Plugins/TCheetah

  1. TCheetah Templating Plugin
    1. self.metadata variables
    2. self.properties
    3. Simple Example
      1. bcfg2/TCheetah/foo/template
      2. bcfg2/TCheetah/foo/info
      3. Output
    4. Example: Replace the crontab plugin
    5. Note: Comments and Cheetah

TCheetah Templating Plugin

This document reflects the TCheetah plugin in bcfg2 0.8.4 and later.

The TCheetah plugin allows you to use the cheetah templating system to create files, instead of the various diff-based methods offered by the Cfg plugin. It also allows you to include the results of probes executed on the client in the created files.

To begin, you will need to download and install the Cheetah templating engine from http://www.cheetahtemplate.org/. Once it is installed, you can enable it by adding TCheetah to the generators line in /etc/bcfg2.conf on your Bcfg server.
For example (Note: The generators line is deprecated in versions 1.0 and greater): 

generators = SSHbase,Cfg,Pkgmgr,Svcmgr,Rules,TCheetah

The TCheetah plugin makes use of a Cfg-like directory structure located in in a TCheetah subdirectory of your repository, usually /var/lib/bcfg2/TCheetah. Each file has a directory containing two files, template and info. The template is a standard Cheetah template with two additions:

  • self.metadata is the client's metadata
  • self.properties is an xml document of unstructured data

The info file is formatted like :info files from Cfg.

Mostly, people will want to use client metadata.

self.metadata variables

The following variables are available for self.metadata :

  • hostname
  • bundles
  • groups
  • toolset
  • categories
  • probes
  • uuid
  • password

self.metadata is an instance of the class ClientMetadata of file Bcfg2/Server/Plugins/Metadata.py.

self.properties

properties is a python ElementTree object, loaded from the data in /var/lib/bcfg2/etc/properties.xml. That file should have a Properties node at its root.

Example properties.xml: 

<Properties>
 <host>
   <www.example.com>
     <rootdev>/dev/sda</rootdev>
   </www.example.com>
 </host>
</Properties>

You may use any of the ElementTree? methods to access data in your template. Several examples follow, each producing an identical result on the host 'www.example.com': 

$self.properties.find('host').find('www.example.com').find('rootdev').text
$self.properties.find('host').find($self.metadata.hostname).find('rootdev').text
${self.properties.xpath('host/www.example.com/rootdev')[0].text}
${self.properties.xpath('host/' + self.metadata.hostname + '/rootdev')[0].text}
#set $path = 'host/' + $self.metadata.hostname + '/rootdev'
${self.properties.xpath($path)[0].text}
${self.properties.xpath(path)[0].text}

Simple Example

bcfg2/TCheetah/foo/template

> buildfile /foo <clientname>
Hostname is $self.metadata.hostname
Groups:
#for $group in $self.metadata.groups:
 * $group
#end for
Categories:
#for $category in $self.metadata.categories:
 * $category -- $self.metadata.categories[$category]
#end for

Probes:
#for $probe in $self.metadata.probes:
 * $probe -- $self.metadata.probes[$probe]
#end for

bcfg2/TCheetah/foo/info

perms: 624

Output

The following output can be generated with bcfg2-info. Note that probe information is not persistent, hence, it only works when clients directly query the server. For this reason, bcfg2-info output doesn't reflect current client probe state. 

<ConfigFile name="/foo" owner="root" perms="0624" group="root">
Hostname is topaz.mcs.anl.gov
Groups:
 * desktop
 * mcs-base
 * ypbound
 * workstation
 * xserver
 * debian-sarge
 * debian
 * a
Categories:
 * test -- a

Probes:
</ConfigFile>

Example: Replace the crontab plugin

In many cases you can use the TCheetah plugin to avoid writing custom plugins in Python. This example replaces the crontab plugin (Bcfg2.Server.Plugins.Crontab). This plugin randomizes the time of cron.daily execution with a stable result. Cron.daily is run at a consistent, randomized time between midnight and 7am. 

#import random
#silent random.seed($self.metadata.hostname)

# /etc/crontab: system-wide crontab
# Unlike any other crontab you don't have to run the `crontab'
# command to install the new version when you edit this file.
# This file also has a username field, that none of the other crontabs do.

SHELL=/bin/sh
PATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin://bin

# m h dom mon dow user  command
17 *    * * *   root    run-parts --report /etc/cron.hourly
$random.randrange(0,59) $random.randrange(0,6)    * * *   root    test -x /usr/sbin/anacron || run-parts --report /etc/cron.daily
47 6    * * 7   root    test -x /usr/sbin/anacron || run-parts --report /etc/cron.weekly
52 6    1 * *   root    test -x /usr/sbin/anacron || run-parts --report /etc/cron.monthly.

Note: Comments and Cheetah

As Cheetah processes your templates it will consider hash "#" style comments to be actual comments in the template and will strip them from the final config file. If you would like to preserve the comment in the final config file you need to escape the hash character '\#' which will tell Cheetah (and Python) that you do in fact want the comment to appear in the final config file. 

# This is a comment in my template which will be stripped when it's processed through Cheetah

\# This comment will appear in the generated config file.