I was trying to set up a Golang server with a MongoDB database for FCOS Pinger Backend using docker-compose (alias docker-compose=podman-compose), but I was struggling to access the MongoDB container from server container with error message saying Connection refused. Tried adding a router, but that didn’t work.

After researching and learning about networks on Openshift/Kubernetes, it turns out that the server code clientOptions := options.Client().ApplyURI("mongodb://localhost:27017") needs to be clientOptions := options.Client().ApplyURI("mongodb://mongodb:27017") since the pods have different Ip addresses and the localhost:27017 in server container will not be the port that MongoDB is listening on.

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17

version: '3'
services:
  mongodb:
    image: mongo
    container_name: pinger-mongodb
    network_mode: host
    ports:
      - "27017:27017"
  backend:
    image: quay.io/zonggen/fcos-pinger-backend:latest
    container_name: pinger-backend
    network_mode: host
    ports:
      - "5000:5000"
    depends_on:
      - mongodb

Note that only the name mongodb will work on Openshift. None of localhost, mongo, or pinger-mongodb would work. However, mongodb, localhost and pinger-mongodb would work with docker-compose up. It makes sense given the meaning of the names: host internet localhost, service name mongodb, image name mongo and container name pinger-mongodb.

Since containers are wrapped inside pods in Openshift/Kubernetes, communications between containers/pods are through services. Therefore, it makes sense to use service name for DNS server instead of localhost/container/image name.

In local docker-compose environment, since containers are using host network directly, it makes sense that localhost will work. Also since there’s no layer between containers in docker-compose environment, accessing another container network through container name or service name makes sense.

After hours of learning networks and Openshift/Kubernetes, the backend is finally up and running. :)