Express container unable to connect to Mongo Image with Docker-Compose

Question

I am trying to connect an express container to MongoDB image with docker compose but the connection is being rejected, I can connect to the db with robomongo. I can't get what is happening, this is the express code that connects it:

 mongoose.connect('mongodb://localhost:27017/database') .then(()=>console.log('connection succesfull to url')) .catch((err)=>console.error(err)); 

This is the docker-compose file

 version: "3" services: backend: build: context: ../backend dockerfile: ${PWD}/images/backend/Dockerfile container_name: backend ports: - "${BACKEND_PORT}:${BACKEND_PORT}" env_file: - ./deploy.env environment: - PORT=3000 - MONGO_CONNECTION=${MONGO_CONNECTION} command: npm start links: - mongodb depends_on: - mongodb front-app: build: context: ../front-app dockerfile: ${PWD}/images/angular/Dockerfile container_name: front-app ports: - "${FRONTEND_PORT}:4200" env_file: - ./deploy.env command: npm start mongodb: image: mongo:3.6 container_name: mongo volumes: - "${MONGO_DB_DATA}:/data/db" - "${MONGO_DB_DATA}:/data/configdb" ports: - "27017:27017" 

This is the error

 backend | { MongoNetworkError: failed to connect to server [localhost:27017] on first connect [MongoNetworkError: connect ECONNREFUSED 127.0.0.1:27017] backend | at Pool.<anonymous> (/usr/src/app/node_modules/mongodb-core/lib/topologies/server.js:564:11) backend | at Pool.emit (events.js:182:13) backend | at Pool.EventEmitter.emit (domain.js:442:20) backend | at Connection.<anonymous> (/usr/src/app/node_modules/mongodb-core/lib/connection/pool.js:317:12) backend | at Object.onceWrapper (events.js:273:13) backend | at Connection.emit (events.js:182:13) backend | at Connection.EventEmitter.emit (domain.js:442:20) backend | at Socket.<anonymous> (/usr/src/app/node_modules/mongodb-core/lib/connection/connection.js:246:50) backend | at Object.onceWrapper (events.js:273:13) backend | at Socket.emit (events.js:182:13) backend | at Socket.EventEmitter.emit (domain.js:442:20) backend | at emitErrorNT (internal/streams/destroy.js:82:8) backend | at emitErrorAndCloseNT (internal/streams/destroy.js:50:3) backend | at process._tickCallback (internal/process/next_tick.js:63:19) backend | name: 'MongoNetworkError', backend | errorLabels: [ 'TransientTransactionError' ], backend | [Symbol(mongoErrorContextSymbol)]: {} } 

Answer 1

Every process running in docker container thinks that he is "the only one in the world". It means that for this process localhost means: my, container's localhost. And your backend is alone in his container so that's why he cannot find mongodb under localhost.

To fix that problem you should place hostname "mongodb" instead of "localhost" as in docker-compose you can access services using theirs names - it means that mongodb container can also access your backend using "backend" domain.

Please note also that "links" is deprecated in docker and shouldn't be used - it's not required in your configuration as docker-compose gives every service within docker-compose file access to each other using method mentioned above.