Mastering CloudForms Automation

Chapter 7. $evm and the Workspace

When we write automation scripts, we access the Automation Engine and all of its objects through a single $evm variable.¹ This is sometimes referred to as the workspace.

As discussed in Chapter 6, the $evm variable is a DRb::DRbObject object representing a dRuby client connection back to the Automation Engine. The object at the dRuby server side of our $evm variable is an instance of an MiqAeService object, which contains over 40 methods. In practice we generally use only a few of these methods, most commonly:

$evm.root
$evm.object
$evm.current (this is equivalent to calling $evm.object(nil))
$evm.parent
$evm.log
$evm.vmdb
$evm.execute
$evm.instantiate

We will look at these methods in more detail in the next sections.

$evm.log

$evm.log is a simple method that we’ve used already. It writes a message to automation.log and accepts two arguments: a log level and the text string to write. The log level can be written as a Ruby symbol (e.g., :info, :warn), or as a text string (e.g., "info", "warn").

$evm.root

$evm.root, illustrated in Figure 7-1, is the method that returns to us the root object in the workspace (environment, variables, linked objects, etc.). This is the instance whose invocation took us into the Automation Engine. From $evm.root we can access other service model objects, such as $evm.root['vm'], $evm.root['user'], or $evm.root['miq_request'] (the actual objects available depend on the context of the Automate tasks that we are performing). ...