Advanced recursion and memoization in Puppet

As a follow-up to my original article on recursion in Puppet, and in my attempt to Push Puppet (to its limit), I’ll now attempt some more advanced recursion techniques in Puppet.

In my original recursion example, the type does recurse, but the callee cannot return any value to the caller because it is a type, and not strictly a function. This limitation immediately limits the usefulness of this technique, but I’ll try to press on! Let’s try to write a Fibonacci series function in native Puppet.

For those who aren’t familiar, the Fibonacci series function is a canonical computer science recursion example. It is very easy to write as pseudo-code or to implement in python:

def F(n):
	if n == 0: return 0
	elif n == 1: return 1
	else: return F(n-1)+F(n-2)

$ ./fibonacci.py
F(0) == 0
F(1) == 1
F(2) == 1
F(3) == 2
F(4) == 3
F(5) == 5
F(6) == 8
F(7) == 13
F(8) == 21
F(9) == 34
F(10) == 55
F(11) == 89
F(12) == 144
F(13) == 233
...

Now, I’ll introduce my Puppet implementation:

#!/usr/bin/puppet apply
class fibdir {
	# memoization directory
	file { '/tmp/fibonacci/':
		ensure => directory,
	}
}

define fibonacci(
	$n,
	$intermediate = false # used for pretty printing
) {
	include fibdir

	$vardir = '/tmp'
	$fibdir = "${vardir}/fibonacci"

	if "${n}" == '0' {
		# 0
		exec { "${name}: F(0)":
			command => "/bin/echo 0 > ${fibdir}/0",
			creates => "${fibdir}/0",
			require => File["${fibdir}/"],
		}

	} elsif "${n}" == '1' {
		# 1
		exec { "${name}: F(1)":
			command => "/bin/echo 1 > ${fibdir}/1",
			creates => "${fibdir}/1",
			require => File["${fibdir}/"],
		}

	} else {

		$minus1 = inline_template('<%= n.to_i - 1 %>')
		fibonacci { "${name}: F(${n}-1)":
			n => $minus1,
			intermediate => true,
		}

		$minus2 = inline_template('<%= n.to_i - 2 %>')
		fibonacci { "${name}: F(${n}-2)":
			n => $minus2,
			intermediate => true,
		}

		# who can figure out what the problem with this is ?
		$fn1 = inline_template('<%= f1=fibdir+"/"+minus1; (File.exist?(f1) ? File.open(f1, "r").read.to_i : -1) %>')
		$fn2 = inline_template('<%= f2=fibdir+"/"+minus2; (File.exist?(f2) ? File.open(f2, "r").read.to_i : -1) %>')

		if (("${fn1}" == '-1') or ("${fn2}" == '-1')) {
			$fn = '-1'
		} else {
			$fn = inline_template('<%= fn1.to_i+fn2.to_i %>')
		}

		if "${fn}" != '-1' { # did the lookup work ?
			# store fibonacci number in 'table' (memoization)
			exec { "${name}: F(${n})":
				command => "/bin/echo ${fn} > ${fibdir}/${n}",
				creates => "${fibdir}/${n}",
				require => [
					File["${fibdir}/"],
					Fibonacci["${name}: F(${n}-1)"],
					Fibonacci["${name}: F(${n}-2)"],
				],
			}

			if ! $intermediate {
				# display...
				notify { "F(${n})":
					message => "F(${n}) == ${fn}",
					require => Exec["${name}: F(${n})"],
				}
			}
		}
	}
}

# kick it off...
fibonacci { 'start':
	n => 8,
}

Each time this runs, Puppet will complete the next step in the execution. To make this successive execution automatic, I’ve written a small bash wrapper to do this, but you can run it manually too. If you do use my wrapper, use it with the fibonacci.pp file provided in git.

The computer scientist might notice that as a side effect, we are actually memoizing. This means that if we run this type again with a larger input value, the previously completed intermediate step values are used as a starting point for the subsequent computations. Cool!

The Puppet wizard might notice that I cheated slightly. Take a minute to try to see where…

(IMAGE OF TIME PASSING)

Have you figured it out? The problem with the current implementation is that it will only work when run locally as a standalone Puppet program. The reason, is that exec types run on the client, and the templates run on the server. This type requires that both of those elements run on the same machine so that the save/load memoization can work correctly. Since this code runs on the same machine, this isn’t a problem! This split execution model is one of the features that can confuse new Puppet users.

To adapt our function (technically a type) to work in any environment, we need to do some more hacking! We can continue to use our exec type for saving, but a fact needs to be used to load in the necessary values:

require 'facter'

fibdir = '/tmp/fibonacci/'
valid_fibdir = fibdir.gsub(/\/$/, '')+'/' # ensure trailing slash

results = {} # create list of values

if File.directory?(valid_fibdir)
	Dir.glob(valid_fibdir+'*').each do |f|
		b = File.basename(f)
		i = b.to_i # invalid returns 0
		if b == '0' or i != 0
			v = File.open(f, 'r').read.strip.to_i # read into int
			results[i] = v
		end
	end
end

results.keys.each do |x|
	Facter.add('pushing_fibonacci_'+x.to_s) do
		setcode {
			results[x]
		}
	end
end

# these are 'fact' lookups
$fn1 = getvar("pushing_fibonacci_${minus1}")
$fn2 = getvar("pushing_fibonacci_${minus2}")

I hope you enjoyed this. Please let me know! Comment below, or send me a message.

Happy hacking,

James

P.S.: While I think this is fun, I wrote this hack to demonstrate some techniques, and to set the stage for future hacks, future techniques, and future Puppet examples. If you’re using this as a good way to actually compute values in the Fibonacci series, you’re insane!

P.P.S.: The word is actually memoization, not memorization, despite the similarities between the two words, and the two concepts.

P.P.P.S: Gluster users get extra points if they can figure out how this will lead to a feature for Puppet-Gluster. It’s a bit tricky to see if you’re not following my git commits.

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November 27, 2013
1058 words

• technical