resources_common_lazy_evaluation.md

In some cases, the value for a property cannot be known until the execution phase of a Chef Infra Client run. In this situation, using lazy evaluation of property values can be helpful. Instead of a property being assigned a value, it may instead be assigned a code block. The syntax for using lazy evaluation is as follows:

property_name lazy { code_block }

where lazy is used to tell Chef Infra Client to evaluate the contents of the code block later on in the resource evaluation process (instead of immediately) and { code_block } is arbitrary Ruby code that provides the value.

For example, a resource that is not doing lazy evaluation:

template 'template_name' do
  # some properties
  path '/foo/bar'
end

and a resource block that is doing lazy evaluation:

template 'template_name' do
  # some properties
  path lazy { ' some Ruby code ' }
end

In the previous examples, the first resource uses the value /foo/bar and the second resource uses the value provided by the code block, as long as the contents of that code block are a valid resource property.

The following example shows how to use lazy evaluation with template variables:

template '/tmp/canvey_island.txt' do
  source 'canvey_island.txt.erb'
  variables(
    lazy do
      { canvey_island: node.run_state['sea_power'] }
    end
  )
end