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Docker quick reference.md

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title: Docker quick reference

Docker quick reference

This document explains how the environment for testing load balancing is built.

The test environment uses a Vagrant virtual machine inside of which run several Docker containers. When the Vagrant VM runs for the first time a provisioning script (provision.sh) is run that provisions the Docker containers.

All the following commands are run on the command line inside the Vagrant VM. To log into the VM type vagrant ssh.

How to build a web application container image

Note: This step should not be necessary in this lab as you are not expected to change the Dockerfile.

To rebuild a container from the Dockerfile, cd to the directory containing the Dockerfile and run

$ docker build -t softengheigvd/webapp .

or if you are in a different directory, run

$ docker build -t softengheigvd/webapp <pathToWebAppDirectory>

The option -t softengheigvd/webapp will tag the image with the name softengheigvd/webapp and in the subsequent commands you will be able to use that name to refer to the image.

How to start a web application container

Note: The image is already built automatically in this lab when you provisioned the Vagrant VM. The name of the image is softengheigvd/webapp.

$ docker run -d -e "TAG=<containerName>" --name <containerName> softengheigvd/webapp
  • containerName: Use a human-readable container name like we did in this lab. We chose s1 and s2.

Note: If you have built your own version of the web app image, replace the image name by yours.

Note 2: If you receive an error message that is saying s1 or s2 is already used, it means that a container with that name was already started (running containers must have unique names). In that case you should stop the offending container.

How to remove a web application container

# If not started
$ docker rm s1

# If started
$ docker rm -f s1

How to start the HAProxy container in interactive mode

Interactive mode allows you to log into the container and directly inspect and manipulate its files. You have to start the two web app containers first.

$ docker run -ti -p 80:80 -p 1936:1936 -p 9999:9999 --link <webappContainerName1> --link <webappContainerName2> --name ha <imageName> bash

In this example, we have few placeholders to fill

  • webappContainerName 1 and 2: Replace by the names you have given to the web app containers (see above). Should be s1 and s2. This will create the required network and env vars plumbing inside the container for later use.
  • containerName: We used ha for the container name.
  • imageName: We used softengheigvd/ha for the image name. You will have to rebuild frequently the image or to use it extensively in interactive mode.

Note: Remember that all the modifications done in the container are only persisted in the container. Once the container is erased, no data will survive and all modifications are lost.

To avoid losing data, add the -v <host_dir>:<container_dir> option when you launch the HAProxy:

$> docker run -ti ... -v /vagrant/ha/config:/config --name ha <imageName> ... bash

The option -v bind-mounts a volume that is shared between the docker host (in our case Vagrant) and the container. The directory /vagrant/ha/config on the docker host becomes accessible to the container at the mount point /config.

Note that Vagrant has a similar mechanism to share files between the host and the Vagrant VM: The host directory where the Vagrantfile is located is automatically bind-mounted inside the VM under the mount point /vagrant.

In our case the container directory /config contains the HAProxy configuration file haproxy.cfg. It is now shared between three parties: the running container (the container sees it at /config/haproxy.cfg), the Vagrant VM (the VM sees it at /vagrant/ha/config/haproxy.cfg) and your host (your host sees it in the Vagrantfile directory under ha/config/haproxy.cfg). If you do a modification in this file on your host, or in the VM or in the container, the modification will be seen in the three places and will persist in time.

How to start HAProxy when running the container in interactive mode

Automatic method

Simply run the /scripts/run-daemon.sh script. The script will take care of updating the container IPs of s1 and s2.

$ /scripts/run-daemon.sh

Manual method

Hint: The first time you run HAProxy manually, you should start the rsyslogd daemon to be able to get logs in files.

$ rsyslogd

Before running the HAProxy process, you need to update the configuration of HAProxy so that it forwards traffic to the s1 and s2 web applications.

Edit in the container the file /config/haproxy.cfg and find and replace the following lines:

# Around lines 30s
    server s1 <s1>:3000 check
    server s2 <s2>:3000 check

If you have used --link s1 and --link s2 when you started the TODO

Replace <s1> with the container IP of s1, same for <s2>. To get the IP of each container, apply the following method. You will get the IP directly. You cannot run this command inside the HAProxy container. You should do it in the Vagrant VM and then copy/paste the IP in the right place.

$ docker inspect --format '{{ .NetworkSettings.IPAddress }}' <containerName>

Where <containerName> is the web application container name.

Finally, you can run the following command to start or restart the HAProxy:

$ haproxy -D -f /usr/local/etc/haproxy/haproxy.cfg -p /var/run/haproxy.pid

How to consult the logs

If you want to consult the logs when you are logged into the container, use the tail command like so:

$ tail -f /logs/haproxy.log

If you want to access the logs from your host or the Vagrant VM, you have to add the -v <host_dir>:<container_dir> bind-mount option to the docker command to run the HAProxy container in interactive mode.

$ docker run ... -v /vagrant/ha/logs:/logs --name ha <imageName> ... bash

This will mount the docker host directory /vagrant/ha/logs in the container under the mount point /logs. In the Vagrant VM you will therefore see the HAProxy log file as /vagrant/ha/logs/haproxy.log and you can do a tail -f on it.

How to remove a HAProxy container

# If not started
$ docker rm ha

# If started
£ docker rm -f ha