Puppet users - Feb 2010 - Traversal of class inheritance - chain

Can one include muliple parts of a (class) - inheritance chain? And if
yes, how does this chain get traversed? Are there any limitations /
sideffects / documentation?

The idea behind is: I''d like to provide a base set of disabled
services. Each of this  service can be enabled through inheriting from
the base class and overriding the ensure / enable attributes.

suppose the following:

node foobar {
  include base
  include syslog
  include ueber_syslog
}

#disables syslog - just as an example ;)
class base {
 service {"svc:/system/system-log:default":
        ensure => stopped,
        enable => false
    }
}

#enables syslog
class syslog inherits base {
    Service["svc:/system/system-log:default"]{
        ensure => running,
        enable => true
    }
}

#disables syslog and enables uebersyslog
class ueber_syslog inherits syslog {
    Service["svc:/system/system-log:default"]{
        ensure => stopped,
        enable => false
    }

   service {"svc:/system/uebersyslog:default":
        ensure => running,
        enable => true
    }
}

Thanks for your input.

Cheers Robert

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On Feb 17, 2:56 am, "robert.gstoehl" <robert.gsto...@gmail.com>
wrote:
> Can one include muliple parts of a (class) - inheritance chain? And if
> yes, how does this chain get traversed? Are there any limitations /
> sideffects / documentation?
You can include classes from multiple parts of an inheritance tree,
including classes from different branches of the tree, provided that
there are no conflicting overrides. Two overrides conflict if they
attempt to set different values for the same resource property, and
they occur in two classes where neither is a descendant of the other.

The two overrides in your "syslog" and "ueber_syslog"
classes are
perfect examples of conflicting overrides.  I see nothing wrong with
your classes individually, but including both "syslog" and
"ueber_syslog" on the same node will fail.  That makes sense, because
doing so declares that the syslog service must be both running and
stopped, which is impossible.  There is no problem, however, with
including class "base" along with either one of the other two.

The solution, quite simply, is to avoid declaring conflicting
resources or resource properties.  In the example, that means each
node may include at most one of the "syslog" and
"ueber_syslog"
classes.  To achieve that you can hard-code per node group which class
to use, use "if" or "case" statements to choose one class
for each
node based on its available facts, or use an external node classifier.

Cheers,

John

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Hey John,

Thanks for the clarification. Makes kind of sense ;)

Cheers Robert

On Feb 17, 3:50 pm, jcbollinger <John.Bollin...@stJude.org>
wrote:
> On Feb 17, 2:56 am, "robert.gstoehl"
<robert.gsto...@gmail.com> wrote:
>
> > Can one include muliple parts of a (class) - inheritance chain? And if
> > yes, how does this chain get traversed? Are there any limitations /
> > sideffects / documentation?
>
> You can include classes from multiple parts of an inheritance tree,
> including classes from different branches of the tree, provided that
> there are no conflicting overrides. Two overrides conflict if they
> attempt to set different values for the same resource property, and
> they occur in two classes where neither is a descendant of the other.
>
> The two overrides in your "syslog" and "ueber_syslog"
classes are
> perfect examples of conflicting overrides.  I see nothing wrong with
> your classes individually, but including both "syslog" and
> "ueber_syslog" on the same node will fail.  That makes sense,
because
> doing so declares that the syslog service must be both running and
> stopped, which is impossible.  There is no problem, however, with
> including class "base" along with either one of the other two.
>
> The solution, quite simply, is to avoid declaring conflicting
> resources or resource properties.  In the example, that means each
> node may include at most one of the "syslog" and
"ueber_syslog"
> classes.  To achieve that you can hard-code per node group which class
> to use, use "if" or "case" statements to choose one
class for each
> node based on its available facts, or use an external node classifier.
>
> Cheers,
>
> John
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