how to link docker container to each other with docker-compose

Question

I have to setup a mongo replica set with docker-compose. For the replica set the containers have to know each other.

I tried in docker-compose.yml

    dbreplicasetpart1:       image: mongo:2.6.8       expose:         - '27018'       links:         - replicasetpart2         - replicasetpart3       cap_add:         - NET_ADMIN      dbreplicasetpart2:       image: mongo:2.6.8       links:         - replicasetpart1         - replicasetpart3       expose:         - '27019'       cap_add:         - NET_ADMIN ... 

I get an circular import message. But if I remove the back-link to dbreplicasetpart1 I can't ping from dbreplicasetpart2 to dbreplicasetpart1. What is the solution?

Answer 1

Updated for Docker 1.10

Docker 1.10 allows the definition of networks within the compose file. Here's the updated code

version: "2"  services:   replica1:     image: mongo:2.6.8     container_name: replica1     networks:       - my-net     ports:       - "27018"     environment:       REPLICA2_URL: "http://replica2:27019"   replica2:     image: mongo:2.6.8     container_name: replica2     networks:       - my-net     ports:       - "27019"     environment:       REPLICA1_URL: "http://replica1:27018"  networks:   my-net:     driver: bridge 

Previous answer for Docker 1.9

As of Docker 1.9, the solution to this is to create a custom network and pass it to the docker-compose up command.

1. Create a network docker network create --driver bridge my-net

2. Reference that network as an environment variable (${NETWORK})in the docker-compose.yml files. Eg:

```

replica1:   image: mongo:2.6.8   container_name: replica1   net: ${NETWORK}   ports:     - "27018"   environment:     REPLICA2_URL: "http://replica2:27019"  replica2:   image: mongo:2.6.8   container_name: replica2   net: ${NETWORK}   ports:     - "27019"   environment:     REPLICA1_URL: "http://replica1:27018" 

```

Note that replica1 in http://replica1:27018 will resolve to the ip address of the replica1 service (container). No need to hardcode ip addresses; An entry for replica1 is automatically added to the /etc/host of the replica2 container. Same goes for the replica1 container. Docker will add an entry for replica2 in its /etc/host file.

3. Call docker-compose, passing it the network you created NETWORK=my-net docker-compose up -d -f docker-compose.yml

I've created a bridge network above which only works within one node (host). Good for dev. If you need to get two nodes to talk to each other, you need to create an overlay network. Same principle though. You pass the network name to the docker-compose up command.