Best practices for excluding certain modules from certain nodes

[Romeo Theriault]

Hi, I'm just getting started with puppet and am looking for some best
practices on how to handle node and module inheritance issues. I'm
planning to start using heira so want to plan my implementation around
hiera specifics.

Specifically, one item I can't seem to find a clean way of dealing
with is one-off nodes. For example, let's say I want to apply a class
called zabbix::agent to my whole infrastructure, so I put it in
common.yaml. But then I find out there are a few nodes that for
whatever reason I can't apply this class to. Short of just not
inheriting anything from common.yaml is there a clean way to say
"inherit everything from common except zabbix::agent"?

How are people dealing with the slight variations in their
infrastructure? I realize it's possible to code some logic into the
classes for these specific one-off hosts but that seems really hackish
and brittle.

Thanks for any insight!

--
Romeo

[Romeo Theriault]

After a bit more googling I found this informative puppet-users thread:

http://groups.google.com/group/puppet-users/browse_thread/thread/6b59ae2470acfa14/810eb8671a5b3cdd

which talks about creating special "disabled" classes which inherit
the widely used class and set certain values to 'undef'. This seems
like it's probably the way to go since it's the best method I've
seen/heard of so far to deal with this.

> I think a lot of shops do this by creating special "disabling" classes
> for those one-off systems.  To use your puppetmaster example (untested
> pseudocode ahead):
>        class puppet::client {
>          file { '/etc/puppet/puppet.conf':
>            ensure => present,
>            source => 'puppet:///puppet/configfile',
>          }
>        }
>        class puppet::client::disabled inherits puppet::client {
>          File['/etc/puppet/puppet.conf'] {
>            ensure => undef,
>            source => undef,
>          }
>        }
>        class puppet::server {
>          include puppet::client::disabled
>        }
> Now it's safe to apply puppet::client to all your nodes, including
> your puppetmaster, because the ::disabled class will override the
> management of puppet.conf on the puppetmaster (which presumably
> includes the puppet::server class).

Anyone else dealing with this in a different way?

Thanks,
Romeo

--
Romeo

[Justin Lloyd]

I just dealt with something similar regarding installing puppet agent vs. master and whether mcollective client (and thus activemq) should be installed. However, I'm including my base class rather than inheriting it. So how about something like this?

# templates.pp (imported into site.pp)
class system_base ( $zabbix = true ) {
    include motd # etc.
    if $zabbix {
        include zabbix::stuff
   }
}

# nodes.pp (imported into site.pp)
node 'basic_host' {
    class { 'system_base': }
}
node 'special_case' {
    class { 'system_base': zabbix => false }
}

--
Romeo

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[jcbollinger]

On Mar 2, 2:12 pm, Romeo Theriault <romeo.theria...@maine.edu> wrote:
> On Fri, Mar 2, 2012 at 08:56, Romeo Theriault <romeo.theria...@maine.edu> wrote:
> > [...] one item I can't seem to find a clean way of dealing

> > with is one-off nodes. For example, let's say I want to apply a class
> > called zabbix::agent to my whole infrastructure, so I put it in
> > common.yaml. But then I find out there are a few nodes that for
> > whatever reason I can't apply this class to. Short of just not
> > inheriting anything from common.yaml is there a clean way to say
> > "inherit everything from common except zabbix::agent"?
>
> > How are people dealing with the slight variations in their
> > infrastructure? I realize it's possible to code some logic into the
> > classes for these specific one-off hosts but that seems really hackish
> > and brittle.
>
> After a bit more googling I found this informative puppet-users thread:
>

> http://groups.google.com/group/puppet-users/browse_thread/thread/6b59...

>
> which talks about creating special "disabled" classes which inherit
> the widely used class and set certain values to 'undef'. This seems
> like it's probably the way to go since it's the best method I've
> seen/heard of so far to deal with this.

That is one of the standard approaches to the kind of problem you
describe, and it is simultaneously one of the few appropriate uses for
class inheritance. The post you referenced provides a rather specific
solution, however, and your description of it suggests that you may
not yet see how that generalizes.

In particular,
1) Overriding resource properties is the entire purpose of class
inheritance.
2) A subclass can override resource properties to any value, not just
undef. In fact, I think overriding to undef is unusual.
3) Although setting a resource property to undef generally means that
*property* is unmanaged, that's a very different thing from making the
entire resource be unmanaged.
4) Not managing a resource (or property) is very different from
managing it to an atypical state. Either might be what you want.

> Anyone else dealing with this in a different way?

Not I, but I can offer some alternatives anyway. Hiera provides
several:

A) Put an if block in Class['zabbix-agent'] around everything else in
the class body. Use hiera to look up the value controlling whether
the condition is satisfied. That provides an opt-out that any node
can be made to use simply by setting an appropriate value in its hiera
data.

B) As I recall (but have not used), hiera provides an ENC-like
function whereby you can cause it to declare classes for your nodes
based on class names it looks up in your data. You could use that to
decide whether to apply Class['zabbix-agent'] instead of declaring it
in a class / node declaration that every node uses. Leverage hiera's
hierarchical structure.

C) Instead of overriding certain resource properties in a subclass,
have the erstwhile parent class use hiera to look up the wanted
property values in the first place. This is a general-purpose
alternative to class inheritance.


John

[Romeo Theriault]

Thank you both for your great replies. They've both given me a great
lead on which direction to head in. I haven't had the time to fully
flesh out how I'm going to handle this yet but I know I'll be trying
to stay away from parametrized classes for the time being. I'll also
be trying to use hiera as much as possible to handle external data and
determining which classes get applied to which nodes.

Thanks again for the help.

Romeo

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