Puppet Best Practices by Chris Barbour

Author:Chris Barbour
Language: eng
Format: mobi, epub
Tags: COMPUTERS / System Administration / General
ISBN: 9781491922965
Publisher: O’Reilly Media
Published: 2015-03-16T16:00:00+00:00

Variable Qualifiecation

Local variables should be unqualified, global variables should be fully qualified, and fully qualified out-of-scope variable references should be avoided if possible.

Fully qualifying variable names accomplishes two goals:

Clarifies your intent

Disambiguates local and global variables

When a variable is fully qualified, it becomes clear that your module is attempting to consume a top level variable, and eliminates the possibility that you simply forgot to define that variable or are attempting to inherit a variable from a higher scope. This disambiguation is important when you revisit your code in the future, either to extend the code or debug a problem.

Many validation tools also assume that unqualified variables are local, and will throw a warning if the variable is not defined in scope. With puppet-lint, this behavior can be disabled, however I recommend against doing so as it’s a useful way to catch subtle bugs.

While you can fully qualify references to local variables, using unqualified names makes it clear at a glance that the variable is local and has been defined in the current scope. This hint again is used by the validators.

Finally, I strongly recommend against creating inter-class variable references using fully qualified variable names. Such references are a useful stop-gap measure when upgrading code that relies on variable inheritance, however it usually violates the principle of separation of concerns. A major issue with inter-class variable references is that there’s no way to tell from the referenced class that such a reference exists. As a result, it’s very easy to break the reference when re-factoring code. Instead, consider parameterizing your classes, using class parameters to pass data from one module to another. A side benefit of this approach is that it tends to reduce and eliminate circular dependencies.

Example 4-20. Passing variables using fully qualified variables

class parent { $foo = 'alpha' } class parent::child { $foo = $::parent::foo alert($foo) #Prints 'alpha' }

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