This week, Chef released a version of the Chef client that can run inside a Linux container. This container-friendly client is called chef-container. In this post we’ll give you an introduction to chef-container, its purpose and its components. We’ll also tell you about a new knife plugin for managing container images. Then, we’ll show you how to launch an instance of Apache2, running inside a Docker container.
Why chef-container?
Linux containers and containerization have been around for some time but the rapid rise in popularity of tools like Docker and the benefits they provide have brought containerization, and the concept of immutable infrastructure, back into the spotlight. For more on this I recommend you check out Julian’s post “Immutable Infrastructure: Practical or Not?”
Hosting your applications inside of Linux containers gives you more speed and flexibility than running them in VMs, but those containers still need proper configuration management. Most people choose something that goes beyond simple shell scripts. Docker’s Dockerfile has some characteristics of infrastructure as code but, deep down, it still defines actions to be taken as shell commands. For systems administrators and developers who want to migrate their existing application stack to containers, the prospect of rewriting all their infrastructure code for application stacks they may not fully understand can be daunting.
This is one of the reasons that we created chef-container. We believe that your automation platform should be able to manage your full stack. In other words, you should be able to use your existing library of cookbooks, whether you are managing the beefiest bare metal servers or the smallest Linux containers. You should be able to do all of this without losing any of the benefits that an automation platform provides.
What is chef-container?
chef-container provides a consistent configuration management experience across container solutions. It does this by bundling the Chef client with runit and chef-init. runit is a lightweight, cross-platform, open-source init system, and chef-init is a RubyGem that acts as the entry point into a container. It also provides the necessary abstractions to make configuring a container image the same as configuring any other piece of infrastructure with Chef. Together, these three pieces of software create a delightful container management workflow takes you from development to production.
The first release of chef-container addresses the unique execution environment inside Docker that, until now, required significant workarounds. By default, a Docker container lacks an init system as well as the ability to attach to a TTY inside the container. These characteristics made it difficult to bootstrap a Docker container or build a Docker image using the existing Chef workflow. chef-container, when used with the knife container plugin, resolves these issues and allows you build and launch Docker images while adhering to Docker best practices.
What is knife container?
The knife container plugin lets you use Chef to manage the lifecycle of container images. The first release of knife container provides the workflow specific to managing Docker images. It allows you to create, delete and manage Docker images.
Example: Launching an Apache2 Docker Instance
We’re going to show you how to launch an Apache2 Docker instance using chef-container and knife container.
Install the Software
You’ll first need to install three pieces of software on your workstation.
- Install Docker. You can do this from the Docker installation page.
- Install Chef DK. You can do this from the Chef Development Kit page.
- Install the knife container plugin with the following command:
chef gem install knife-container
Plan Your Deployment
Once you’ve installed the software, there are four decisions to make.
- What do you want to name your Docker image?
- What run list will you use for the Docker image? In other words, what recipes will you use to configure it?
- Will you manage the image in local mode or server mode?
- Will you manage your cookbooks manually or with Berkshelf?
The Demo
To give you a concrete example, we’re going to create a Docker image and then launch it. We have a run list made up of a single recipe named apache2. We’ll manage the image locally and we’ll use Berkshelf to manage our cookbooks.
Initialize the Docker Context
To initialize the Docker context, use the init command. The knife container uses a folder called dockerfiles to organize all the Docker contexts that you manage. By default, the dockerfiles folder is created in your chef repo.
To initialize the Docker context, type the following command:
knife container docker init demo/apache2 -r 'recipe[apache2]' -z -b
Pass in your image name (in this example,demo/apache2), a run list, a –z and a –b. The –z is for local mode and the –b says to generate a Berksfile.
Define a Node Attribute
You need to define a node attribute for any service resource that you want Chef container to manage. chef-container is distributed with runit because Docker containers lack an init scheme. The chef-init gem provides the container_service resource that takes over the service resource and shims it to runit. Because runit works differently than your normal upstart service would, you need to provide an additional string value, which is the command that runit uses to run the service.
The command string is passed in through a node attribute and automatically recognized by chef-init, so you don’t need to modify existing cookbooks. You can specify the value anywhere node attributes can be specified. For this example we will enter it in the first-boot.jsn file that is created as part of the initialization process. Here’s the node attribute for our demo.
{
"run\_list": [
"recipe[apache2]"
],
"container_service": {
"apache2": {
"command": "/usr/sbin/apache2 -k start"
}
}
}Build the Docker Image
Use the build command to create a Docker image. The only value you’ll need to pass in is the image name you specified when you ran the init command. In our example cookbook dependencies are first automatically resolved because we’re using Berkshelf. Next, the command performs a docker build on the Docker context and generates a Docker image.
In this example, the name of the image was demo/apache2.
knife container docker build demo/apache2
Once the container is successfully built, you can see that the image exists with the following command:
docker images
Now you can push the image to a registry and launch it on any machine that you’d like.
docker push demo/apache2
Launch the Docker Image
You’re now ready to do a docker run on the Docker image to launch it as a running instance. Type the following command:
docker run –d demo/apache2
When the instance launches, it first runs the Chef client again for any last mile configuration changes that it needs. That run is convergent so it should be very fast. An important point to note is that, with Chef, you can configure containers after they’ve been launched. For example, if you have two containers that need to communicate with each other, you can only configure this after the launch because you won’t know their IP addresses until then.
To see a list of running containers, type:
docker ps
To see the processes running inside a container, type:
docker top container ID
Watch the Video
Watch a video that shows the demo.
Learn More
To learn more about Chef’s support for containers, read Chef for Containers.
To provide feedback on the chef-init or knife-container projects, please submit an issue via Github.