Setting up more than one MQTT broker with Docker - docker

Question

Using Docker, I was able to use eclipse-mosquitto to set up an MQTT broker with my app, which subscribes to messages. I'm learning Docker right now, so wanted to try adding two brokers to Docker-compose with different ports mapped like this:
version: '3'
services:
myapp:
...
links:
- mqtt
- mqtt2
depends_on:
- mqtt
- mqtt2
mqtt:
image: eclipse-mosquitto:latest
container_name: mqtt-iot
ports:
- 1883:1883
mqtt2:
image: eclipse-mosquitto:latest
container_name: mqtt2-iot
ports:
- 1884:1883
From outside of the myapp container (i.e. from my OS X terminal), both mqtt and mqtt2 are working; I can publish and subscribe to messages as expected.
const mqtt = require('mqtt')
mqtt.connect('mqtt://mqtt', {port: 1883}) // Success
mqtt.connect('mqtt://mqtt2', {port: 1884}) // Success
However, when I'm inside the container of myapp, I can only connect to mqtt. mqtt2 connection fires the offline event right away, and no connection fails. What do I need to do to for myapp to be using both of those brokers properly?

Answer 1

Two issues here
links:
- mqtt
- mqtt2
Links is deprecated now and is not even required in your compose. Next when you use below
const mqtt = require('mqtt')
mqtt.connect('mqtt://mqtt', {port: 1883}) // Success
mqtt.connect('mqtt://mqtt2', {port: 1884}) // Success
From outside. This is based on the ports on the host. When you do it from app container you should do it like below
const mqtt = require('mqtt')
mqtt.connect('mqtt://mqtt', {port: 1883}) // Success
mqtt.connect('mqtt://mqtt2', {port: 1883}) // Success
The container cannot see mapped port on host. It will see what is inside the network. And in local network both are listen on 1883