Puppet - Template

Templating is a method of getting things in a standard format, which can be used in multiple locations. In Puppet, templating and templates are supported using erb which comes as a part of standard Ruby library, which can be used on other projects apart from Ruby like in Ruby on Rails projects. As a standard practice, one needs to have a basic understanding of Ruby. Templating is very helpful when the user is trying to manage content of a template file. Templates plays a key role when configurations cannot be managed by a built-in Puppet type.

Evaluating Templates

Templates are evaluated using simple functions.

$value = template ("testtemplate.erb")

One can specify the full path of a template or one can pull all templates in Puppet’s templatedir, which is usually located at /var/puppet/templates. One can find the directory location by running the puppet –-configprint templatedir.

Templates are always evaluated by the parser, not the client which means that if one is using puppetmasterd, then the template only needs to be on the server and one never needs to download them to the client. There’s no difference on how the client sees between using a template and specifying all the content of a file as a string. This clearly indicates that client-specific variables are learned first by puppetmasterd during the puppet startup phase.

Using Templates

Following is an example of generating the tomcat configuration for testing sites.

define testingsite($cgidir, $tracdir) { 
   file { "testing-$name": 
   path => "/etc/tomcat/testing/$name.conf", 
   owner => superuser, 
   group => superuser, 
   mode => 644, 
   require => File[tomcatconf], 
   content => template("testsite.erb"), 
   notify => Service[tomcat] 
}  
   symlink { "testsym-$name": 
      path => "$cgidir/$name.cgi", 
      ensure => "/usr/share/test/cgi-bin/test.cgi" 
   } 
}

Following is the template definition.

<Location "/cgi-bin/ <%= name %>.cgi"> 
   SetEnv TEST_ENV "/export/svn/test/<%= name %>" 
</Location>  

# You need something like this to authenticate users 
<Location "/cgi-bin/<%= name %>.cgi/login"> 
   AuthType Basic 
   AuthName "Test" 
   AuthUserFile /etc/tomcat/auth/svn 
   Require valid-user 
</Location>

This pushes each template file into a separate file and then one needs to just tell Apache to load these configuration files.

Include /etc/apache2/trac/[^.#]*

Combining Templates

Two templates can be easily combined using the following command.

template('/path/to/template1','/path/to/template2')

Iteration in Templates

Puppet template also supports array iteration. If the variable one is accessing is an array, then one can iterate over it.

$values = [val1, val2, otherval]

We can have templates like the following.

<% values.each do |val| -%> 
Some stuff with <%= val %> 
<% end -%>

The above command will produce the following result.

Some stuff with val1 
Some stuff with val2 
Some stuff with otherval

Conditions in Templates

The erb templating supports conditionals. The following construct is a quick and easy way to conditionally put a content in a file.

<% if broadcast != "NONE" %> broadcast <%= broadcast %> <% end %>

Templates and Variables

One can use templates to fill in variables in addition to filling out the file content.

testvariable = template('/var/puppet/template/testvar')

Undefined Variable

If one needs to check if the variable is defined before using it, the following command works.

<% if has_variable?("myvar") then %> 
myvar has <%= myvar %> value 
<% end %>

Out of Scope Variable

One can look for out of scope variable explicitly with the lookupvar function.

<%= scope.lookupvar('apache::user') %>

Sample Project Template

<#Autogenerated by puppet. Do not edit. 
[default] 
#Default priority (lower value means higher priority) 
priority = <%= @priority %> 
#Different types of backup. Will be done in the same order as specified here. 
#Valid options: rdiff-backup, mysql, command 
backups = <% if @backup_rdiff %>rdiff-backup, 
<% end %><% if @backup_mysql %>mysql, 
<% end %><% if @backup_command %>command<% end %> 
<% if @backup_rdiff -%>  

[rdiff-backup]  

<% if @rdiff_global_exclude_file -%> 
   global-exclude-file = <%= @rdiff_global_exclude_file %> 
<% end -%> 
   <% if @rdiff_user -%> 
      user = <%= @rdiff_user %> 
<% end -%> 
<% if @rdiff_path -%> 
   path = <%= @rdiff_path %> 
<% end -%>  

#Optional extra parameters for rdiff-backup  

extra-parameters = <%= @rdiff_extra_parameters %>  

#How long backups are going to be kept 
keep = <%= @rdiff_keep %> 
<% end -%> 
<% if @backup_mysql -%>%= scope.lookupvar('apache::user') %>  

[mysql]  

#ssh user to connect for running the backup 
sshuser =  <%= @mysql_sshuser %>

#ssh private key to be used 
   sshkey = <%= @backup_home %>/<%= @mysql_sshkey %> 
   <% end -%> 
<% if @backup_command -%>  
[command] 

#Run a specific command on the backup server after the backup has finished  

command = <%= @command_to_execute %> 
<% end -%>
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