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Merge pull request #7361 from SvenDowideit/merge-edit_dockerlinks

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First pass at replacing parent-child metaphor to describe docker links.
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Sven Dowideit 2014-08-01 11:12:05 +10:00
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docs/sources/userguide/dockerlinks.md
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@ -4,48 +4,47 @@ page_keywords: Examples, Usage, user guide, links, linking, docker, documentatio
# Linking Containers Together
In [the Using Docker section](/userguide/usingdocker) we touched on
connecting to a service running inside a Docker container via a network
port. This is one of the ways that you can interact with services and
applications running inside Docker containers. In this section we're
going to give you a refresher on connecting to a Docker container via a
network port as well as introduce you to the concepts of container
linking.
In [the Using Docker section](/userguide/usingdocker), you saw how you can
connect to a service running inside a Docker container via a network
port. But a port connection is only one way you can interact with services and
applications running inside Docker containers. In this section, we'll briefly revisit
connecting via a network port and then we'll introduce you to another method of access:
container linking.
## Network port mapping refresher
In [the Using Docker section](/userguide/usingdocker) we created a
container that ran a Python Flask application.
In [the Using Docker section](/userguide/usingdocker), you created a
container that ran a Python Flask application:
$ sudo docker run -d -P training/webapp python app.py
> **Note:**
> Containers have an internal network and an IP address
> (remember we used the `docker inspect` command to show the container's
> (as we saw when we used the `docker inspect` command to show the container's
> IP address in the [Using Docker](/userguide/usingdocker/) section).
> Docker can have a variety of network configurations. You can see more
> information on Docker networking [here](/articles/networking/).
When we created that container we used the `-P` flag to automatically map any
network ports inside that container to a random high port from the range 49000
to 49900 on our Docker host. When we subsequently ran `docker ps` we saw that
port 5000 was bound to port 49155.
When that container was created, the `-P` flag was used to automatically map any
network ports inside it to a random high port from the range 49000
to 49900 on our Docker host. Next, when `docker ps` was run, you saw that
port 5000 in the container was bound to port 49155 on the host.
$ sudo docker ps nostalgic_morse
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
bc533791f3f5 training/webapp:latest python app.py 5 seconds ago Up 2 seconds 0.0.0.0:49155->5000/tcp nostalgic_morse
We also saw how we can bind a container's ports to a specific port using
the `-p` flag.
You also saw how you can bind a container's ports to a specific port using
the `-p` flag:
$ sudo docker run -d -p 5000:5000 training/webapp python app.py
And we saw why this isn't such a great idea because it constrains us to
And you saw why this isn't such a great idea because it constrains you to
only one container on that specific port.
There are also a few other ways we can configure the `-p` flag. By
There are also a few other ways you can configure the `-p` flag. By
default the `-p` flag will bind the specified port to all interfaces on
the host machine. But we can also specify a binding to a specific
the host machine. But you can also specify a binding to a specific
interface, for example only to the `localhost`.
$ sudo docker run -d -p 127.0.0.1:5000:5000 training/webapp python app.py
@ -53,20 +52,19 @@ interface, for example only to the `localhost`.
This would bind port 5000 inside the container to port 5000 on the
`localhost` or `127.0.0.1` interface on the host machine.
Or to bind port 5000 of the container to a dynamic port but only on the
`localhost` we could:
Or, to bind port 5000 of the container to a dynamic port but only on the
`localhost`, you could use:
$ sudo docker run -d -p 127.0.0.1::5000 training/webapp python app.py
We can also bind UDP ports by adding a trailing `/udp`, for example:
You can also bind UDP ports by adding a trailing `/udp`. For example:
$ sudo docker run -d -p 127.0.0.1:5000:5000/udp training/webapp python app.py
We also saw the useful `docker port` shortcut which showed us the
current port bindings, this is also useful for showing us specific port
configurations. For example if we've bound the container port to the
`localhost` on the host machine this will be shown in the `docker port`
output.
You also learned about the useful `docker port` shortcut which showed us the
current port bindings. This is also useful for showing you specific port
configurations. For example, if you've bound the container port to the
`localhost` on the host machine, then the `docker port` output will reflect that.
$ docker port nostalgic_morse 5000
127.0.0.1:49155
@ -78,38 +76,39 @@ output.
Network port mappings are not the only way Docker containers can connect
to one another. Docker also has a linking system that allows you to link
multiple containers together and share connection information between
them. Docker linking will create a parent child relationship where the
parent container can see selected information about its child.
multiple containers together and send connection information from one to another.
When containers are linked, information about a source container can be sent to a
recipient container. This allows the recipient to see selected data describing
aspects of the source container.
## Container naming
To perform this linking Docker relies on the names of your containers.
We've already seen that each container we create has an automatically
created name, indeed we've become familiar with our old friend
To establish links, Docker relies on the names of your containers.
You've already seen that each container you create has an automatically
created name; indeed you've become familiar with our old friend
`nostalgic_morse` during this guide. You can also name containers
yourself. This naming provides two useful functions:
1. It's useful to name containers that do specific functions in a way
1. It can be useful to name containers that do specific functions in a way
that makes it easier for you to remember them, for example naming a
container with a web application in it `web`.
container containing a web application `web`.
2. It provides Docker with a reference point that allows it to refer to other
containers, for example link container `web` to container `db`.
containers, for example, you can specify to link the container `web` to container `db`.
You can name your container by using the `--name` flag, for example:
$ sudo docker run -d -P --name web training/webapp python app.py
You can see we've launched a new container and used the `--name` flag to
call the container `web`. We can see the container's name using the
This launches a new container and uses the `--name` flag to
name the container `web`. You can see the container's name using the
`docker ps` command.
$ 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
We can also use `docker inspect` to return the container's name.
You can also use `docker inspect` to return the container's name.
$ sudo docker inspect -f "{{ .Name }}" aed84ee21bde
/web
@ -117,67 +116,70 @@ We can also use `docker inspect` to return the container's name.
> **Note:**
> Container names have to be unique. That means you can only call
> one container `web`. If you want to re-use a container name you must delete
> the old container with the `docker rm` command before you can create a new
> the old container (with `docker rm`) before you can create a new
> container with the same name. As an alternative you can use the `--rm`
> flag with the `docker run` command. This will delete the container
> immediately after it stops.
> immediately after it is stopped.
## Container Linking
Links allow containers to discover and securely communicate with each
other. To create a link you use the `--link` flag. Let's create a new
container, this one a database.
Links allow containers to discover each other and securely transfer information about one
container to another container. When you set up a link, you create a conduit between a
source container and a recipient container. The recipient can then access select data
about the source. To create a link, you use the `--link` flag. First, create a new
container, this time one containing a database.
$ sudo docker run -d --name db training/postgres
Here we've created a new container called `db` using the `training/postgres`
This creates a new container called `db` from the `training/postgres`
image, which contains a PostgreSQL database.
We need to delete the `web` container we created previously so we can replace it
Now, you need to delete the `web` container you created previously so you can replace it
with a linked one:
$ docker rm -f web
Now let's create a new `web` container and link it with our `db` container.
Now, create a new `web` container and link it with your `db` container.
$ sudo docker run -d -P --name web --link db:db training/webapp python app.py
This will link the new `web` container with the `db` container we created
This will link the new `web` container with the `db` container you created
earlier. The `--link` flag takes the form:
--link name:alias
Where `name` is the name of the container we're linking to and `alias` is an
alias for the link name. We'll see how that alias gets used shortly.
alias for the link name. You'll see how that alias gets used shortly.
Let's look at our linked containers using `docker ps`.
Next, look at your linked containers using `docker ps`.
$ 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
We can see our named containers, `db` and `web`, and we can see that the `db`
containers also shows `web/db` in the `NAMES` column. This tells us that the
`web` container is linked to the `db` container in a parent/child relationship.
You can see your named containers, `db` and `web`, and you can see that the `db`
container also shows `web/db` in the `NAMES` column. This tells you that the
`web` container is linked to the `db` container, which allows it to access information
about the `db` container.
So what does linking the containers do? Well we've discovered the link creates
a parent-child relationship between the two containers. The child container,
here `web`, can access information on the parent container `db`. To do this
Docker creates a secure tunnel between the containers without the need to
expose any ports externally on the container. You'll note when we started the
`db` container we did not use either of the `-P` or `-p` flags. As we're
linking the containers we don't need to expose the PostgreSQL database via the
network.
So what does linking the containers actually do? You've learned that a link creates a
source container that can provide information about itself to a recipient container. In
our example, the recipient, `web`, can access information about the source `db`. To do
this, Docker creates a secure tunnel between the containers that doesn't need to
expose any ports externally on the container; you'll note when we started the
`db` container we did not use either the `-P` or `-p` flags. That's a big benefit of
linking: we don't need to expose the source container, here the PostgreSQL database, to
the network.
Docker exposes connectivity information for the parent container inside the
child container in two ways:
Docker exposes connectivity information for the source container to the
recipient container in two ways:
* Environment variables,
* Updating the `/etc/hosts` file.
Let's look first at the environment variables Docker sets. Let's run the `env`
command to list the container's environment variables.
Docker can set a number of environment variables. You run the `env`
command to list the specified container's environment variables.
```
$ sudo docker run --rm --name web2 --link db:db training/webapp env
@ -196,17 +198,17 @@ command to list the container's environment variables.
> container. Similarly, some daemons (such as `sshd`)
> will scrub them when spawning shells for connection.
We can see that Docker has created a series of environment variables with
useful information about our `db` container. Each variable is prefixed with
`DB_` which is populated from the `alias` we specified above. If our `alias`
were `db1` the variables would be prefixed with `DB1_`. You can use these
You can see that Docker has created a series of environment variables with
useful information about the source `db` container. Each variable is prefixed with
`DB_`, which is populated from the `alias` you specified above. If the `alias`
were `db1`, the variables would be prefixed with `DB1_`. You can use these
environment variables to configure your applications to connect to the database
on the `db` container. The connection will be secure, private and only the
on the `db` container. The connection will be secure and private; only the
linked `web` container will be able to talk to the `db` container.
In addition to the environment variables Docker adds a host entry for the
linked parent to the `/etc/hosts` file. Let's look at this file on the `web`
container now.
In addition to the environment variables, Docker adds a host entry for the
source container to the `/etc/hosts` file. Here's an entry for the `web`
container:
$ sudo docker run -t -i --rm --link db:db training/webapp /bin/bash
root@aed84ee21bde:/opt/webapp# cat /etc/hosts
@ -214,9 +216,9 @@ container now.
. . .
172.17.0.5 db
We can see two relevant host entries. The first is an entry for the `web`
You can see two relevant host entries. The first is an entry for the `web`
container that uses the Container ID as a host name. The second entry uses the
link alias to reference the IP address of the `db` container. Let's try to ping
link alias to reference the IP address of the `db` container. You can ping
that host now via this host name.
root@aed84ee21bde:/opt/webapp# apt-get install -yqq inetutils-ping
@ -227,21 +229,22 @@ that host now via this host name.
56 bytes from 172.17.0.5: icmp_seq=2 ttl=64 time=0.256 ms
> **Note:**
> We had to install `ping` because our container didn't have it.
> In the example, you'll note you had to install `ping` because it was not included
> in the container initially.
We've used the `ping` command to ping the `db` container using it's host entry
which resolves to `172.17.0.5`. We can make use of this host entry to configure
an application to make use of our `db` container.
Here, you used the `ping` command to ping the `db` container using its host entry,
which resolves to `172.17.0.5`. You can use this host entry to configure an application
to make use of your `db` container.
> **Note:**
> You can link multiple child containers to a single parent. For
> example, we could have multiple web containers attached to our `db`
> container.
> You can link multiple recipient containers to a single source. For
> example, you could have multiple (differently named) web containers attached to your
>`db` container.
# Next step
Now we know how to link Docker containers together the next step is
learning how to manage data, volumes and mounts inside our containers.
Now that you know how to link Docker containers together, the next step is
learning how to manage data, volumes and mounts inside your containers.
Go to [Managing Data in Containers](/userguide/dockervolumes).