Connecting Containers

Parameters#

Parameter	Details
`tty:true`	In docker-compose.yml, the `tty: true` flag keeps the container’s `sh` command running waiting for input.

Remarks#

The host and bridge network drivers are able to connect containers on a single docker host. To allow containers to communicate beyond one machine, create an overlay network. Steps to create the network depend on how your docker hosts are managed.

Swarm Mode: docker network create --driver overlay
docker/swarm: requires an external key-value store

Docker network

Containers in the same docker network have access to exposed ports.

docker network create sample
docker run --net sample --name keys consul agent -server -client=0.0.0.0 -bootstrap

Consul’s Dockerfile exposes 8500, 8600, and several more ports. To demonstrate, run another container in the same network:

docker run --net sample -ti alpine sh
/ # wget -qO- keys:8500/v1/catalog/nodes

Here the consul container is resolved from keys, the name given in the first command. Docker provides dns resolution on this network, to find containers by their --name.

Docker-compose

Networks can be specified in a compose file (v2). By default all the containers are in a shared network.

Start with this file: example/docker-compose.yml:

version: '2'
services:
  keys:
    image: consul
    command: agent -server -client=0.0.0.0 -bootstrap
  test:
    image: alpine
    tty: true
    command: sh

Starting this stack with docker-compose up -d will create a network named after the parent directory, in this case example_default. Check with docker network ls

 > docker network ls
NETWORK ID          NAME                    DRIVER              SCOPE
719eafa8690b        example_default         bridge              local

Connect to the alpine container to verify the containers can resolve and communicate:

 > docker exec -ti example_test_1 sh
/ # nslookup keys
...
/ # wget -qO- keys:8500/v1/kv/?recurse
...

A compose file can have a networks: top level section to specify the network name, driver, and other options from the docker network command.

Container Linking

The docker --link argument, and link: sections docker-compose make aliases to other containers.

docker network create sample
docker run -d --net sample --name redis redis

With link either the original name or the mapping will resolve the redis container.

> docker run --net sample --link redis:cache -ti python:alpine sh -c "pip install redis && python"
>>> import redis
>>> r = redis.StrictRedis(host='cache')
>>> r.set('key', 'value')
True

Before docker 1.10.0 container linking also setup network connectivity - behavior now provided by docker network. Links in later versions only provide legacy effect on the default bridge network.