service not ready. --- ## ✅ Common Mistake: Port Mapping Confusion You might have this in `docker-compose.yml`: ```yaml ports: - "3306:3306" ``` This **only** exposes the port to the **host machine**, not within Docker networks. **Inside Docker networks**, you **do NOT need to publish ports** to communicate between containers. So even if you remove `ports:` entirely, containers in the same network can still reach each other on `mysql:3306`. > 🔥 **Best practice**: Remove `ports:` unless you specifically need external access. Use service names for inter-container comms. --- ## ✅ Summary: What to Do | Situation | What to Use | |----------|-------------| | App on **host machine** | `localhost` or `127.0.0.1` | | App running **in Docker container**, connecting to MySQL service | `mysql` (service name) | | App in container, connecting to **host’s Docker daemon** | Use `host.docker.internal` (Docker Desktop) or configure `--network host` | > ✅ **For Docker Compose containers: Always use service names as hostnames.** --- ## ✅ Example docker-compose.yml (correct setup) ```yaml version: '3.8' services: mysql: image: mysql:8.0 environment: MYSQL_ROOT_PASSWORD: root MYSQL_DATABASE: mydb # No ports needed unless accessed from host # ports: # - "3306:3306" app: build: . depends_on: - mysql environment: DB_HOST: mysql # <-- Use service name DB_PORT: 3306 ``` Then in your app code: ```python host = os.getenv('DB_HOST', 'mysql') # Defaults to service name ``` --- ## ❗ Also Check: Service Startup Order Even if names work, MySQL might not be ready when your app starts. Fix with `healthcheck` + `depends_on` + retry logic: ```yaml mysql: image: mysql:8.0 environment: MYSQL_ROOT_PASSWORD: root healthcheck: test: ["CMD", "mysqladmin", "ping", "-h", "localhost", "-u", "root", "--password=root"] retries: 20 timeout: 5s interval: 10s ``` And in your app, add a retry loop for database connection on startup. --- Let me know if you want help debugging your exact `docker-compose.yml` — paste it here and I’ll fix it for you!

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