SocketException when trying to access to a mysql docker container from shelf server container

Question

Context

I am trying to create a Dart server with the library shelf.
I want it a bit scalable in order to get to production when my project will be ready.

And to facilitate the production release, I decided to use Docker in order to deploy it easily.

---

Dart Shelf Server

Here is my Dockerfile :

# Use latest stable channel SDK.
FROM dart:stable AS build

# Resolve app dependencies.
WORKDIR /app
COPY pubspec.* ./
RUN dart pub get

# Copy app source code (except anything in .dockerignore) and AOT compile app.
COPY . .
RUN dart compile exe bin/server.dart -o bin/server

# Build minimal serving image from AOT-compiled `/server`
# and the pre-built AOT-runtime in the `/runtime/` directory of the base image.
FROM scratch
COPY --from=build /runtime/ /
COPY --from=build /app/bin/server /app/bin/

# Start server.
EXPOSE 8080
CMD ["/app/bin/server"]

(it is the basic dockerfile provided by Shelf readme)

---

Docker-compose file with Database

Now, I need a database.
I put it in a separate container.
Here is my docker-compose.yaml file :

version: "3.7"
services:
  db:
    restart: always
    image: mysql
    environment:
      MYSQL_USER: admin
      MYSQL_PASS: secret
      MYSQL_ROOT_PASSWORD: secret
      MYSQL_PASSWORD: secret
      MYSQL_DATABASE: mydatabase
    # volumes:
    #   - toast-datavolume:/var/lib/mysql
    ports:
      - "3306:3306"
    networks:
      - mynetwork

  server:
    build: .
    ports:
      - "8000:8080"
    depends_on:
      - db
    environment:
      - MYSQL_HOST=db
    links:
      - db:mysql
    networks:
      - mynetwork

networks:
  mynetwork:
# volumes:
#   toast-datavolume:

(I commented out the persistence part, as I just want this to work first)

---

Dart code

Finally, here is the dart code responsible of the connection :

try {
  print('try');
  var settings = ConnectionSettings(
    host: 'db',
    port: 3306,
    user: 'admin',
    password: 'secret',
    db: 'mydatabase',
    timeout: Duration(seconds: 30),
  );
  var conn = await MySqlConnection.connect(settings);
  var result = await conn.query('insert into users (name, email, age) values (?, ?, ?)', ['Bob', 'bob@bob.com', 25]);

  return 1;
} catch (exception) {
  print(exception);
  return -1;
}

---

Error

Now, when I launch my containers, everything goes correctly. However, when I try to launch this part of the code (with a curl request, redirected here by some code above), here is my exception (on the server docker logs) :

2023-06-07 11:45:14 2023-06-07T09:45:14.465441  0:00:00.011712 POST    [400] /register
2023-06-07 11:46:42 try
2023-06-07 11:46:42 SocketException: Broken pipe (OS Error: Broken pipe, errno = 32), address = db, port = 33206

The POST line shows my curl command.
The second line is my print('try') line.

Here are what I already tried :

• Remove the network in docker-compose.yaml to use default network
• Use mysql://db:3306 as host in my dart code
• Log into my mysql container from my local machine (works perfectly, so the issue is the connection between dart server and mysql server)
• I tried a lot of things I don't recall, as I spent 2 days tweaking around to make this work.

Thanks for your help !

Answer 1

All of your docker networking setup looks correct. You shouldn't need the link entry FWIW, but it won't hurt anything. The mysql1 package you are using may have an issue with the latest version of mysql server. I duplicated your setup with my best guess based on your post and was getting a "packets out of order" error which brought me here Flutter mysql1 got packets out of order. I swapped the mysql1 package to mysql_client and was then able to connect successfully.