Cannot connect from inside docker swarm cluster to external mongodb service

Question

If I run single docker container of my backend, it runs well and connects to mongodb which is running on host. But when I run my backend using docker-compose, it doesn't connect to mongodb and prints to console:

MongoError: failed to connect to server [12.345.678.912:27017] on first connect [MongoError: connection 0 to 12.345.678.912:27017 timed out]

docker-compose.yml contents:

version: "3"
services:
  web:
    image: __BE-IMAGE__
    deploy:
      replicas: 1
      restart_policy:
        condition: on-failure
      resources:
        limits:
          cpus: "0.1"
          memory: 2048M
    ports:
      - "1337:8080"
    networks:
      - webnet
  visualizer:
    image: dockersamples/visualizer:stable
    ports:
      - "1340:8080"
    volumes:
      - "/var/run/docker.sock:/var/run/docker.sock"
    deploy:
      placement:
        constraints: [node.role == manager]
    networks:
      - webnet
networks:
  webnet:

how I run single docker container:

docker run -p 1337:8080 BE-IMAGE

Answer 1

you need to link the mongo port since localhost is not the same from inside versus outside the containers

ports:
  - "1337:8080"
  - "27017:27017"

On port definitions left hand side is outside, right side is internal to your container ... Your error says internal to your container it cannot see port 27017 ... above is just linking that mongo port so the container can access that port outside of docker