Forum: Cfengine Help
Subject: Re: Random variable persistence problem -- crontab randomization
Author: Justin
Link to topic: https://cfengine.com/forum/read.php?3,20077,20081#msg-20081
I wanted a randomization of calls to a web server and used a dist class. Here's
an excerpt from a policy file of ours:
#=========================================================================
# The "dist" class operator creates probabilistic classes. Given a class
# name and a list of numbers, it will always define the base class name
# and it will create a class with one of the numbers in the list
appended,
# based on that number's weight in the whole list. In this case, the
# dist_percent class will always be defined, while dist_percent_1 will
# be defined 1% of the time (1/(1+99)) and dist_percent_99 will be
# defined 99% of the time (99/(1+99)). (The numbers don't need to total
# 100. That is just a convenience for this particular case.)
#=========================================================================
any::
"dist_percent" dist => { "1", "99" },
comment => "Probability distribution class whose values are
percentages";
#=========================================================================
# Fetch the host's information from Asset Tracker if the cache file does
# not exist, the force_asset_fetch class is defined (meaning the
external
# class file had very recently been created to force the update) or in
the
# ~1% of cf-agent executions that the dist_percent_1 class (see above)
is
# defined. Since Cfengine runs every 5 minutes, which is 288 times per
day,
# on average the dist_percent_1 class will be defined randomly about
# 3 (2.88) times per day. This method allows for random retrieval of
# asset tracker information to spread the load on the asset tracker web
# server.
#
# This is used mainly for the side effect of the data fetch, rather than
# for the actual class definition. However, the class can be used to
# implement special logic, if desired, for immediately after the data is
# retrieved and locally cached in a file.
#=========================================================================
asset_command_exists.(!asset_cache_exists|dist_percent_1|force_asset_fetch)::
"fetched_asset_data" expression => returnszero(
"$(asset_command) -o $(asset_cache_file) data -d '|' -q ''
$(sys.uqhost)",
"noshell"
), comment => "Fetch asset data and store it in a local cache
file.";
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