Facter 1.6: Custom Facts

This version of Facter is not included in Puppet Enterprise. The latest version of PE includes Facter 3.6. A newer version is available; see the version menu above for details.

Extend Facter by writing your own custom facts to provide information to Puppet.

Adding Custom Facts to Facter

Sometimes you need to be able to write conditional expressions based on site-specific data that just isn’t available via Facter, or perhaps you’d like to include it in a template.

Since you can’t include arbitrary Ruby code in your manifests, the best solution is to add a new fact to Facter. These additional facts can then be distributed to Puppet clients and are available for use in manifests and templates, just like any other fact would be.

The Concept

You can add new facts by writing snippets of Ruby code on the Puppet master. Puppet then uses Plugins in Modules to distribute the facts to the client.

Loading Custom Facts

Facter offers a few methods of loading facts:

  • $LOAD_PATH, or the Ruby library load path
  • The environment variable FACTERLIB
  • Facts distributed using pluginsync

You can use these methods of loading facts to do things like test files locally before distributing them, or you can arrange to have a specific set of facts available on certain machines.

Facter searches all directories in the Ruby $LOAD_PATH variable for subdirectories named ‘facter’, and loads all Ruby files in those directories. If you had some directory in your $LOAD_PATH like ~/lib/ruby, set up like this:

#~/lib/ruby
└── facter
    ├── rackspace.rb
    ├── system_load.rb
    └── users.rb

Facter tries to load ‘facter/system_load.rb’, ‘facter/users.rb’, and ‘facter/rackspace.rb’.

Facter also checks the environment variable FACTERLIB for a colon-delimited set of absolute paths to directories, then tries to load all Ruby files in those directories. This allows you to do something like this:

$ pwd
/home/user
$ ls my_facts
system_load.rb
$ ls my_other_facts
users.rb
$ export FACTERLIB="/home/user/my_facts:/home/user/my_other_facts"
$ facter system_load users
system_load => 0.25
users => thomas,pat

Facter can also easily load fact files distributed using pluginsync. Running facter -p loads all the facts that have been distributed via pluginsync, so if you’re using a lot of custom facts inside Puppet, you can easily use these facts with standalone Facter.

Custom facts can be distributed to clients using the Plugins in Modules method.

Two Parts of Every Fact

Setting aside external facts for now, every fact has at least two elements:

  1. A call to Facter.add('fact_name'), which determines the name of the fact.
  2. A setcode statement, which Facter evaluates to determine the fact’s value.

Facts can get a lot more complicated than that, but every fact contains at least those two parts.

Executing Shell Commands in Facts

Puppet gets information about a system from Facter, and the most common way for Facter to get that information is by executing shell commands. You can then parse and manipulate the output from those commands using standard Ruby code. The Facter API gives you two ways to execute shell commands:

  • If all you want to do is run the command and use the verbatim output as your fact’s value, you can pass the command into setcode directly. For example: setcode "uname --hardware-platform".
  • If your fact is any more complicated than that, you can call Facter::Util::Resolution.exec('uname --hardware-platform') from within the setcode doend block. As always, Facter uses whatever the setcode statement returns as the fact’s value.

It’s important to note that not everything that works in the terminal will work in a fact. You can use the pipe (|) and similar operators just as you normally would, but Bash-specific syntax like if statements do not work. The best way to handle this limitation is to write your conditional logic in Ruby.

An Example

Let’s say you need to get the output of uname --hardware-platform to single out a specific type of workstation. To do this, create a new custom fact. Start by giving the fact a name, in this case hardware_platform, and create your new fact in a file, hardware_platform.rb, on the Puppet master server:

# hardware_platform.rb

Facter.add('hardware_platform') do
  setcode do
    Facter::Util::Resolution.exec('/bin/uname --hardware-platform')
  end
end

Note: Prior to Facter 1.5.8, values returned by Facter::Util::Resolution.exec often had trailing newlines. If your custom fact will also be used by older versions of Facter, you may need to call chomp on these values. (In the example above, this would look like Facter::Util::Resolution.exec('/bin/uname --hardware-platform').chomp.)

You can then use the instructions in the Plugins in Modules guide to copy the new fact to a module and distribute it. The value of the new fact is then available to use in your manifests and templates during your next Puppet run.

The best place to find ideas on writing your own custom facts is to look at the code for Facter’s core facts, which contains a wealth of examples of how to retrieve different types of system data and return useful facts.

Using Other Facts

You can write a fact that uses other facts by accessing Facter.value('somefact').

For example:

Facter.add('osfamily') do
  setcode do
    distid = Facter.value('lsbdistid')
    case distid
    when /RedHatEnterprise|CentOS|Fedora/
      'redhat'
    when 'ubuntu'
      'debian'
    else
      distid
    end
  end
end

Configuring Facts

Facts have a few properties that you can use to customize how facts are evaluated.

Confining Facts

One of the more commonly used properties is the confine statement, which restricts the fact to only run on systems that matches another given fact.

An example of the confine statement would be something like the following:

Facter.add(:powerstates) do
  confine :kernel => 'Linux'
  setcode do
    Facter::Util::Resolution.exec('cat /sys/power/states')
  end
end

This fact uses sysfs on Linux to get a list of the power states that are available on the given system. Since this is only available on Linux systems, we use the confine statement to ensure that this fact isn’t needlessly run on systems that don’t support this type of enumeration.

Fact Precedence

A single fact can have multiple resolutions, each of which is a different way of ascertaining what the value of the fact should be. It’s very common to have different resolutions for different operating systems, for example. It’s easy to confuse facts and resolutions because they are superficially identical — to add a new resolution to a fact, you simply add the fact again, only with a different setcode statement.

When a fact has more than one resolution, make sure that Facter executes only one of them. Otherwise, each subsequent resolution overrides the one before it, and you might not get the value that you want.

Facter decides the issue of precedence using the weight property. Once Facter rules out any resolutions that are excluded because of confine statements, it executes the resolution with the highest weight. If that resolution doesn’t return a value, Facter moves on to the next resolution (by descending weight) until it gets a suitable value for the fact.

By default, the weight of a fact is the number of confines for that resolution, so more specific resolutions take priority over less specific resolutions.

# Check to see if this server has been marked as a postgres server.
Facter.add(:role) do
  has_weight 100
  setcode do
    if File.exist? '/etc/postgres_server'
      'postgres_server'
    end
  end
end

# Guess if this is a server by the presence of the pg_create binary.
Facter.add(:role) do
  has_weight 50
  setcode do
    if File.exist? '/usr/sbin/pg_create'
      'postgres_server'
    end
  end
end

# If this server doesn't look like a server, it must be a desktop.
Facter.add(:role) do
  setcode do
    'desktop'
  end
end

Timing Out

If you have facts that are unreliable and may not finish running, you can use the timeout property. If a fact is defined with a timeout and the evaluation of the setcode block exceeds the timeout, Facter will halt the resolution of that fact and move on.

# Sleep
Facter.add(:sleep, :timeout => 10) do
  setcode do
      sleep 999999
  end
end

Viewing Fact Values

If your Puppet masters are configured to use PuppetDB or the inventory service, you can view and search all of the facts for any node, including custom facts. See the PuppetDB or inventory service documentation for more information.

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