Detaching Resources is as series about my personal findings and experiments on building web applications in a service-oriented fashion. The title and content are based on “Backing Services” from the Twelve-Factor App methodology.
A Quick Introduction on Virtualization
Virtualization, as the name suggests, is the act of creating a virtual version of something. It can be used to create virtual hardware, operating system, memory (swap), network (VPN), storage (RAID), etc. We will focus on the first two.
Hardware virtualization is probably the most known type. You certainly have heard of virtual machines (VM) and softwares like Parallels, VMware and VirtualBox. It allows you run, for instance, Linux inside Mac OS X. Since it creates an entire virtual computer, it can become very heavyweight, CPU and memory hungry.
Operating system (OS) virtualization acts in the OS kernel layer. It creates lightweight virtual environments (VE or containers), because they use the host OS’ system call. There is no need of an intermediate VM; and still being completely isolated. On the other hand, it is not as flexible as the hardware virtualization, since the guest and host OS must be the same. Linux-VServer, OpenVZ and Linux Containers (LXC) are examples of implementations of this technology.
LXC is the newest implementation of OS virtualization in Linux. It is based on the Linux kernel cgroups (control groups) to limit, account and isolate hardware resources of process groups. LXC is considered as something between a “chroot on steroids” and a full VM. Its goal is to create an environment as close as possible to a standard Linux installation, but without the need of a separate kernel.
Docker provides a high level API on top of LXC. It automates the creation of containers based on a Dockerfile, making it repeatable and consistent. It allows container versioning with git-like capabilities. Allows reuse. Any container can be used as a base image to create new ones. It encourages sharing. Anyone can upload a container to their public registry. The registry can also be used in your private servers.
Hands on Docker
To install Docker, check its documentation for your OS. It is comprehensive and easy to follow.
$ git clone https://github.com/dotcloud/docker.git $ cd docker $ vagrant up $ vagrant ssh
The docker daemon should be already running as root. Since the user
vagrant is in the group
docker, it is not necessary to run the
docker CLI with sudo.
To get the basics on Docker, I suggest you follow their interactive
tutorial. You will
learn how to
pull container images from the index
run the container,
your changes and
push it back to the repository. I will skip straight
to the use of a
Dockerfile, where things get more interesting.
Dockerfile contains the set of
steps docker will
use to reproduce the system images.
# Dockerfile FROM ubuntu:latest MAINTAINER Vinicius Horewicz <email@example.com> RUN sed -i.bak "s/main$/main universe/" /etc/apt/sources.list RUN apt-get update RUN apt-get upgrade -y RUN locale-gen en_US en_US.UTF-8 RUN echo "root:root" | chpasswd # initctl workaround RUN dpkg-divert --local --rename --add /sbin/initctl RUN ln -s /bin/true /sbin/initctl # prevent services from starting automatically RUN echo exit 101 > /usr/sbin/policy-rc.d RUN chmod +x /usr/sbin/policy-rc.d RUN apt-get install -y curl monit openssh-server rabbitmq-server # disable upstart for SSH RUN mv /etc/init/ssh.conf /etc/init/ssh.conf.dist ADD ./etc/monit/sshd.conf /etc/monit/conf.d/sshd.conf ADD ./etc/monit/rabbitmq.conf /etc/monit/conf.d/rabbitmq.conf # run monit in foreground CMD ["/usr/bin/monit", "-I"] EXPOSE 22 :5672
Dockerfiles follow the format
INSTRUCTION arguments. The first
instruction must be
FROM. It specifies the base image from which we
Images names are in the format
user is the
username of its creator [in the repository]. When it is not specified,
means you are using official images from the Docker team.
image is the
image name. Images can be tagged, just like git. That is the reason of
tag argument. When omitted, the value
latest is assumed.
RUN instruction runs a command and commits the result at the build
RUN means a new image is committed, which serves as a base
for the next instruction. Thanks to the this
layered approach and to
AuFS, Docker can cache build steps,
making them blazing fast.
ADD commands will copy files from
<dest>. Source can be a
file or directory relative to the source directory being built or a
remote file URL. Destination is the path in the container.
ADD is not
CMD specifies the command to be executed when the container is started.
There can exist only one in a Dockerfile. If there are more than one,
only the last will take effect.
EXPOSE sets port to be publicly exposed when running the container. It
is specified as
[PUBLIC:]PRIVATE. When PUBLIC is omitted, a random port
is assigned. If you want to use the same port for public and private,
you can specify only
Running the Container
You start the container with the
docker run command. It will run the
CMD. When this process completes, the container is fully
stopped. Think about containers as “process in a box”. This is why we
must run the
CMD in foreground instead of using a daemon or running
/etc/init.d/monit start in
One might ask: Why monit? Remember a container run only one process and stops right after this process ends. Even though a container can start with /sbin/init, docker still has some issues with upstart, so we will use monit to start and keep our process running.
The Final Result
The final result is available in this repository. I have created basic monitrc files for SSH and RabbitMQ, so monit can start these services. If you want to give it a try:
$ git clone https://github.com/wicz/detaching-resources.git $ cd detaching-resources/v1/docker-rabbitmq $ docker build -t <yourusername>/rabbitmq . $ docker run -t <yourusername>/rabbitmq
Finally, we have set up the architecture foundation. In the next post of the series we will start implementing services which will exchange messages through this message queue container.