Linking containers and volume for datastore

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Note

How to create one container that can access a service on another?

In this article, we'll show how Docker does it. We'll launch WordPress site using Docker. In this article, we'll create 3 different Docker images:

One image for WordPress itself.
It's going to be linked to MySQL container.
Then, it will be storing data. We call it "Data Container". Unlike the WordPress and MySQL containers, Data container can be run only once, and does not have to be running. It's enough to just define the volume and share the volume with MySQL. Then, the WordPress will be linked to the volume so that it can have an access to MySQL database's port, locally instead of remotely.

Image source : Riding The Next Wave: Docker Containers

Where Docker stores its files?

Let's look inside /var/lib/docker folder in Ubuntu:

$ sudo ls /var/lib/docker
aufs	    execdriver	init	      repositories-aufs  trust	volumes
containers  graph	linkgraph.db  tmp		 vfs

When Docker starts an image and create a container that has a volume defined, it will store those in a UUID named directory inside two directories (volumes and vsf). Now the two directories are empty since we haven't defined any volume:

$ sudo ls /var/lib/docker/volumes
$ sudo ls /var/lib/docker/vfs/dir

Creating a volume

We're going to create a volume using tiny linux distro called busybox.

Docker system should maintain the consistency of the data store we're creating now in /var/lib/mysql directory.

For reference, here are the docker run args we're going to use to create a volume:

-v
Bind mount a volume (e.g., from the host: -v /host:/container, from Docker: -v /container)
--name
Assign a name to the container
-d
Detached mode: run the container in the background and print the new container ID.

Then, run docker. The following command just runs a container and exits as we can see from the output of docker ps -a:.

$ docker run -v /var/lib/mysql --name=my_datastore -d busybox echo "My Datastore"
$ docker ps -a
CONTAINER ID        IMAGE               COMMAND                CREATED             STATUS                     PORTS               NAMES
6a7e05a1e120        busybox             "echo 'My Datastore'   3 minutes ago       Exited (0) 3 minutes ago                       my_datastore

However, by running once, it has created a volume and this is the key: data container does not have to be running. If we inspect my_datastore:

$ docker inspect my_datastore
[
{
    ...
    "Volumes": {
        "/var/lib/mysql": "/var/lib/docker/volumes/7ec328ed9c22b7190c9998d13d3dc49d8d8249e336fd84667c5bf7fc724b70bc/_data"
    },
    "VolumesRW": {
        "/var/lib/mysql": true
    },
    ...
    }
}
]
$

So, from now on, whenever anything write to will be stored in this area of local host system!

MySQL container

Now, we need to start MySQL container (OFFICIAL REPOSITORY mysql).

$ docker run --name my_mysql -e MYSQL_ROOT_PASSWORD=my-secret-pw --volumes-from my_datastore -d mysql
...
Status: Downloaded newer image for mysql:latest
9173d3457bc8ec440fbf5df30d30db8102ede04d4e344c9593a1421057f99786

Note that we added my_mysql for the container name, passed environment variable MYSQL_ROOT_PASSWORD, and most importantly, added our datastore for --volumes-from, and running it in detached mode.

$ docker ps 
CONTAINER ID        IMAGE               COMMAND                CREATED             STATUS              PORTS               NAMES
9173d3457bc8        mysql               "/entrypoint.sh mysq   2 minutes ago       Up 2 minutes        3306/tcp            my_mysql

Now, we have MySQL up and running.

If we inspect my_mysql container, we can see it's using the volume we specified:

$ docker inspect my_mysql
    ...
    "Volumes": {
        "/var/lib/mysql": "/var/lib/docker/volumes/7ec328ed9c22b7190c9998d13d3dc49d8d8249e336fd84667c5bf7fc724b70bc/_data"
    },
    "VolumesRW": {
        "/var/lib/mysql": true
    },
    ...

Container for WordPress site

Note our WordPress container should have a link to our MySQL container!

Let's look into the inspection in the previous section, and we can locate the "ExposedPorts":

$ docker inspect my_mysql
        ...
        "ExposedPorts": {
            "3306/tcp": {}
        },
        ...

We'll link to this port to WordPress so that it's available for WordPress internally:

$ docker run --link=my_mysql:mysql -p 8888:80 -d wordpress

Here, we used --link to add link to WordPress container in the form of container-name:alias, specified the ports: hostPort:containerPort, and run it in detached mode as before.

Now, we have two containers running:

$ docker ps
CONTAINER ID        IMAGE               COMMAND                CREATED             STATUS              PORTS               NAMES
9b149061e09b        wordpress           "/entrypoint.sh apac   6 seconds ago       Up 5 seconds        0.0.0.0:8888->80/tcp   jovial_noyce        
9173d3457bc8        mysql               "/entrypoint.sh mysq   30 minutes ago      Up 30 minutes       3306/tcp               my_mysql

Actually, we have three containers: two are running, and another holds the volume for datastore:

$ docker ps -a
CONTAINER ID        IMAGE               COMMAND                CREATED             STATUS                         PORTS               NAMES
9b149061e09b        wordpress           "/entrypoint.sh apac   2 minutes ago       Up 2 minutes                   0.0.0.0:8888->80/tcp   jovial_noyce             
9173d3457bc8        mysql               "/entrypoint.sh mysq   32 minutes ago      Up 32 minutes                  3306/tcp               my_mysql            
6a7e05a1e120        busybox             "echo 'My Datastore'   About an hour ago   Exited (0) About an hour ago                          my_datastore

Now, we're able to access the instance from the host without the container's IP, since we used standard port mappings.

Then, access it via http://localhost:8888 or http://host-ip:8888 in a browser: