First pass at replacing parent-child metaphor to describe docker links.
Addresses issue #7141 Docker-DCO-1.1-Signed-off-by: Fred Lifton <fred.lifton@docker.com> (github: fredlf) a little git by Docker-DCO-1.1-Signed-off-by: Fred Lifton <fred.lifton@docker.com> (github: SvenDowideit)
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Fred Lifton
Sven Dowideit
parent
5395cecc48
commit
631ea726e4
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@ -4,48 +4,47 @@ page_keywords: Examples, Usage, user guide, links, linking, docker, documentatio
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# Linking Containers Together
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In [the Using Docker section](/userguide/usingdocker) we touched on
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connecting to a service running inside a Docker container via a network
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port. This is one of the ways that you can interact with services and
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applications running inside Docker containers. In this section we're
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going to give you a refresher on connecting to a Docker container via a
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network port as well as introduce you to the concepts of container
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linking.
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In [the Using Docker section](/userguide/usingdocker), you saw how you can
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connect to a service running inside a Docker container via a network
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port. But a port connection is only one way you can interact with services and
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applications running inside Docker containers. In this section, we'll briefly revisit
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connecting via a network port and then we'll introduce you to another method of access:
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container linking.
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## Network port mapping refresher
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In [the Using Docker section](/userguide/usingdocker) we created a
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container that ran a Python Flask application.
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In [the Using Docker section](/userguide/usingdocker), you created a
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container that ran a Python Flask application:
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$ sudo docker run -d -P training/webapp python app.py
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> **Note:**
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> Containers have an internal network and an IP address
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> (remember we used the `docker inspect` command to show the container's
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> (as we saw when we used the `docker inspect` command to show the container's
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> IP address in the [Using Docker](/userguide/usingdocker/) section).
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> Docker can have a variety of network configurations. You can see more
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> information on Docker networking [here](/articles/networking/).
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When we created that container we used the `-P` flag to automatically map any
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network ports inside that container to a random high port from the range 49000
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to 49900 on our Docker host. When we subsequently ran `docker ps` we saw that
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port 5000 was bound to port 49155.
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When that container was created, the `-P` flag was used to automatically map any
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network ports inside it to a random high port from the range 49000
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to 49900 on our Docker host. Next, when `docker ps` was run, you saw that
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port 5000 in the container was bound to port 49155 on the host.
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$ sudo docker ps nostalgic_morse
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CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
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bc533791f3f5 training/webapp:latest python app.py 5 seconds ago Up 2 seconds 0.0.0.0:49155->5000/tcp nostalgic_morse
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We also saw how we can bind a container's ports to a specific port using
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the `-p` flag.
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You also saw how you can bind a container's ports to a specific port using
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the `-p` flag:
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$ sudo docker run -d -p 5000:5000 training/webapp python app.py
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And we saw why this isn't such a great idea because it constrains us to
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And you saw why this isn't such a great idea because it constrains you to
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only one container on that specific port.
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There are also a few other ways we can configure the `-p` flag. By
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There are also a few other ways you can configure the `-p` flag. By
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default the `-p` flag will bind the specified port to all interfaces on
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the host machine. But we can also specify a binding to a specific
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the host machine. But you can also specify a binding to a specific
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interface, for example only to the `localhost`.
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$ sudo docker run -d -p 127.0.0.1:5000:5000 training/webapp python app.py
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@ -53,20 +52,19 @@ interface, for example only to the `localhost`.
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This would bind port 5000 inside the container to port 5000 on the
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`localhost` or `127.0.0.1` interface on the host machine.
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Or to bind port 5000 of the container to a dynamic port but only on the
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`localhost` we could:
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Or, to bind port 5000 of the container to a dynamic port but only on the
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`localhost`, you could use:
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$ sudo docker run -d -p 127.0.0.1::5000 training/webapp python app.py
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We can also bind UDP ports by adding a trailing `/udp`, for example:
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You can also bind UDP ports by adding a trailing `/udp`. For example:
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$ sudo docker run -d -p 127.0.0.1:5000:5000/udp training/webapp python app.py
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We also saw the useful `docker port` shortcut which showed us the
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current port bindings, this is also useful for showing us specific port
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configurations. For example if we've bound the container port to the
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`localhost` on the host machine this will be shown in the `docker port`
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output.
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You also learned about the useful `docker port` shortcut which showed us the
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current port bindings. This is also useful for showing you specific port
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configurations. For example, if you've bound the container port to the
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`localhost` on the host machine, then the `docker port` output will reflect that.
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$ docker port nostalgic_morse 5000
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127.0.0.1:49155
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@ -78,38 +76,39 @@ output.
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Network port mappings are not the only way Docker containers can connect
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to one another. Docker also has a linking system that allows you to link
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multiple containers together and share connection information between
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them. Docker linking will create a parent child relationship where the
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parent container can see selected information about its child.
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multiple containers together and send connection information from one to another.
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When containers are linked, information about a source container can be sent to a
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recipient container. This allows the recipient to see selected data describing
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aspects of the source container.
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## Container naming
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To perform this linking Docker relies on the names of your containers.
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We've already seen that each container we create has an automatically
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created name, indeed we've become familiar with our old friend
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To establish links, Docker relies on the names of your containers.
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You've already seen that each container you create has an automatically
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created name; indeed you've become familiar with our old friend
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`nostalgic_morse` during this guide. You can also name containers
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yourself. This naming provides two useful functions:
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1. It's useful to name containers that do specific functions in a way
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1. It can be useful to name containers that do specific functions in a way
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that makes it easier for you to remember them, for example naming a
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container with a web application in it `web`.
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container containing a web application `web`.
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2. It provides Docker with a reference point that allows it to refer to other
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containers, for example link container `web` to container `db`.
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containers, for example, you can specify to link the container `web` to container `db`.
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You can name your container by using the `--name` flag, for example:
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$ sudo docker run -d -P --name web training/webapp python app.py
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You can see we've launched a new container and used the `--name` flag to
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call the container `web`. We can see the container's name using the
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This launches a new container and uses the `--name` flag to
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name the container `web`. You can see the container's name using the
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`docker ps` command.
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$ sudo docker ps -l
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CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
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aed84ee21bde training/webapp:latest python app.py 12 hours ago Up 2 seconds 0.0.0.0:49154->5000/tcp web
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We can also use `docker inspect` to return the container's name.
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You can also use `docker inspect` to return the container's name.
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$ sudo docker inspect -f "{{ .Name }}" aed84ee21bde
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/web
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@ -117,67 +116,70 @@ We can also use `docker inspect` to return the container's name.
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> **Note:**
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> Container names have to be unique. That means you can only call
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> one container `web`. If you want to re-use a container name you must delete
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> the old container with the `docker rm` command before you can create a new
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> the old container (with `docker rm`) before you can create a new
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> container with the same name. As an alternative you can use the `--rm`
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> flag with the `docker run` command. This will delete the container
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> immediately after it stops.
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> immediately after it is stopped.
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## Container Linking
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Links allow containers to discover and securely communicate with each
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other. To create a link you use the `--link` flag. Let's create a new
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container, this one a database.
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Links allow containers to discover each other and securely transfer information about one
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container to another container. When you set up a link, you create a conduit between a
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source container and a recipient container. The recipient can then access select data
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about the source. To create a link, you use the `--link` flag. First, create a new
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container, this time one containing a database.
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$ sudo docker run -d --name db training/postgres
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Here we've created a new container called `db` using the `training/postgres`
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This creates a new container called `db` from the `training/postgres`
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image, which contains a PostgreSQL database.
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We need to delete the `web` container we created previously so we can replace it
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Now, you need to delete the `web` container you created previously so you can replace it
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with a linked one:
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$ docker rm -f web
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Now let's create a new `web` container and link it with our `db` container.
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Now, create a new `web` container and link it with your `db` container.
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$ sudo docker run -d -P --name web --link db:db training/webapp python app.py
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This will link the new `web` container with the `db` container we created
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This will link the new `web` container with the `db` container you created
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earlier. The `--link` flag takes the form:
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--link name:alias
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Where `name` is the name of the container we're linking to and `alias` is an
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alias for the link name. We'll see how that alias gets used shortly.
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alias for the link name. You'll see how that alias gets used shortly.
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Let's look at our linked containers using `docker ps`.
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Next, look at your linked containers using `docker ps`.
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$ docker ps
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CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
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349169744e49 training/postgres:latest su postgres -c '/usr About a minute ago Up About a minute 5432/tcp db, web/db
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aed84ee21bde training/webapp:latest python app.py 16 hours ago Up 2 minutes 0.0.0.0:49154->5000/tcp web
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We can see our named containers, `db` and `web`, and we can see that the `db`
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containers also shows `web/db` in the `NAMES` column. This tells us that the
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`web` container is linked to the `db` container in a parent/child relationship.
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You can see your named containers, `db` and `web`, and you can see that the `db`
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container also shows `web/db` in the `NAMES` column. This tells you that the
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`web` container is linked to the `db` container, which allows it to access information
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about the `db` container.
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So what does linking the containers do? Well we've discovered the link creates
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a parent-child relationship between the two containers. The child container,
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here `web`, can access information on the parent container `db`. To do this
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Docker creates a secure tunnel between the containers without the need to
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expose any ports externally on the container. You'll note when we started the
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`db` container we did not use either of the `-P` or `-p` flags. As we're
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linking the containers we don't need to expose the PostgreSQL database via the
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network.
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So what does linking the containers actually do? You've learned that a link creates a
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source container that can provide information about itself to a recipient container. In
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our example, the recipient, `web`, can access information about the source `db`. To do
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this, Docker creates a secure tunnel between the containers that doesn't need to
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expose any ports externally on the container; you'll note when we started the
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`db` container we did not use either the `-P` or `-p` flags. That's a big benefit of
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linking: we don't need to expose the source container, here the PostgreSQL database, to
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the network.
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Docker exposes connectivity information for the parent container inside the
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child container in two ways:
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Docker exposes connectivity information for the source container to the
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recipient container in two ways:
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* Environment variables,
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* Updating the `/etc/hosts` file.
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Let's look first at the environment variables Docker sets. Let's run the `env`
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command to list the container's environment variables.
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Docker can set a number of environment variables. You run the `env`
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command to list the specified container's environment variables.
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```
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$ sudo docker run --rm --name web2 --link db:db training/webapp env
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@ -196,17 +198,17 @@ command to list the container's environment variables.
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> container. Similarly, some daemons (such as `sshd`)
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> will scrub them when spawning shells for connection.
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We can see that Docker has created a series of environment variables with
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useful information about our `db` container. Each variable is prefixed with
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`DB_` which is populated from the `alias` we specified above. If our `alias`
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were `db1` the variables would be prefixed with `DB1_`. You can use these
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You can see that Docker has created a series of environment variables with
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useful information about the source `db` container. Each variable is prefixed with
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`DB_`, which is populated from the `alias` you specified above. If the `alias`
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were `db1`, the variables would be prefixed with `DB1_`. You can use these
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environment variables to configure your applications to connect to the database
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on the `db` container. The connection will be secure, private and only the
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on the `db` container. The connection will be secure and private; only the
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linked `web` container will be able to talk to the `db` container.
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In addition to the environment variables Docker adds a host entry for the
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linked parent to the `/etc/hosts` file. Let's look at this file on the `web`
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container now.
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In addition to the environment variables, Docker adds a host entry for the
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source container to the `/etc/hosts` file. Here's an entry for the `web`
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container:
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$ sudo docker run -t -i --rm --link db:db training/webapp /bin/bash
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root@aed84ee21bde:/opt/webapp# cat /etc/hosts
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@ -214,9 +216,9 @@ container now.
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. . .
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172.17.0.5 db
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We can see two relevant host entries. The first is an entry for the `web`
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You can see two relevant host entries. The first is an entry for the `web`
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container that uses the Container ID as a host name. The second entry uses the
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link alias to reference the IP address of the `db` container. Let's try to ping
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link alias to reference the IP address of the `db` container. You can ping
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that host now via this host name.
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root@aed84ee21bde:/opt/webapp# apt-get install -yqq inetutils-ping
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@ -227,21 +229,22 @@ that host now via this host name.
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56 bytes from 172.17.0.5: icmp_seq=2 ttl=64 time=0.256 ms
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> **Note:**
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> We had to install `ping` because our container didn't have it.
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> In the example, you'll note you had to install `ping` because it was not included
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> in the container initially.
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We've used the `ping` command to ping the `db` container using it's host entry
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which resolves to `172.17.0.5`. We can make use of this host entry to configure
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an application to make use of our `db` container.
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Here, you used the `ping` command to ping the `db` container using its host entry,
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which resolves to `172.17.0.5`. You can use this host entry to configure an application
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to make use of your `db` container.
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> **Note:**
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> You can link multiple child containers to a single parent. For
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> example, we could have multiple web containers attached to our `db`
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> container.
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> You can link multiple recipient containers to a single source. For
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> example, you could have multiple (differently named) web containers attached to your
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>`db` container.
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# Next step
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Now we know how to link Docker containers together the next step is
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learning how to manage data, volumes and mounts inside our containers.
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Now that you know how to link Docker containers together, the next step is
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learning how to manage data, volumes and mounts inside your containers.
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Go to [Managing Data in Containers](/userguide/dockervolumes).
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Reference in New Issue