How To Clean Up Your Docker Environment Using CloudSlang on a CoreOS Cluster

How To Clean Up Your Docker Environment Using CloudSlang on a CoreOS Cluster


CoreOS is a Linux distribution focused on quickly spinning up clustered environments by utilizing Docker containers and service discovery. If you’re new to CoreOS, check out this Getting Started with CoreOS tutorial series.

However, Docker images can occupy quite a lot of disk space on the Docker host. A base image can be hundreds of MB in size and custom images can easily reach 1 GB. If you have many releases of new Docker images for your app, they can easily stockpile on the server storage; the server can run out of disk space if you don’t clear out old or unused images from time to time.

CloudSlang is an open source orchestration solution that makes it easy to automate processes using workflows, or flows for short. A flow contains a list of tasks and navigation logic. A task can call an operation, which contains an action that runs a Python script or Java method, or another flow. The CloudSlang language allows you to define flows in a textual, reusable manner, and you can either use the existing content (Docker, OpenStack, and utilities) to manage your deployed applications or create your own custom flows.

In this tutorial, we will clean up the Docker environment for each machine deployed in a CoreOS cluster using CloudSlang. We will use the already existing content, so you will not need to edit any CloudSlang files.


Before you begin, you will need:

Step 1 — Installing unzip

In this step, we will install unzip on the CloudSlang server.

First, make sure the package list is up to date.

  1. sudo apt-get update

Then, install unzip.

  1. sudo apt-get install unzip

Step 2 — Downloading CloudSlang

In this section, we will download the CloudSlang CLI tool and the available content (predefined operations and flows). The CloudSlang CLI is a command line interface tool that can be used to run flows.

First, download the CloudSlang CLI archive.

  1. wget https://github.com/CloudSlang/cloud-slang/releases/download/cloudslang-0.7.29/cslang-cli-with-content.zip

Unzip the archive.

  1. unzip cslang-cli-with-content.zip

This will create a cslang directory. If you list the contents of that directory,

  1. ls ~/cslang

You will notice three directories in it:

  • python-lib, which is used for external Python libraries.

  • cslang, which contains the CloudSlang CLI files. The cslang/bin folder contains a file named cslang which is used to start up the CLI. The cslang/lib contains the necessary dependencies for the application.

  • content, which contains the ready-made CloudSlang content. The flow we are going to run is located at content/io/cloudslang/coreos and it is called cluster_docker_images_maintenance.sl. This flow iterates over all the machines in the cluster and deletes unused Docker images.

Step 3 — Adding the Private Key

CloudSlang needs SSH key access to your CoreOS cluster. In this step, we will add this by creating a new key pair on the CloudSlang server and adding the public key to the CoreOS cluster.

First, create a key pair without a passphrase by following steps 1 and 2 of this tutorial. After you have a key pair, you’ll need to add your public key to each of the machines in your CoreOS cluster.

First, get the public key on your CloudSlang server.

cat ~/.ssh/id_rsa.pub

You’ll see a long output which begins with ssh-rsa and ends with username@hostname. Copy this to use in the next command.

SSH into one of your CoreOS servers (the default username is core), then run the following command to add your public key.

echo "your_public_key" >> ~/.ssh/authorized_keys

You’ll need to do this for each server in your CoreOS cluster.

Step 4 — Running the Flow

In this section we will run the flow and verify its behavior.

In order to run the flow, on the CloudSlang server, first change to the /cslang/bin directory.

  1. cd ~/cslang/cslang/bin/

Run the executable called cslang in order to start up the CLI.

  1. ./cslang

After a moment, you’ll see the CloudSlang welcome screen.

Welcome to CloudSlang. For assistance type help.

Enter the following command in the CLI, replacing your_coreos_server_ip with the IP address of one of the CoreOS servers in your cluster.

run --f  ../../content/io/cloudslang/coreos/cluster_docker_images_maintenance.sl --i coreos_host=your_coreos_server_ip,coreos_username=core,private_key_file=~/.ssh/id_rsa --cp ../../content/

The run command triggers the flow. --f specifies the path to the flow. --i specifies the flow inputs: a CoreOS host and username, and the associated private SSH key. --cp specifies the classpath when the flow depends upon other operations and flows. Because this flow has many different dependencies, we can specify the parent content folder; the scanning is recursive so subdirectories are also scanned.

The flow logic first retrieves the IP addresses of the machines from the cluster, then iterates over the machines and clear unused images. First, it gets all the images, leaving only unused ones by checking running/stopped containers. Next, it deletes the unused images. Finally, it does the same for dangling images.

While the flow is running, the CLI displays the task names that are executed. Once the flow finishes, the CLI outputs some useful information like the flow outputs and the flow result.

In our case, the flow result will either be SUCCESS (which means unused Docker images were cleared in the cluster) or FAILURE (which means a problem occurred). This flow has one output: number_of_deleted_images_per_host, which is how many images were deleted on every host in the cluster.

If everything went well you should see an output similar to this:


Flow : cluster_docker_images_maintenance finished with result : SUCCESS
Execution id: 101600001, duration: 0:02:06.180

If you want more information on the execution, look in the execution.log file which is created by the CLI in the bin folder.


Now all the unused Docker images are deleted in your CoreOS cluster!

In this tutorial, you have seen how to run CloudSlang on your Ubuntu machine and how to use the CloudSlang CLI to trigger flows. You also used a ready-made workflow to clean up your Docker environment.

Copyright June 9, 2015, Hewlett-Packard Development Company, L.P. Reproduced with Permission.

Thanks for learning with the DigitalOcean Community. Check out our offerings for compute, storage, networking, and managed databases.

Learn more about us

About the authors

Default avatar

staff technical writer

hi! i write do.co/docs now, but i used to be the senior tech editor publishing tutorials here in the community.

Still looking for an answer?

Ask a questionSearch for more help

Was this helpful?
Leave a comment

This textbox defaults to using Markdown to format your answer.

You can type !ref in this text area to quickly search our full set of tutorials, documentation & marketplace offerings and insert the link!

Try DigitalOcean for free

Click below to sign up and get $200 of credit to try our products over 60 days!

Sign up

Join the Tech Talk
Success! Thank you! Please check your email for further details.

Please complete your information!

Featured on Community

Get our biweekly newsletter

Sign up for Infrastructure as a Newsletter.

Hollie's Hub for Good

Working on improving health and education, reducing inequality, and spurring economic growth? We'd like to help.

Become a contributor

Get paid to write technical tutorials and select a tech-focused charity to receive a matching donation.

Welcome to the developer cloud

DigitalOcean makes it simple to launch in the cloud and scale up as you grow — whether you're running one virtual machine or ten thousand.

Learn more
DigitalOcean Cloud Control Panel