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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 you’re going to make use of the process management tool, Supervisor, to manage multiple processes in a container. Using Supervisor allows you to better control, manage, and restart the processes inside the container. 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:16.04
MAINTAINER examples@docker.com

Installing Supervisor

You can now install the SSH and Apache daemons as well as Supervisor in the 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

The first RUN instruction installs the openssh-server, apache2 and supervisor (which provides the Supervisor daemon) packages. The next RUN instruction creates four new directories that are needed to run the 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 the 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. The nodaemon directive is used, 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 the 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"]

These instructions tell Docker that ports 22 and 80 are exposed by the container and that the /usr/bin/supervisord binary should be executed when the container launches.

Building our image

Your completed Dockerfile now looks like this:

FROM ubuntu:16.04
MAINTAINER examples@docker.com

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

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

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

And your supervisord.conf file looks like this;

[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"

You can now build the image using this command;

$ docker build -t mysupervisord .

Running your Supervisor container

Once you have built your image you can launch a container from it.

$ docker run -p 22 -p 80 -t -i mysupervisord
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
...

You 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.