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Thursday, December 17, 2015

Managing Kubernetes Pods, Services and Replication Controllers with Puppet

Today’s guest post is written by Gareth Rushgrove, Senior Software Engineer at Puppet Labs, a leader in IT automation. Gareth tells us about a new Puppet module that helps manage resources in Kubernetes. 

People familiar with Puppet might have used it for managing files, packages and users on host computers. But Puppet is first and foremost a configuration management tool, and config management is a much broader discipline than just managing host-level resources. A good definition of configuration management is that it aims to solve four related problems: identification, control, status accounting and verification and audit. These problems exist in the operation of any complex system, and with the new Puppet Kubernetes module we’re starting to look at how we can solve those problems for Kubernetes.

The Puppet Kubernetes Module


The Puppet Kubernetes module currently assumes you already have a Kubernetes cluster up and running. Its focus is on managing the resources in Kubernetes, like Pods, Replication Controllers and Services, not (yet) on managing the underlying kubelet or etcd services. Here’s a quick snippet of code describing a Pod in Puppet’s DSL.

kubernetes_pod { 'sample-pod':
  ensure   => present,
  metadata => {
    namespace => 'default',
  },
  spec     => {
    containers => [{
      name  => 'container-name',
      image => 'nginx',
    }]
  },
}


If you’re familiar with the YAML file format, you’ll probably recognise the structure immediately. The interface is intentionally identical to aid conversion between different formats — in fact, the code powering this is autogenerated from the Kubernetes API Swagger definitions. Running the above code, assuming we save it as pod.pp, is as simple as:

puppet apply pod.pp

Authentication uses the standard kubectl configuration file. You can find complete installation instructions in the module's README.

Kubernetes has several resources, from Pods and Services to Replication Controllers and Service Accounts. You can see an example of the module managing these resources in the Kubernetes guestbook sample in Puppet post. This demonstrates converting the canonical hello-world example to use Puppet code.

One of the main advantages of using Puppet for this, however, is that you can create your own higher-level and more business-specific interfaces to Kubernetes-managed applications. For instance, for the guestbook, you could create something like the following:

guestbook { 'myguestbook':
  redis_slave_replicas => 2,
  frontend_replicas    => 3,
  redis_master_image   => 'redis',
  redis_slave_image    => 'gcr.io/google_samples/gb-redisslave:v1',
  frontend_image       => 'gcr.io/google_samples/gb-frontend:v3',     
}

You can read more about using Puppet’s defined types, and see lots more code examples, in the Puppet blog post, Building Your Own Abstractions for Kubernetes in Puppet.

Conclusions


The advantages of using Puppet rather than just the standard YAML files and kubectl are:

  • The ability to create your own abstractions to cut down on repetition and craft higher-level user interfaces, like the guestbook example above. 
  • Use of Puppet’s development tools for validating code and for writing unit tests. 
  • Integration with other tools such as Puppet Server, for ensuring that your model in code matches the state of your cluster, and with PuppetDB for storing reports and tracking changes.
  • The ability to run the same code repeatedly against the Kubernetes API, to detect any changes or remediate configuration drift. 

It’s also worth noting that most large organisations will have very heterogenous environments, running a wide range of software and operating systems. Having a single toolchain that unifies those discrete systems can make adopting new technology like Kubernetes much easier.

It’s safe to say that Kubernetes provides an excellent set of primitives on which to build cloud-native systems. And with Puppet, you can address some of the operational and configuration management issues that come with running any complex system in production. Let us know what you think if you try the module out, and what else you’d like to see supported in the future.

 - Gareth Rushgrove, Senior Software Engineer, Puppet Labs

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