Docker-compose mongoose - mongodb

Question

I'm new to Docker, and I'm trying the simplest of setups with docker-compose, but don't succeed to connect to Mongodb.
My docker-compose.local.yaml file:
version: "2"
services:
posts-api:
build:
dockerfile: Dockerfile.local
context: ./
volumes:
- ".:/app"
ports:
- "6820:6820"
depends_on:
- mongodb
mongodb:
image: mongo:3.5
ports:
- "27018:27018"
command: mongod --port 27018
My Docker file:
FROM node:7.8.0
MAINTAINER Livefeed 'project.livefeed#gmail.com'
RUN mkdir /app
VOLUME /app
WORKDIR /app
ADD package.json yarn.lock ./
RUN eval rm -rf node_modules && \
yarn
ADD server.js .
RUN mkdir config src
ADD config config/
ADD src src/
EXPOSE 6820
EXPOSE 27018
CMD yarn run local
In server.js I try to connect with:
mongoose.connect('mongodb://localhost:27018');
I also tried:
mongoose.connect('mongodb://mongodb:27018');
To run docker-compose:
docker-compose -f docker-compose.local.yaml up --build
And I receive the error:
connection error: { MongoError: failed to connect to server [localhost:27018] on first connect [MongoError: connect ECONNREFUSED 127.0.0.1:27018]
What am I missing?

Answer 1

In server.js use mongodb instead of localhost:
mongoose.connect('mongodb://mongodb:27018');
Because containers in the same network can communicate using their service name.
Bear in mind that each container and your host have their own localhost. Each localhost is a different host: container A, container B, your host (each one has its own network interface).
Edit:
Be sure to get your mongo up:
docker-compose logs mongodb
docker-compose ps
Sometimes it doesn't get up because of disk space.
Edit 2:
With newer versions of mongo, you need to specify to listen to all interfaces too:
command: mongod --port 27018 --bind_ip_all

Answer 2

I think, that you should add links option in your config. Like this:
ports:
- "6820:6820"
depends_on:
- mongodb
links:
- mongodb
update
As I promised
version: '2.1'
services:
pm2:
image: keymetrics/pm2-docker-alpine:6
restart: always
container_name: pm2
volumes:
- ./pm2:/app
links:
- redis_db
- db
environment:
REDIS_CONNECTION_STRING: redis://redis_db:6379
nginx:
image: firesh/nginx-lua
restart: always
volumes:
- ./nginx:/etc/nginx
- /var/run/docker.sock:/tmp/docker.sock:ro
ports:
- 80:80
links:
- pm2
s3: # mock for development
image: lphoward/fake-s3:latest
redis_db:
container_name: redis_db
image: redis
ports:
- 6379:6379
db: # for scorebig-syncer
image: mysql:5.7
ports:
- 3306:3306