How Docker containers are interconnected

A container's connection system is another way to interact with the application in the container, in addition to port mapping.

The system creates a tunnel between the source and the receiving container, which can see the information specified by the source container.

Custom container naming

The connection system is performed according to the name of the container. Therefore, you first need to customize the naming of a well-remembered container.

Although when the container is created, a name is assigned by default. Custom named containers have two benefits:

• Custom naming, better remember, such as a web application container we can name it web
• When you want to connect other containers, you can use them as a useful reference point, such as connecting web containers to db containers

Use --name to customize the name for the container.

$ sudo docker run -d -P --name web training/webapp python app.py

Use docker ps verify the name of the setting.

$ sudo docker ps -l
CONTAINER ID  IMAGE                  COMMAND        CREATED       STATUS       PORTS                    NAMES
aed84ee21bde  training/webapp:latest python app.py  12 hours ago  Up 2 seconds 0.0.0.0:49154->5000/tcp  web

You can also docker inspect see the name of the container

$ sudo docker inspect -f "{{ .Name }}" aed84ee21bde
/web

Note: The name of the container is unique. If you have named a container called the web, when you want to use the name web again, you need docker rm remove the container of the same name that you created earlier.

If you add a --rm tag when you execute docker run the container is deleted as soon as it terminates. Note that --rm -d cannot be used at the same time.

Containers are interconnected

Using --link parameter allows safe interaction between containers.

Let's start by creating a new database container.

$ sudo docker run -d --name db training/postgres

Delete the web container you created earlier

$ docker rm -f web

Then create a new web container and connect it to the db container

$ sudo docker run -d -P --name web --link db:db training/webapp python app.py

At this point, the db container and the web container are interconnected.

--link the --link name:alias name is the name of the container to be linked, and alias of the connection.

Use docker ps view the connection to the container

$ docker ps
CONTAINER ID  IMAGE                     COMMAND               CREATED             STATUS             PORTS                    NAMES
349169744e49  training/postgres:latest  su postgres -c '/usr  About a minute ago  Up About a minute  5432/tcp                 db, web/db
aed84ee21bde  training/webapp:latest    python app.py         16 hours ago        Up 2 minutes       0.0.0.0:49154->5000/tcp  web

You can see custom named containers, db and web, and the names columns of db containers have both db and web/db. This means that the web container is linked to the db container, and the web container will be allowed access to the information of the db container.

Docker creates a secure tunnel between two interconnected containers without mapping their ports to the host host. The db container is not started with -p -P thus avoiding exposing database ports to external networks.

Docker exposes connection information to containers in 2 ways:

• Environment variables
• Update /etc/hosts file

Use env to view the environment variables of the web container

$ sudo docker run --rm --name web2 --link db:db training/webapp env
. . .
DB_NAME=/web2/db
DB_PORT=tcp://172.17.0.5:5432
DB_PORT_5000_TCP=tcp://172.17.0.5:5432
DB_PORT_5000_TCP_PROTO=tcp
DB_PORT_5000_TCP_PORT=5432
DB_PORT_5000_TCP_ADDR=172.17.0.5
. . .

Where DB_ at the beginning of the web container connection db container, the prefix uses a capital connection alias.

In addition to the environment variables, Docker adds host information to the /etc/hosts file. Below is the hosts file for the parent container web

$ sudo docker run -t -i --rm --link db:db training/webapp /bin/bash
root@aed84ee21bde:/opt/webapp# cat /etc/hosts
172.17.0.7  aed84ee21bde
. . .
172.17.0.5  db

There are 2 hosts, the first is the web container, which uses id as his host name, and the second is the ip and host name of the db container. You can install ping commands in a web container to test connectivity to the db container.

root@aed84ee21bde:/opt/webapp# apt-get install -yqq inetutils-ping
root@aed84ee21bde:/opt/webapp# ping db
PING db (172.17.0.5): 48 data bytes
56 bytes from 172.17.0.5: icmp_seq=0 ttl=64 time=0.267 ms
56 bytes from 172.17.0.5: icmp_seq=1 ttl=64 time=0.250 ms
56 bytes from 172.17.0.5: icmp_seq=2 ttl=64 time=0.256 ms

Test the db container with ping, which resolves 172.17.0.5 Note: The official ubuntu image does not have pings installed by default and needs to be installed on its own.

Users can link multiple parent containers to child containers, such as multiple webs to db containers.