How to connect a socket with Socket.io in a Docker container?

Question

I have a tiny socket server in a docker container the server looks like

var app = require('express')();
var server = require('http').Server(app);
var io = require('socket.io')(server, {origins: 'localhost:*'});
io.on('connection', function (socket) {
console.log('Connected');
});
const PORT = 8081;
const HOST = '0.0.0.0';
server.listen(PORT, HOST);

and the docker file is

FROM keymetrics/pm2-docker-alpine:latest
WORKDIR /root
RUN apk update && \
   apk upgrade && \
   apk add git
ENV HOME /root
COPY socket.js ./
COPY package.json ./
RUN npm install
COPY pm2.json ./
EXPOSE 8081
CMD [ "pm2-docker", "start", "pm2.json" ]

pm2.json looks like

{
"apps": [{
    "name": "socket-server",
    "script": "socket.js",
    "exec_mode" : "cluster",
    "instances"  : 2,
    "env": {
    "production": true
    }
}]
}

package.json

{
"name": "socket-server",
"version": "1.0.0",
"description": "",
"main": "socket.js",
"scripts": {
  "test": "echo \"Error: no test specified\" && exit 1"
},
"author": "",
"license": "ISC",
"dependencies": {
"express": "^4.15.3",
"socket.io": "^2.0.3"
}
}

It all runs just fine with

docker run -d -p 8081:8081 socket-server

until I try to connect to it from a website running in another container, the website connects like this...

<script src="socket.io.js"></script>
<script>
  var socket = io.connect('http://localhost:8081');
  socket.on('connect', function(data) {
    console.log('Connected Client')
  });
</script>  

and in the console, it shows that it polls just fine with

Request URL:http://localhost:8081/socket.io/?
EIO=3&transport=polling&t=LthQCgI&sid=93sOyTiSOe5RVOdEAAAL
Request Method:POST
Status Code:200 OK

but fails to get a socket connection

Request URL:ws://localhost:8081/socket.io/?
EIO=3&transport=websocket&sid=93sOyTiSOe5RVOdEAAAL
Request Method:GET
Status Code:400 Bad Request

Now if I run the socket server, not in the docker container it's fine and the socket connects.

I have tried getting the IP of the container that the socket server is running and using that in the connection script but even the polling doesn't work when I configure it like that.

I really need this inside a Docker container.

Any help is most appreciated

Answer 1

Although this is an old question I figured I would elaborate a little bit on it for anyone else that is wondering how you would go about connecting containers since the previous answer was a bit slim.

Using swarm in this case would be overkill especially for something like running the containers locally in a manner that would allow them to talk. Instead you simply want to establish the containers on the same docker network.

version: '3.5'

services:
  app:
    build:
      context: .
      dockerfile: Dockerfile
    command: app
    ports:
      - "4000:4000"
    volumes:
      - .:/app
    networks:
      - app-network

  pgsql:
    image: postgres:latest
    ports:
      - "5432:5432"
    volumes:
      - postgres_data:/var/lib/postgresql/data
    networks:
      - app-network

networks:
  app-network:
    driver: bridge

volumes:
  postgres_data:
    driver: local

In the docker-compose.yml file example above you can see that I am defining a network via:

networks:
      app-network:
        driver: bridge

Then on both the app and pgsql containers I am assigning them to that network via:

networks:
      - app-network

This allows me to access the containers from one another via the container "name". So in my code I am now able to use pgsql:5432 and communicate with the postgres service. The app container is also reachable from the pgsql container via app:4000.

While this can get much more complex than the above example I figured I leave a working docker-compose.yml example above. You can find out more about docker networks at https://docs.docker.com/compose/networking/