Using Juju to orchestrate CentOS-based cloud services

Earlier this week I had the opportunity to meet Kyle MacDonald, head of Ubuntu Cloud, during FISL, and he was surprised when we told him we are using Juju with CentOS at Then I decided to write this post explaining how we came up with a patched version of Juju that allows us to have CentOS clouds managed by Juju.

For those who doesn't know Juju, it's a service orchestration tool, focused on devops "development method". It allows you to deploy services on clouds, local machine and even bare metal machines (using Canonical's MAAS).

It's based on charms and very straightforward to use. Here is a very basic set of commands with which you can deploy a Wordpress related to a MySQL service:

% juju bootstrap
% juju deploy mysql
% juju deploy wordpress
% juju add-relation wordpress mysql
% juju expose wordpress

These commands will boostrap the environment, setting up a bootstrap machine which will manage your services; deploy mysql and wordpress instances; add a relation between them; and expose the wordpress port. The voilĂ , we have a wordpress deployed, and ready to serve our posts. Amazing, huh?

But there is an issue: although you can install the juju command line tool in almost any OS (including Mac OS), right now you are able do deploy only Ubuntu-based services (you must use an Ubuntu instance or container).

To change this behavior, and enable Juju to spawn CentOS instances (and containers, if you have a CentOS lxc template), we need to develop and apply some changes to Juju and cloud-init. Juju uses cloud-init to spawn machines with proper dependencies set up, and it's based on modules. All we need to do, is add a module able to install rpm packages using yum.

cloud-init modules are Python modules that starts with cc_ and implement a handle function (for example, a module called "yum_packages" would be written to a file called So, here is the code for the module yum_packages:

import subprocess
import traceback

from cloudinit import CloudConfig, util

frequency = CloudConfig.per_instance

def yum_install(packages):
    cmd = ["yum", "--quiet", "--assumeyes", "install"]

def handle(_name, cfg, _cloud, log, args):
    pkglist = util.get_cfg_option_list_or_str(cfg, "packages", [])

    if pkglist:
        except subprocess.CalledProcessError:
            log.warn("Failed to install yum packages: %s" % pkglist)

    return True

The module installs all packages listed in cloud-init yaml file. If we want to install emacs-nox package, we would write this yaml file and use it as user data in the instance:

 - yum_packages
packages: [emacs-nox]

cloud-init already works on Fedora, with Python 2.7, but to work on CentOS 6, with Python 2.6, it needs a patch:

--- cloudinit/ 2012-05-22 12:18:21.000000000 -0300
+++ cloudinit/ 2012-05-31 12:44:24.000000000 -0300
@@ -227,7 +227,7 @@
         stderr=subprocess.PIPE, stdin=subprocess.PIPE)
     out, err = sp.communicate(input_)
     if sp.returncode is not 0:
-        raise subprocess.CalledProcessError(sp.returncode, args, (out, err))
+        raise subprocess.CalledProcessError(sp.returncode, args)
     return(out, err)

I've packet up this module and this patch in a RPM package that must be pre-installed in the lxc template and AMI images. Now, we need to change Juju in order to make it use the yum_packages module, and include all RPM packages that we need to install when the machine borns.

Is Juju, there is a class that is responsible for building and rendering the YAML file used by cloud-init. We can extend it and change only two methods: _collect_packages, that returns the list of packages that will be installed in the machine after it is spawned; and render that returns the file itself. Here is our CentOSCloudInit class (within the patch):

diff -u juju-0.5-bzr531.orig/juju/providers/common/ juju-0.5-bzr531/juju/providers/common/
--- juju-0.5-bzr531.orig/juju/providers/common/ 2012-05-31 15:42:17.480769486 -0300
+++ juju-0.5-bzr531/juju/providers/common/ 2012-05-31 15:55:13.342884919 -0300
@@ -324,3 +324,32 @@
             "machine-id": self._machine_id,
             "juju-provider-type": self._provider_type,
             "juju-zookeeper-hosts": self._join_zookeeper_hosts()}
+class CentOSCloudInit(CloudInit):
+    def _collect_packages(self):
+        packages = [
+            "bzr", "byobu", "tmux", "python-setuptools", "python-twisted",
+            "python-txaws", "python-zookeeper", "python-devel", "juju"]
+        if self._zookeeper:
+            packages.extend([
+                "zookeeper", "libzookeeper", "libzookeeper-devel"])
+        return packages
+    def render(self):
+        """Get content for a cloud-init file with appropriate specifications.
+        :rtype: str
+        :raises: :exc:`juju.errors.CloudInitError` if there isn't enough
+            information to create a useful cloud-init.
+        """
+        self._validate()
+        return format_cloud_init(
+            self._ssh_keys,
+            packages=self._collect_packages(),
+            repositories=self._collect_repositories(),
+            scripts=self._collect_scripts(),
+            data=self._collect_machine_data(),
+            modules=["ssh", "yum_packages", "runcmd"])

The other change we need is in the format_cloud_init function, in order to make it recognize the modules parameter that we used above, and tell cloud-init to not run apt-get (update nor upgrade). Here is the patch:

diff -ur juju-0.5-bzr531.orig/juju/providers/common/ juju-0.5-bzr531/juju/providers/common/
--- juju-0.5-bzr531.orig/juju/providers/common/ 2012-05-31 15:42:17.480769486 -0300
+++ juju-0.5-bzr531/juju/providers/common/ 2012-05-31 15:44:06.605014021 -0300
@@ -85,7 +85,7 @@

 def format_cloud_init(
-    authorized_keys, packages=(), repositories=None, scripts=None, data=None):
+    authorized_keys, packages=(), repositories=None, scripts=None, data=None, modules=None):
     """Format a user-data cloud-init file.

     This will enable package installation, and ssh access, and script
@@ -117,8 +117,8 @@
     cloud_config = {
-        "apt-update": True,
-        "apt-upgrade": True,
+        "apt-update": False,
+        "apt-upgrade": False,
         "ssh_authorized_keys": authorized_keys,
         "packages": [],
         "output": {"all": "| tee -a /var/log/cloud-init-output.log"}}
@@ -136,6 +136,11 @@
     if scripts:
         cloud_config["runcmd"] = scripts

+    if modules:
+        cloud_config["modules"] = modules
     output = safe_dump(cloud_config)
     output = "#cloud-config\n%s" % (output)
     return output

This patch is also packed up within juju-centos-6 repository, which provides sources for building RPM packages for juju, and also some pre-built RPM packages.

Now just build an AMI image with cloudinit pre-installed, configure your juju environments.yaml file to use this image in the environment and you are ready to deploy cloud services on CentOS machines using Juju!

Some caveats:

It's important to notice that we are going to put some effort to make the Go version of juju born supporting multiple OSes, ideally through an interface that makes it extensible to any other OS, not Ubuntu and CentOS only.