This presentation was given at ThoughtWorks pune during meetup. It describes about Docker, Docker general commands and underhood of it.

1. Docker Tech Talk with Demo
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Sandeep Karnawat
Principal S/W Engineer

2. Sample Agenda
1 Docker Background
2 How to Use Docker
3 Docker Inside
4 Docker Demo
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3. What is docker?
• Docker is:
– An open platform for developers and sysadmins to develop, ship, and run
distributed applications [docker.io]
– An open-source project that automates the deployment of applications
inside software containers by providing an additional layer of abstraction
and automation of operating system-level virtualization on Linux
[Wikipedia]
– A tool that can package an application and its dependencies in a virtual
container that can run on any Linux server [451 Research]
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4. How is it different from VM?
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5. What does Docker look like?
• Docker is a client-server application.
– Docker client and the daemon can run on the same system, or on different
machines
– They communicate via sockets (or through a RESTful API).
– Users interact with the client to command the daemon
– The daemon, receiving those commands, does the job
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6. Sample Agenda
1 Docker Background
2 How to Use Docker
3 Docker Inside
4 Docker Application
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7. How to download a docker image
• docker pull <image_name>
– Pull: fetch the image from the Docker Hub Registry
(registry.hub.docker.com)
– Image_name: usually consist of user_name/image_name
– E.g., sudo docker pull skarnawat/mytest_docker
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8. How to launch a docker container
• docker run –it <image_name> [command_name]
– Option –t: allocate pseudo-terminal
– Option –i: interactive mode
– Eg: docker run –it mingwei/mytest_docker bash
– Note: when you run “bash” option -i and –t are both needed
– When you do not need “terminal”; use “–d” instead of “-it”
• Where is my container?
– docker ps
– docker ps –a (all containers you have run)
• How can I name my own container?
– docker run –it –n mycontainer mingwei/mytest_docker bash
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9. How to generate a new image?
• Manually Create Image
– A container is a running instance of an image
– When all processes inside container exit, container is stopped
– One way to create a new image:
• Create a new container using “docker run –it <image> bash”
• Issue commands: “apt-get install <software>”
• Transform your container to an image:
– docker commit <your_container_name> <your_new_image_name>
– What if I didn’t name my container?
• Use the automatically assigned container id instead.
• Container id could be found using “docker ps “
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10. How to generate your own image
• Dockerfile
– Like a makefile, you use it to automate the building of an image:
• docker build –t <your_img_name> .
• Your image is built using the Dockerfile in current directory
• Docker file contains a sequence of commands
– Inside Dockerfile:
• Updates will be applied to new image
• FROM: base image you specify
• RUN: run a command inside container
• ADD: copy files into new image
– Note: tar, gzip bzip2 and etc will be decompressed
• Other Directives:
– http://docs.docker.com/reference/builder/
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From ubuntu:14.04
Author: skarnawat
RUN apt-get install binutils
ADD myfile.tar /app
VOLUME [“/yourdata”]
…

11. Docker Tech Talk
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12. Login to Existing Container
• You can’t login to an existing container, if
– The container does not have terminal (launched with –d but not –it)
– E.g., docker run –d skdocker/apache apache2ctl -D FOREGROUND
• Option1: sshd server
– Using a sshd, you could login to existing container
– Issues: manage passwords, keys
• Option2: use docker attach
– Docker attach <container_name>
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13. Docker Tech Talk
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Docker: Under the Hood
Implementation and Details

14. Docker Container Implementation
• Namespaces
– Docker takes advantage of a technology called namespaces to provide the
isolated workspace we call the container.
– One container cannot see names in another container’s namespace
– The pid namespace: virtualized process names (PID: Process ID).
– The net namespace: virtualized network interfaces, routing tables, etc.
(NET: Networking).
– The mnt namespace: virtualized file system mount points (MNT: Mount).
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15. Linux Container Implementation
• Namespaces
• Control groups
– provide a mechanism for performance isolation
– Cgroup allows you to control the resource usage of:
• CPUSET and CPU USAGE
• Memory
• Disk I/O
• Device visibility
– Cgroup is maintained as a virtual file system (like proc): cgroupfs
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16. Your diff
(rw)
Your
view
image #1
Base image
(readonly)
File System (AUFS)
• AUFS: advanced union file system
– Union of all images
– Less storage
– Maximum layers 127
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image #2
Your diff
(rw)
Your diff
(ro)
Your diff
(rw)
Your diff
(ro)

17. Docker and LXC
• Docker containers are in linux
container format.
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18. Comparison
Docker
• Application container
– Only application process is running
• Ship with file system support
• Use cgroups and namespace
• Has docker image repository
• Support versioning and commit
• Has API support
• Support SELinux and Apparmor
LXC
• Light weight virtual machine
– A set of system daemons are running
• User need to config their file system
• Use cgroups and namespace
• No repository support (can’t move!)
• No support on image version
• No API, only configuration
• Support SELinux and Apparmor
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It is still unclear which style of containers will win in the future

19. Docker Start-up Process
• Command: docker run –it ubuntu bash
• What happened?
– Mount aufs (all diffs)
• Transform image name to ID
• Get all diffs required and merge them
– Prepare cgroup file system
– Launch container process (using clone)
• Clone(2) specify using new namespace
• Exec docerinit (launch docker binary)
• Prepare other file systems
– Devfs, tmpfs, proc and etc
– Symlinks for standard I/O
• Change root file system (pivot_root)
• Drop capabilities
– Capget(2)
– Prctl(PR_CAPBSET_DROP, 0x.., 0, 0, 0)
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Base image
Base image
(readonly)
Diff (ro)
Diff (ro)
Diff (ro)
Diff (ro)
Diff (ro)
/var/lib/docker/aufs/diff/ID:
/var/lib/docker/aufs/mnt/Container_ID:

20. Docker Start-up Process
• Command: Docker run –it ubuntu bash
• What happened?
– Mount aufs (all diffs)
– Prepare cgroup (resource management)
– Launch container process (using clone)
• Clone(2) specify using new namespace
• Exec docerinit (launch docker binary)
• Prepare other file systems
– Devfs, tmpfs, proc and etc
– Symlinks for standard I/O and etc
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Container File System View
base=/var/lib/docker/aufs/mnt/Container_ID:
$base/dev
$base/etc/hostname
$base/etc/resolv.conf
$base/etc/hosts
$base/proc/proc/fd/0

21. Docker Start-up Process
• Command: Docker run –it ubuntu bash
• What happened?
– Mount aufs (all diffs)
– Prepare cgroup (resource management)
– Launch container process (using clone)
• Clone(2) specify using new namespace
• Exec docerinit (launch docker binary)
• Prepare other file systems
– Devfs, tmpfs, proc and etc
– Symlinks for standard I/O
• Change root file system (pivot_root)
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Container File System View
base=/var/lib/docker/aufs/mnt/Container_ID:
/dev
/etc/hostname
/etc/resolv.conf
/etc/hosts
/proc

22. Sample Agenda
1 Docker Background
2 How to Use Docker
3 Docker Inside
4 Docker Demo
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23. Thank You!
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Karnawat.sk@gamil.com
https://in.linkedin.com/in/sandeepkarnawat