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Work with a development container
In this section, you learn to develop like the Moby Engine core team.
The moby/moby
repository includes a Dockerfile
at its root. This file defines
Moby's development environment. The Dockerfile
lists the environment's
dependencies: system libraries and binaries, Go environment, Go dependencies,
etc.
Moby's development environment is itself, ultimately a Docker container.
You use the moby/moby
repository and its Dockerfile
to create a Docker image,
run a Docker container, and develop code in the container.
If you followed the procedures that set up Git for contributing, you should have a fork of the moby/moby
repository. You also created a branch called dry-run-test
. In this section,
you continue working with your fork on this branch.
Task 1. Remove images and containers
Moby developers run the latest stable release of the Docker software. They clean their local hosts of unnecessary Docker artifacts such as stopped containers or unused images. Cleaning unnecessary artifacts isn't strictly necessary, but it is good practice, so it is included here.
To remove unnecessary artifacts:
-
Verify that you have no unnecessary containers running on your host.
$ docker ps -a
You should see something similar to the following:
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
There are no running or stopped containers on this host. A fast way to remove old containers is the following:
You can now use the
docker system prune
command to achieve this:$ docker system prune -a
Older versions of the Docker Engine should reference the command below:
$ docker rm $(docker ps -a -q)
This command uses
docker ps
to list all containers (-a
flag) by numeric IDs (-q
flag). Then, thedocker rm
command removes the resulting list. If you have running but unused containers, stop and then remove them with thedocker stop
anddocker rm
commands. -
Verify that your host has no dangling images.
$ docker images
You should see something similar to the following:
REPOSITORY TAG IMAGE ID CREATED SIZE
This host has no images. You may have one or more dangling images. A dangling image is not used by a running container and is not an ancestor of another image on your system. A fast way to remove dangling image is the following:
$ docker rmi -f $(docker images -q -a -f dangling=true)
This command uses
docker images
to list all images (-a
flag) by numeric IDs (-q
flag) and filter them to find dangling images (-f dangling=true
). Then, thedocker rmi
command forcibly (-f
flag) removes the resulting list. If you get a "docker: "rmi" requires a minimum of 1 argument." message, that means there were no dangling images. To remove just one image, use thedocker rmi ID
command.
Task 2. Start a development container
If you followed the last procedure, your host is clean of unnecessary images and containers. In this section, you build an image from the Engine development environment and run it in the container. Both steps are automated for you by the Makefile in the Engine code repository. The first time you build an image, it can take over 15 minutes to complete.
-
Open a terminal.
For Docker Toolbox users, use
docker-machine status your_vm_name
to make sure your VM is running. You may need to runeval "$(docker-machine env your_vm_name)"
to initialize your shell environment. If you use Docker for Mac or Docker for Windows, you do not need to use Docker Machine. -
Change into the root of the
moby-fork
repository.$ cd ~/repos/moby-fork
If you are following along with this guide, you created a
dry-run-test
branch when you set up Git for contributing. -
Ensure you are on your
dry-run-test
branch.$ git checkout dry-run-test
If you get a message that the branch doesn't exist, add the
-b
flag (git checkout -b dry-run-test
) so the command both creates the branch and checks it out. -
Use
make
to build a development environment image and run it in a container.$ make BIND_DIR=. shell
Using the instructions in the
Dockerfile
, the build may need to download and / or configure source and other images. On first build this process may take between 5 - 15 minutes to create an image. The command returns informational messages as it runs. A successful build returns a final message and opens a Bash shell into the container.Successfully built 3d872560918e Successfully tagged docker-dev:dry-run-test docker run --rm -i --privileged -e BUILDFLAGS -e KEEPBUNDLE -e DOCKER_BUILD_GOGC -e DOCKER_BUILD_PKGS -e DOCKER_CLIENTONLY -e DOCKER_DEBUG -e DOCKER_EXPERIMENTAL -e DOCKER_GITCOMMIT -e DOCKER_GRAPHDRIVER=devicemapper -e DOCKER_INCREMENTAL_BINARY -e DOCKER_REMAP_ROOT -e DOCKER_STORAGE_OPTS -e DOCKER_USERLANDPROXY -e TESTDIRS -e TESTFLAGS -e TIMEOUT -v "home/ubuntu/repos/docker/bundles:/go/src/github.com/moby/moby/bundles" -t "docker-dev:dry-run-test" bash root@f31fa223770f:/go/src/github.com/docker/docker#
At this point, your prompt reflects the container's BASH shell.
-
List the contents of the current directory (
/go/src/github.com/docker/docker
).You should see the image's source from the
/go/src/github.com/docker/docker
directory. -
Make a
dockerd
binary.root@a8b2885ab900:/go/src/github.com/docker/docker# hack/make.sh binary Removing bundles/ ---> Making bundle: binary (in bundles/binary) Building: bundles/binary-daemon/dockerd-17.06.0-dev Created binary: bundles/binary-daemon/dockerd-17.06.0-dev Copying nested executables into bundles/binary-daemon
-
Run
make install
, which copies the binary to the container's/usr/local/bin/
directory.root@a8b2885ab900:/go/src/github.com/docker/docker# make install
-
Start the Engine daemon running in the background.
root@a8b2885ab900:/go/src/github.com/docker/docker# dockerd -D & ...output snipped... DEBU[0001] Registering POST, /networks/{id:.*}/connect DEBU[0001] Registering POST, /networks/{id:.*}/disconnect DEBU[0001] Registering DELETE, /networks/{id:.*} INFO[0001] API listen on /var/run/docker.sock DEBU[0003] containerd connection state change: READY
The
-D
flag starts the daemon in debug mode. The&
starts it as a background process. You'll find these options useful when debugging code development. You will need to hitreturn
in order to get back to your shell prompt.Note
: The following command automates the
build
,install
, andrun
steps above. Once the command below completes, hitctrl-z
to suspend the process, then runbg 1
and hitenter
to resume the daemon process in the background and get back to your shell prompt.hack/make.sh binary install-binary run
-
Inside your container, check your Docker version.
root@5f8630b873fe:/go/src/github.com/docker/docker# docker --version Docker version 1.12.0-dev, build 6e728fb
Inside the container you are running a development version. This is the version on the current branch. It reflects the value of the
VERSION
file at the root of yourdocker-fork
repository. -
Run the
hello-world
image.root@5f8630b873fe:/go/src/github.com/docker/docker# docker run hello-world
-
List the image you just downloaded.
root@5f8630b873fe:/go/src/github.com/docker/docker# docker images REPOSITORY TAG IMAGE ID CREATED SIZE hello-world latest c54a2cc56cbb 3 months ago 1.85 kB
-
Open another terminal on your local host.
-
List the container running your development container.
ubuntu@ubuntu1404:~$ docker ps CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES a8b2885ab900 docker-dev:dry-run-test "hack/dind bash" 43 minutes ago Up 43 minutes hungry_payne
Notice that the tag on the container is marked with the
dry-run-test
branch name.
Task 3. Make a code change
At this point, you have experienced the "Moby inception" technique. That is, you have:
- forked and cloned the Moby Engine code repository
- created a feature branch for development
- created and started an Engine development container from your branch
- built a binary inside of your development container
- launched a
docker
daemon using your newly compiled binary - called the
docker
client to run ahello-world
container inside your development container
Running the make BIND_DIR=. shell
command mounted your local Docker repository source into
your Docker container.
Note
: Inspecting the
Dockerfile
shows aCOPY . /go/src/github.com/docker/docker
instruction, suggesting that dynamic code changes will not be reflected in the container. However inspecting theMakefile
shows that the current working directory will be mounted via a-v
volume mount.
When you start to develop code though, you'll want to iterate code changes and builds inside the container. If you have followed this guide exactly, you have a bash shell running a development container.
Try a simple code change and see it reflected in your container. For this
example, you'll edit the help for the attach
subcommand.
-
If you don't have one, open a terminal in your local host.
-
Make sure you are in your
moby-fork
repository.$ pwd /Users/mary/go/src/github.com/moxiegirl/moby-fork
Your location should be different because, at least, your username is different.
-
Open the
cmd/dockerd/docker.go
file. -
Edit the command's help message.
For example, you can edit this line:
Short: "A self-sufficient runtime for containers.",
And change it to this:
Short: "A self-sufficient and really fun runtime for containers.",
-
Save and close the
cmd/dockerd/docker.go
file. -
Go to your running docker development container shell.
-
Rebuild the binary by using the command
hack/make.sh binary
in the docker development container shell. -
Stop Docker if it is running.
-
Copy the binaries to /usr/bin by entering the following commands in the docker development container shell.
hack/make.sh binary install-binary
-
To view your change, run the
dockerd --help
command in the docker development container shell.
root@b0cb4f22715d:/go/src/github.com/docker/docker# dockerd --help
Usage: dockerd COMMAND
A self-sufficient and really fun runtime for containers.
Options:
...
You've just done the basic workflow for changing the Engine code base. You made your code changes in your feature branch. Then, you updated the binary in your development container and tried your change out. If you were making a bigger change, you might repeat or iterate through this flow several times.
Where to go next
Congratulations, you have successfully achieved Docker inception. You've had a small experience of the development process. You've set up your development environment and verified almost all the essential processes you need to contribute. Of course, before you start contributing, you'll need to learn one more piece of the development process, the test framework.