Building a Django App Server with Chef: Part 1

Alternate title: Fucking Chef, How does it work?

When I started looking at Chef, I found it incredibly complex and lacking in fundamental documentation. This is going to be my experience understanding Chef while setting up a single server. This strategy can be used across multiple servers, with a little tweaking.

I’d like to thank Jacob for the ideas and some of the inspiration behind the code and ideas. The code for this blog post can be found on github. It will be expanded as I write updates.

Today’s code will be using tag blog-post-1 in the repo.

Goal

I’m hoping to write a blog series that goes from explaining what Chef is, to having a working Django server, and to release all this code so that you can tweak and use it with your own servers. I’ll be doing this to set up a new and configuration managed server for ReadTheDocs.

There are a couple of other good chef intros available. However, they only get you to the super basic first step of setting up the server. Hopefully this series will be more in depth and useful to actually getting a real server running under Chef.

Basic terminology

Chef has some basic terminology that you need to understand before we get into things. I’m going to purposefully leave out a ton of stuff, because it isn’t really important for me now.

  • Cookbook (cookbooks/*)

A cookbook, not surprisingly, contains recipes. With my configuration, we’re only going to use one cookbook, that has multiple recipes. This is probably wrong and horrible, but it’s simple, which is the goal for now.

  • Recipe (cookbooks/*/recipes/*.rb)

A recipe is the basic building block of Chef. It is what does the meat of the work that you want done.

  • Resources

A resources is an abstraction that defines a specific piece of your configuration. It can be a file, a user, or just about anything you want to talk about on your system.

  • Attributes (node.json)

Attributes are just a blob of JSON that Chef uses to pass around data. It will be (slightly) different for each server that you want to set up. I really like this approach, because I think a data-driven approach is the correct way to solve this problem.

Bootstrapping chef

We’ll be running what is called chef solo. This means that chef will be run independently of a server, and just execute on our one host. This seems to be the easiest way to get things running.

When you first run chef-solo, it will look for a solo.rb file. The documentation about configuring chef-solo does a decent job of explaining this. By default it looks in /etc/chef/solo.rb, so lets go ahead and use that. It has 2 required fields, a cookbook_path and json_attribs. cookbook_path tells chef where to find the “cookbooks” it uses to run your code, and json_attribs tells it where to load in your data dictionary.

Note: The docs say that file_cache_path is required, but it just defaults to /var/chef/cache.

For simplicity, we’re going to keep our cookbooks and json data file in /etc/chef. On my local machine I keep everything in a ~/projects/chef directory, and then sync that to the remote box.

In production you’ll probably want to set up your remote server to pull from a repository somewhere, so that you can have a stable deployment base, but again, syncing from the local filesystem is simple and works.

Bootstrapping your new server

I’ll be using a fabric script to run the commands on a remote server, which also allows me to run them again later on a new machine in an automated fashion. So this is the first simple script that we’ll use to bootstrap our new server, fabfile.py:

from fabric.api import env, local, sudo
env.user = 'root'
env.hosts = ['204.232.205.196']

env.chef_executable = '/var/lib/gems/1.8/bin/chef-solo'

def install_chef():
    sudo('apt-get update', pty=True)
    sudo('apt-get install -y git-core rubygems ruby ruby-dev', pty=True)
    sudo('gem install chef', pty=True)

def sync_config():
    local('rsync -av . %s@%s:/etc/chef' % (env.user, env.hosts[0]))

def update():
    sync_config()
    sudo('cd /etc/chef && %s' % env.chef_executable, pty=True)

A couple notes: the chef executable is version-dependent, but that’s because I don’t know enough ruby to query it dynamically. You will also need to change the value of the env.hosts to a server that you actually control.

This will install the ruby dependencies, and get chef up and running for you. You’ll need to install fabric (pip install fabric, presumably in a virtualenv), and then run fab install_chef to get it up and running. Then you can go ahead and run fab sync_config to get your chef configuration onto the remote server.

Now we need to go ahead and figure out how to make chef actually do something. You’ll see in the cookbooks/main/recipes/default.rb file we have a simple package declaration. This simply means to make sure that the package is installed on the remote system. This is as simple as it gets, so lets go ahead and run it. With the fab update command, it will sync your local directory, and run chef on the remote server.

-> fab update
[localhost] run: rsync -av . root@204.232.205.196:/etc/chef
[204.232.205.196] sudo: cd /etc/chef && /var/lib/gems/1.8/gems/chef-0.9.12/bin/chef-solo
[204.232.205.196] err: stdin: is not a tty
[204.232.205.196] out: [Tue, 09 Nov 2010 01:42:01 +0000] INFO: Setting the run_list to ["main::default"] from JSON
[204.232.205.196] out: [Tue, 09 Nov 2010 01:42:01 +0000] INFO: Starting Chef Run (Version 0.9.12)
[204.232.205.196] out: [Tue, 09 Nov 2010 01:42:01 +0000] INFO: Installing package[curl] version 7.21.0-1ubuntu1
[204.232.205.196] out: [Tue, 09 Nov 2010 01:42:04 +0000] INFO: Chef Run complete in 2.574963 seconds
[204.232.205.196] out: [Tue, 09 Nov 2010 01:42:04 +0000] INFO: cleaning the checksum cache
[204.232.205.196] out: [Tue, 09 Nov 2010 01:42:04 +0000] INFO: Running report handlers
[204.232.205.196] out: [Tue, 09 Nov 2010 01:42:04 +0000] INFO: Report handlers complete

You now have Chef running on your server. That was pretty easy, eh? For tomorrow’s lesson, we’ll be making it actually do something, like installed nginx and gunicorn, and keeping track of config files.

This Post

Published 2010-11-08

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