cfe-docker

Create Docker containers with managed processes.

Docker monitors one process in each running container and the container lives or dies with that process. By introducing CFEngine inside Docker containers, we can alleviate a few of the issues that may arise:

• It is possible to easily start multiple processes within a container, all of which will be managed automatically, with the normal docker run command.
• If a managed process dies or crashes, CFEngine will start it again within 1 minute.
• The container itself will live as long as the CFEngine scheduling daemon (cf-execd) lives. With CFEngine, we are able to decouple the life of the container from the uptime of the service it provides.

How it works

CFEngine, together with the cfe-docker integration policies, are installed as part of the Dockerfile. This builds CFEngine into our Docker image.

The Dockerfile's ENTRYPOINT takes an arbitrary amount of commands (with any desired arguments) as parameters. When we run the Docker container these parameters get written to CFEngine policies and CFEngine takes over to ensure that the desired processes are running in the container.

CFEngine scans the process table for the basename of the commands given to the ENTRYPOINT and runs the command to start the process if the basename is not found. For example, if we start the container with

docker run "/path/to/my/application parameters"

CFEngine will look for a process named application and run the command. If an entry for application is not found in the process table at any point in time, CFEngine will execute /path/to/my/application parameters to start the application once again. The check on the process table happens every minute.

Note that it is therefore important that the command to start your application leaves a process with the basename of the command. This can be made more flexible by making some minor adjustments to the CFEngine policies, if desired.

Usage

This example assumes you have Docker installed and working. Installation instructions can be found at the Docker web site. We will install and manage apache2 and sshd in a single container.

There are three steps:

1. Install CFEngine into the container.
2. Copy the CFEngine Docker process management policy into the containerized CFEngine installation.
3. Start your application processes as part of the docker run command.

Building the container image

The first two steps can be done as part of a Dockerfile, as follows.

FROM ubuntu
MAINTAINER Eystein Måløy Stenberg <eytein.stenberg@gmail.com>

RUN apt-get -y install wget lsb-release unzip

# install latest CFEngine
RUN wget -qO- http://cfengine.com/pub/gpg.key | apt-key add -
RUN echo "deb http://cfengine.com/pub/apt $(lsb_release -cs) main" > /etc/apt/sources.list.d/cfengine-community.list
RUN apt-get update
RUN apt-get install cfengine-community

# install cfe-docker process management policy
RUN wget --no-check-certificate https://github.com/estenberg/cfe-docker/archive/master.zip -P /tmp/ && unzip /tmp/master.zip -d /tmp/
RUN cp /tmp/cfe-docker-master/cfengine/bin/* /var/cfengine/bin/
RUN cp /tmp/cfe-docker-master/cfengine/inputs/* /var/cfengine/inputs/
RUN rm -rf /tmp/cfe-docker-master /tmp/master.zip

# apache2 and openssh are just for testing purposes, install your own apps here
RUN apt-get -y install openssh-server apache2
RUN mkdir -p /var/run/sshd
RUN echo "root:password" | chpasswd  # need a password for ssh

ENTRYPOINT ["/var/cfengine/bin/docker_processes_run.sh"]

By saving this file as Dockerfile to a working directory, you can then build your container with the docker build command, e.g.

docker build -t managed_image .

Testing the container

Start the container with apache2 and sshd running and managed, forwarding a port to our SSH instance:

docker run -p 127.0.0.1:222:22 -d managed_image "/usr/sbin/sshd" "/etc/init.d/apache2 start"

We now clearly see one of the benefits of the cfe-docker integration: it allows to start several processes as part of a normal docker run command.

We can now log in to our new container and see that both apache2 and sshd are running. We have set the root password to "password" in the Dockerfile above and can use that to log in with ssh:

ssh -p222 root@127.0.0.1

# ps -ef
UID        PID  PPID  C STIME TTY          TIME CMD
root         1     0  0 07:48 ?        00:00:00 /bin/bash /var/cfengine/bin/docker_processes_run.sh /usr/sbin/sshd /etc/init.d/apache2 start
root        18     1  0 07:48 ?        00:00:00 /var/cfengine/bin/cf-execd -F
root        20     1  0 07:48 ?        00:00:00 /usr/sbin/sshd
root        32     1  0 07:48 ?        00:00:00 /usr/sbin/apache2 -k start
www-data    34    32  0 07:48 ?        00:00:00 /usr/sbin/apache2 -k start
www-data    35    32  0 07:48 ?        00:00:00 /usr/sbin/apache2 -k start
www-data    36    32  0 07:48 ?        00:00:00 /usr/sbin/apache2 -k start
root        93    20  0 07:48 ?        00:00:00 sshd: root@pts/0 
root       105    93  0 07:48 pts/0    00:00:00 -bash
root       112   105  0 07:49 pts/0    00:00:00 ps -ef

If we stop apache2, it will be started again within a minute by CFEngine.

# service apache2 status
Apache2 is running (pid 32).
# service apache2 stop
 * Stopping web server apache2                                                                                                                         /usr/sbin/apache2ctl: 87: ulimit: error setting limit (Operation not permitted)
apache2: Could not reliably determine the server's fully qualified domain name, using 127.0.0.1 for ServerName
 ... waiting                                                                                                                                    [ OK ]
# service apache2 status
Apache2 is NOT running.
# ... wait up to 1 minute...
# service apache2 status
Apache2 is running (pid 173).

Adapting to your applications

To make sure your applications get managed in the same manner, there are just two things you need to adjust from the above example:

• In the Dockerfile used above, install your applications instead of apache2 and sshd
• When you start the container with docker run, specify the command line arguments to your applications rather than apache2 and sshd

That's it!