Puppet users - Aug 2011 - Parameterized classes vs defined-types

Out of curiosity, how are people using parameterized classes in a way that is
distinct from defined-types?

-- 
Digant C Kasundra <digant@stanford.edu>
Infrastructure Systems Software Developer, ITS:IDG, Stanford University

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On Aug 23, 1:00 pm, Digant C Kasundra <dig...@stanford.edu>
wrote:
> Out of curiosity, how are people using parameterized classes in a way that
is distinct from defined-types?
<snark>I am _using_ defined types, that''s how.</snark>

Although I disfavor parameterized classes and do not use them, the
pattern of my usage of defined types could not be implemented via
parameterized classes.  In particular, I typically do not define a
type unless I plan to instantiate it multiple times for the same
node.  You cannot do that with parameterized classes.

If you have an OO background then the words "class" and
"type" may
have connotations and implied similarity for you that just don''t apply
in Puppet.  Puppet classes are not "types" in the type theory sense.
Defined types are closer to that, but it may help to use a fuller name
when you think about them: defined *resource* types.  Classes,
parameterized or not, are not resource types; rather, they are
resource _collections_.


John

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On 8/24/11 3:46 PM, jcbollinger wrote:
> If you have an OO background then the words "class" and
"type" may
> have connotations and implied similarity for you that just don''t
apply
> in Puppet.  Puppet classes are not "types" in the type theory
sense.
> Defined types are closer to that, but it may help to use a fuller name
> when you think about them: defined *resource* types.  Classes,
> parameterized or not, are not resource types; rather, they are
> resource _collections_.
I find it helps me to think of classes as *host*-class = a type of host 
classification (i.e. like measurements Weight and Height) - it is 
nonsensical to classify it multiple times e.g size = big, size = small, 
at best it would be redundant (size = big, size = big) to classify a 
host with the same puppet class multiple times (but I think that special 
case is also flagged as an error - which it should because the structure 
is clearly wrong).

Please correct me if I am wrong - I am also learning puppet.

- henrik


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----- Original Message -----
> On Aug 23, 1:00 pm, Digant C Kasundra <dig...@stanford.edu> wrote:
> > Out of curiosity, how are people using parameterized classes in a
> > way that is distinct from defined-types?
> 
> <snark>I am _using_ defined types, that''s how.</snark>
> 
> Although I disfavor parameterized classes and do not use them, the
> pattern of my usage of defined types could not be implemented via
> parameterized classes. In particular, I typically do not define a
> type unless I plan to instantiate it multiple times for the same
> node. You cannot do that with parameterized classes.
> 
> If you have an OO background then the words "class" and
"type" may
> have connotations and implied similarity for you that just don''t
apply
> in Puppet. Puppet classes are not "types" in the type theory
sense.
> Defined types are closer to that, but it may help to use a fuller name
> when you think about them: defined *resource* types. Classes,
> parameterized or not, are not resource types; rather, they are
> resource _collections_.
I agree with you.  I think that''s why I''m curious.  We also
overrides on defined types, which is why we prefer them as well.  I think while
it may be possible to do what we are currently doing with parameterized classes,
it would at least involve a lot of restructuring how we think of things in our
manifests.


-- 
Digant C Kasundra <digant@stanford.edu>
Infrastructure Systems Software Developer, ITS:IDG, Stanford University

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