Can't connect to mongo in Docker

Question

I'm practicing Docker with a dummy app but I can't connect with the database. If anyone can give a clue about what the problem could be, please. Thanks in advance for any help. If any more info is needed please let me know.

Here is my Dockerfile for api:

Dockerfile in server

FROM node:14-alpine

WORKDIR usr/app

COPY package*.json ./

RUN npm install

COPY . .

EXPOSE 3400

CMD ["npm", "start"]

Here is my connection to db in db.js

const mongoose = require('mongoose');

const uri = 'mongodb://mongo:27017/prueba-docker';
const connect = async () => {
  try {
    await mongoose.connect(uri, {});
    console.log('conected to db');
  } catch (err) {
    console.log('error connecting to db', err);
  }
};
module.exports = { connect };

And here docker-compose

version: "3"
services:
  prueba-react:
    image: prueba-react
    build: ./client/
    stdin_open: true
    ports: 
      - "3000:3000"
    networks: 
      - mern-app
  
  prueba-api:
    image: prueba-api
    build: ./server/
    ports: 
      - "3400:3400"
    networks:
      - mern-app
    depends_on:
      - db
  db:
    image: mongo:4.4-bionic
    ports:
      - "27017:27017"
    networks:
      - mern-app
    volumes:
      - mongo-data:/data/db

networks:
  mern-app:
    driver: bridge

volumes:
  mongo-data:
    driver: local

First I found out that I should stop mongo wiht the command because I had a port conflict.

sudo systemctl stop mongod

But now I don't understand why It gives an error when I do docker-compose up

This is the error I get

prueba-api_1    | error connecting to db MongooseServerSelectionError: getaddrinfo EAI_AGAIN mongo
prueba-api_1    |     at NativeConnection.Connection.openUri (/usr/app/node_modules/mongoose/lib/connection.js:796:32)
prueba-api_1    |     at /usr/app/node_modules/mongoose/lib/index.js:328:10
prueba-api_1    |     at /usr/app/node_modules/mongoose/lib/helpers/promiseOrCallback.js:32:5
prueba-api_1    |     at new Promise (<anonymous>)
prueba-api_1    |     at promiseOrCallback (/usr/app/node_modules/mongoose/lib/helpers/promiseOrCallback.js:31:10)
prueba-api_1    |     at Mongoose._promiseOrCallback (/usr/app/node_modules/mongoose/lib/index.js:1149:10)
prueba-api_1    |     at Mongoose.connect (/usr/app/node_modules/mongoose/lib/index.js:327:20)
prueba-api_1    |     at Object.connect (/usr/app/db.js:6:20)
prueba-api_1    |     at Object.<anonymous> (/usr/app/app.js:6:4)
prueba-api_1    |     at Module._compile (internal/modules/cjs/loader.js:1072:14) {
prueba-api_1    |   reason: TopologyDescription {
prueba-api_1    |     type: 'Unknown',
prueba-api_1    |     servers: Map(1) { 'mongo:27017' => [ServerDescription] },
prueba-api_1    |     stale: false,
prueba-api_1    |     compatible: true,
prueba-api_1    |     heartbeatFrequencyMS: 10000,
prueba-api_1    |     localThresholdMS: 15,
prueba-api_1    |     logicalSessionTimeoutMinutes: undefined
prueba-api_1    |   }

Answer 1

In your docker-compose.yml you are creating a network called mern-app where all services are assigned to that network . To communicate between containers in a docker.network all you have to do is use the service name. Instead of mongodb://mongo:27017/prueba-docker try connecting to mongodb://db:27017/prueba-docker .

You also mentioned a port conflict. It seems like you are already running mongodb . If you don't want to stop your mongodb everytime you want to try out your docker-compose , you could just map to another port:

    ports:
      - "27018:27017"

Or you could just don't expose your mongodb at all if no external application is using your db . This only introduces security threats .