I recently found myself in a situation where I had to manage a XenServer resource pool over SSH. XenServer's 'xsconsole' tool provided me with a lot of options, but none that would allow me to boot a guest which was powered off.

User Story:

As a sysadmin
I need to start a XenServer guest from the command line
so that I can start guests without having to use a GUI.

When using Resource Pools, there is no way within the console to boot machines which are powered off - attempting to view all machines results in the error message: "This feature is unavailable in Pools with more than 100 Virtual Machines", even if you have less than 100 virtual machines.

Here's where the XenServer command line applications come in handy. In order to boot a machine which is powered off, you can ssh to any machine in the pool and run `xe vm-list`

[root@ubio-vmh07 ~]# xe vm-list
uuid ( RO): 0ebb9d7d-1743-f9a0-f5b8-692930cc3ad0
name-label ( RW): app10
power-state ( RO): halted
[root@ubio-vmh07 ~]#

This will show you all of your machines by name, state and uuid. Once you find the machine you want to boot, simply run `xe vm-start` and pass it the uuid you want to boot:

[root@ubio-vmh07 ~]# xe vm-start uuid=0ebb9d7d-1743-f9a0-f5b8-692930cc3ad0

and the machine will boot right up. You can then continue to manage the VM using xe or via the xsconsole on the host which is running the VM.

The xe help will show you a list of available commands:

[root@ubio-vmh07 ~]# xe help --all

Assumptions: You have a working knife configuration.

The initial user story that prompted me to figure out how to interact with Chef Server REST API was this:

As a System Administrator
I want to be able to see what IP addresses are being used
In order for me to correctly assign a free IP to a new machine.

In order to access the Chef Server programmatically I first had to figure out authentication. Unfortunately, I couldn't find any good examples of how to actually do this. The Chef Server API wiki page has some basic requirements and concepts, but no concrete examples.

However, it gave me a good starting place--the Chef::REST library. The REST library that comes bundled with Chef in every version > 0.9.0. Let's just make sure that we actually have that version:

$ knife --version
Chef: 0.9.12

Now that we know the version is ok we’re basically done. All we have to do is load the knife configuration file and then we can use the built in rest library. Here’s the code I ended up with:

require 'bundler/setup'
require 'chef'

Chef::Config.from_file("/path/to/knife.rb")
rest = Chef::REST.new(“http://host:port”)

nodes = rest.get_rest(“/nodes”)

Now you can do something like:

nodes.keys.each do |key|
  puts rest.get_rest(“/nodes/#{key}”)[:name]
end

So now we just print each IP address that is being used and we can figure out the next free IP address in any given range. That completes this user story!

There was only one gotcha in the whole process. Trying to print a node just throws an ArgumentError:

puts node #=> ArgumentError: Attribute to_ary is not defined!