Using Supervisor with Docker

Note: - If you don’t like sudo then see Giving non-root access

Traditionally a Docker container runs a single process when it is launched, for example an Apache daemon or a SSH server daemon. Often though you want to run more than one process in a container. There are a number of ways you can achieve this ranging from using a simple Bash script as the value of your container’s CMD instruction to installing a process management tool.

In this example we’re going to make use of the process management tool, Supervisor, to manage multiple processes in our container. Using Supervisor allows us to better control, manage, and restart the processes we want to run. To demonstrate this we’re going to install and manage both an SSH daemon and an Apache daemon.

Creating a Dockerfile

Let’s start by creating a basic Dockerfile for our new image.

FROM ubuntu:13.04
MAINTAINER [email protected]

Installing Supervisor

We can now install our SSH and Apache daemons as well as Supervisor in our container.

RUN apt-get update && apt-get install -y openssh-server apache2 supervisor
RUN mkdir -p /var/lock/apache2 /var/run/apache2 /var/run/sshd /var/log/supervisor

Here we’re installing the openssh-server, apache2 and supervisor (which provides the Supervisor daemon) packages. We’re also creating four new directories that are needed to run our SSH daemon and Supervisor.

Adding Supervisor’s configuration file

Now let’s add a configuration file for Supervisor. The default file is called supervisord.conf and is located in /etc/supervisor/conf.d/.

COPY supervisord.conf /etc/supervisor/conf.d/supervisord.conf

Let’s see what is inside our supervisord.conf file.

[supervisord]
nodaemon=true

[program:sshd]
command=/usr/sbin/sshd -D

[program:apache2]
command=/bin/bash -c "source /etc/apache2/envvars && exec /usr/sbin/apache2 -DFOREGROUND"

The supervisord.conf configuration file contains directives that configure Supervisor and the processes it manages. The first block [supervisord] provides configuration for Supervisor itself. We’re using one directive, nodaemon which tells Supervisor to run interactively rather than daemonize.

The next two blocks manage the services we wish to control. Each block controls a separate process. The blocks contain a single directive, command, which specifies what command to run to start each process.

Exposing ports and running Supervisor

Now let’s finish our Dockerfile by exposing some required ports and specifying the CMD instruction to start Supervisor when our container launches.

EXPOSE 22 80
CMD ["/usr/bin/supervisord"]

Here We’ve exposed ports 22 and 80 on the container and we’re running the /usr/bin/supervisord binary when the container launches.

Building our image

We can now build our new image.

$ docker build -t <yourname>/supervisord .

Running our Supervisor container

Once We’ve got a built image we can launch a container from it.

$ docker run -p 22 -p 80 -t -i <yourname>/supervisord
2013-11-25 18:53:22,312 CRIT Supervisor running as root (no user in config file)
2013-11-25 18:53:22,312 WARN Included extra file "/etc/supervisor/conf.d/supervisord.conf" during parsing
2013-11-25 18:53:22,342 INFO supervisord started with pid 1
2013-11-25 18:53:23,346 INFO spawned: 'sshd' with pid 6
2013-11-25 18:53:23,349 INFO spawned: 'apache2' with pid 7
. . .

We’ve launched a new container interactively using the docker run command. That container has run Supervisor and launched the SSH and Apache daemons with it. We’ve specified the -p flag to expose ports 22 and 80. From here we can now identify the exposed ports and connect to one or both of the SSH and Apache daemons.