Docker

@marekgoldmann

Poznań JUG, 3rd June 2014

Applications are complex

http://www.reddit.com/r/pics/comments/nf8la/im_always_tempted_to_switch_two_random_cables_to/

Different deployment environments

http://www.shedworking.co.uk/2012/03/nuts-and-bolts-cable-ties.html

But how?

Unified deployment format

A project to manage containers.

Containers?

Yes, a lightweight operating system virtualization.

How this compares to a virtual machine?

It's completely different.

Comparison

	Virtual Machine (KVM, VMware)	Container (LXC, OpenVZ)
Hardware	Simulated	Uses it (almost) directly
Supported OS'es	Almost any	Only Linux
Space	User space	Kernel space
Separation	Full	Control Groups (cgroups)
Startup time	Seconds to minutes	Miliseconds
Scalability	A few	Sky is the limit (thousands)
Custom kernel	Yes	No
Enterprise features (live migration, etc)	Yes	No
Ease of creation	Moderate	Easy
Time consumption of creation	High	Low
Size	HUGE	Small

Who's behind Docker?

Docker, Inc. and a powerful community.

Speaking of community...

GitHub
- >800 watchers
- >11000 stars
- >1800 forks
- >500 open issues
- >80 pull requests
Over 400 contributors
- avg 17 commits / day
- 90% are external
Many meetups all over the world
- 50 active meetup groups

Achieved after 15 months since first commit to GitHub

Glossary

Image: a (read-only) template with filesystem and data
Container: a running image
Registry: place where images are hosted/shared

Main concepts

An image can be linked to a parent image
Image not having a parent is called base image
Container has a state, whereas image doesn't
You can launch multiple containers from an image

How does it work?

cgroups
- groups processes in trees, single entity
namespaces
- process isolation; network, pid, filesystem, etc
filesystem
- aufs, btrfs, devicemapper

Show it!

Creating Docker images

It's simple! Use Dockerfiles.

Dockerfile

A plain text file with instructions that automates building the image.

Dockerfile example


# Base on the Fedora 20 image
FROM fedora

# Upgrade the image
RUN yum -y update

# Install WildFly application server
RUN yum -y install wildfly

# Run WildFly after container boot
ENTRYPOINT /usr/share/wildfly/bin/standalone.sh -c standalone-ha.xml -b 0.0.0.0

This should be saved as Dockerfile

Building the image

docker build .

Building the image


$ docker build .
Uploading context  2.56 kB
Uploading context 
Step 0 : FROM fedora
 ---> 58394af37342

[SNIP]

Step 3 : ENTRYPOINT /usr/share/wildfly/bin/standalone.sh -c standalone-ha.xml -b 0.0.0.0
 ---> Running in 72f63daffbd1
 ---> e91c26099a67
Removing intermediate container 72f63daffbd1
Successfully built e91c26099a67

We've built image e91c26099a67