Writing facts with simple resolutions

Main components of simple resolutions

Simple facts are typically made up of the following parts:

1. A call to Facter.add(:fact_name):

   • This introduces a new fact or a new resolution for an existing fact with the same name.

   • The name can be either a symbol or a string.

   • The rest of the fact is wrapped in the add call’s do ... end block.

2. Zero or more confine statements:

   • Determine whether the resolution is suitable (and therefore is evaluated).

   • Can either match against the value of another fact or evaluate a Ruby block.

   • If given a symbol or string representing a fact name, a block is required and the block receives the fact’s value as an argument.

   • If given a hash, the keys are expected to be fact names. The values of the hash are either the expected fact values or an array of values to compare against.

   • If given a block, the confine is suitable if the block returns a value other than nil or false.

3. An optional has_weight statement:

   • When multiple resolutions are available for a fact, resolutions are evaluated from highest weight value to lowest.

   • Must be an integer greater than 0.

   • Defaults to the number of confine statements for the resolution.

4. A setcode statement that determines the value of the fact:

   • Can take either a string or a block.

   • If given a string, Facter executes it as a shell command. If the command succeeds, the output of the command is the value of the fact. If the command fails, the next suitable resolution is evaluated.

   • If given a block, the block’s return value is the value of the fact unless the block returns nil. If nil is returned, the next suitable resolution is evaluated.

   • Can execute shell commands within a setcode block, using the Facter::Core::Execution.exec function.

   • If multiple setcode statements are evaluated for a single resolution, only the last setcode block is used.

   Set all code inside the sections outlined above ⁠— there must not be any code outside setcodeand confine blocks other than an optional has_weight statement in a custom fact.

How to format facts

The format of a fact is important because of the way that Facter evaluates them — by reading all the fact definitions. If formatted incorrectly, Facter can execute code too early. You need to use the setcode correctly. Below is a good example and a bad example of a fact, showing you where to place the setcode.

Good:

Facter.add('phi') do
  confine owner: "BTO"
  confine :kernel do |value|
    value == "Linux"
  end
 
  setcode do
    bar=Facter.value('theta')
    bar + 1
  end
end

In this example, the bar=Facter.value('theta') call is guarded by setcode, which means it is not executed unless or until it is appropriate to do so. Facter loads all Facter.add blocks first, use any OS or confine/weight information to decide which facts to evaluate, and once it chooses, it selectively executes setcode blocks for each fact that it needs.

Bad:

Facter.add('phi') do
  confine owner: "BTO"
  confine :kernel do |value|
    value == "Linux"
  end
  
  bar = Facter.value('theta')
 
  setcode do
    bar + 1
  end
end

In this example, the Facter.value('theta') call is outside of the guarded setcode block and in the unguarded part of the Facter.add block. This means that the statement always executes, on every system, regardless of confine, weight, or which resolution of phi is appropriate. Any code with possible side-effects, or code pertaining to figuring out the value of a fact, must be kept inside the setcode block. The only code left outside setcode is code that helps Facter choose which resolution of a fact to use.

Examples

The following example shows a minimal fact that relies on a single shell command:

Facter.add(:rubypath) do
  setcode 'which ruby'
end

The following example shows different resolutions for different operating systems:

Facter.add(:rubypath) do
  setcode 'which ruby'
end

Facter.add(:rubypath) do
  confine 'os' do |os_fact|
    os_fact['family'] == "Windows"
  end
  # Windows uses 'where' instead of 'which'
  setcode 'where ruby'
end

The following example shows a more complex fact, confined to Linux with a block:

Facter.add(:jruby_installed) do
  confine :kernel do |value|
    value == "Linux"
  end

  setcode do
    # If jruby is present, return true. Otherwise, return false.
    Facter::Core::Execution.which('jruby') != nil
  end
end