How to link containers in docker? - docker

Question

Here's the result when I type docker ps :
I have 3 docker containers: webapps, redis and rabbitmq. I want to link container webapps to container redis and rabbitmq container. In non docker apps, mywebapps can send message to rabbitmq and write/read redis.
I tried using command like this
docker run --name rabbitmq -p 8080:80 --link webapps:nimmis/apache-php7 -d rabbitmq
but it does not work.
Here is my config.php on webapps where I am trying to send messages via rabbitmq:
define('HOST', 'localhost');
define('PORT', 5672);
I tried to change localhost with hostname
define('HOST', 'rabbitmq');
define('PORT', 5672);
Error message says connection refused.
It seems that in my three containers needs to be configured in the same network namespace.

Answer 1

Linking is a legacy feature. Please use "user defined networks":
sudo docker network create mynetwork
Then rerun your containers using this network:
sudo docker run --name rabbitmq -p 8080:80 -d --network mynetwork rabbitmq
Do the same for other containers that you want connected with each other.
Using "user defined networks", you have an "internal name resolution" at your disposal (somewhat like domain name resolution when visiting websites). You can use the names of the container that you want to refer to, in order to resolve the IP addresses of containers, as long as they are running on the same "user defined network". With this, you can resolve the IP address of the rabbitmq container with its name, within other containers, on the same network.
All containters on the same "user defined network" will have network connectivity. There is no need for "legacy linking".

Answer 2

For inter-container dependencies and links, you'll want to use docker-compose where you can define the links between containers.
In your root directory where you store your Docker files, just make a new file called docker-compose.yml and here you can define your containers as services which rely on each other like this:
version: '2'
services:
webapps:
build: .
links:
- "rabbitmq:rabmq"
- "redis"
rabbitmq:
image: rabbitmq
redis:
image: redis
so here in the definition of the webapps service, you see it links the other two services rabbitmq and redis. What this means is that when the webapps container is build, an entry to it's hosts file is made such that the domain name redis is translated to the IP and port number of the actual container.
You have the option to change the name of how this container is address by using the service:alias notation, like how I defined the rabbitmq to
use the alias rabmq inside the container webapps.
To now build and start your containers using docker-compose just type:
docker-compose up -d
So connecting to another container is as simple as using this alias as the name of the host.
Since you are using docker-compose in this case, it creates a docker network automatically to connect all the containers so you shouldn't have to worry about that. But for more information have a look at the docs:
https://docs.docker.com/compose/networking/#/specifying-custom-networks

Answer 3

You need to link rabbitmq and redis to your webapps container and not the other way arround.
#run redis container
docker run --name some-redis -d redis
#run rabbitmq container
docker run -d --hostname my-rabbit --name some-rabbit rabbitmq
#run webapps container
docker run --name webapps -p 8080:80 --link some-redis:redis --link some-rabbit:rabbitmq nimmis/apache-php7
First run redis and rabbitmq containers.
Then run webapps container with links to the 2 containers.
Now, to configure redis host in the webapps - its easy. You can simply use env variable 'REDIS_PORT_6379_TCP_ADDR'. Because once a container is linked you get its env variables. and redis exports that variable.
Regarding the rabbitmq host - you can get the ip after the rabbit container is up by:
RABBITMQ_IP=$(docker inspect --format '{{ .NetworkSettings.IPAddress }}' some-rabbit)
and then pass it in --env when you run the webapps container.

Answer 4

In my experience working with declaratives such as docker-compose.yml is okay, but simply you can use
docker run -d -P -link nimmis/apache-php7 rabbitmq redis

Answer 5

You can define your services to use a user-defined network in your docker-compose.yml
version: "3"
services:
webapps:
image: nimmis/apache-php7
ports:
- "80:8080"
networks:
- my-network
rabbitmq:
image: rabbitmq
networks:
- my-network
redis:
image: redis
networks:
- my-network
networks:
my-network:
driver: overlay
Then do:
docker swarm init
docker stack deploy -c docker-compose.yml my-stack
Check out the full example at https://docs.docker.com/get-started/part3/

Answer 6

You could access the IP Address of your Redis Container.
Start rabbitmq and get the internal IP Adress:
docker inspect -f '{{range.NetworkSettings.Networks}}{{.IPAddress}}{{end}}' rabbitmq > .rabbitmq.ip
Now, you can add an Apache configuration and add the internal IP Address for rabbitmq while starting the webapps container. Or simply add an entry in the Apache container's /etc/hosts like:
// the dynamic internal IP of rabbitmq is known once rabbitmq starts:
172.30.20.10 rabbitmq.redis.local