Syntax question

[Rodrigo Severo]

Hi,


I have the following class definitions on a bundle:

bundle agent basic_classes
{
....
classes:
...
"server" or => { "@(server_types)" };
"workstation" expression => "!server";
}

How could I define workstation from server_types without creating an
intermediate server class?


Regards,

Rodrigo Severo

[Nick Anderson]

Maybe

"workstation" expression => none(".*",  server_types);

https://docs.cfengine.com/latest/reference-functions-none.html



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[Brian Bennett]

This would work, but your original example is appropriate on its own. On a host where none of the server types match, the class server doesn't actually get defined. So `expression => "!server"` or `not => "server"` doesn't actually depend on an intermediary class.

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[Rodrigo Severo]

On Sat, Dec 20, 2014 at 4:49 PM, Nick Anderson
<nick.a...@cfengine.com> wrote:
> Maybe
>
> "workstation" expression => none(".*", server_types);
>
> https://docs.cfengine.com/latest/reference-functions-none.html

Great. Thanks.


Rodrigo

[Rodrigo Severo]

On Sun, Dec 21, 2014 at 6:31 AM, Brian Bennett
<brian....@verticalsysadmin.com> wrote:
> This would work, but your original example is appropriate on its own. On a
> host where none of the server types match, the class server doesn't actually
> get defined. So `expression => "!server"` or `not => "server"` doesn't
> actually depend on an intermediary class.

Good to know. I really need to learn to think the cfengine way.


Thanks,

Rodrigo

[Alex Georgopoulos]

While I will agree with Brian's assessment I just want to chime in and say that I feel Nicks solutions is "safer".  I find using a "!class" can sometimes get you into a negative knowledge situation.  A !class is always true unless class is defined.  If those server classes are not defined by the time you call this bundle you could end up defining both classes.  I have been bitten by doing something very similar to what you have in your code snippet.  Coming from a positive point of view seems to have more deterministic results.  YMMV of course.

[Nick Anderson]

Yes, not knowing, and not knowing yet are slightly different things. Not knowing for sure is slightly better than not knowing indirectly. It's one of those differences that doesn't matter until it does.

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-----Original Message-----

[Brian Bennett]

On Dec 23, 2014, at 12:29 PM, Alex Georgopoulos <ageo...@gmail.com> wrote:

While I will agree with Brian's assessment I just want to chime in and say that I feel Nicks solutions is "safer".  I find using a "!class" can sometimes get you into a negative knowledge situation.  A !class is always true unless class is defined.  If those server classes are not defined by the time you call this bundle you could end up defining both classes.  I have been bitten by doing something very similar to what you have in your code snippet.  Coming from a positive point of view seems to have more deterministic results.  YMMV of course.

But if server_types is not defined (or empty) then `none(".*",  server_types)` will still yield a positive result.

The following workstation class definitions are identical. They will always be defined (or not) together since they are all a negative assertion, directly or indirectly, of any element in the list server_types.

"server" or => { "@{server_types}" };

"workstation1" expression => "!server";

"workstation2" not => "server";

"workstation3" expression => none(".*", server_types);

In order to have a positive assertion for workstation classes there needs to be a positive identifier for workstations, I agree, but none() won't give that to you.

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[Nick Anderson]

Well the regex could be changed to something sensible.

Sent from my mobile device.

-----Original Message-----

From: Brian Bennett <brian....@verticalsysadmin.com>
To: Alex Georgopoulos <ageo...@gmail.com>
Cc: help-c...@googlegroups.com, Nick Anderson <nick.a...@cfengine.com>
Sent: Tue, 23 Dec 2014 3:11 PM
Subject: Re: [help-cfengine] Syntax question

[Alex Georgopoulos]

Correct me if I am wrong but I thought If the RHS doesn't expand  it won't evaluate the left hand side.  Either way you're point is taken if we have an empty list.  

If this was my policy I would have and get rid of the not or the regex thingy or using !class and just do this.

"workstation" or => { "@{workstation_types}" };

[Alex Georgopoulos]

This little snippet demonstrates how this all can go wrong and not quite give the results you expect.

  body common control

{

 bundlesequence => {

                 "basic_classes_a",

                 "basic_classes_b",

                };

}

bundle agent basic_classes_a

{

        vars:

                "server_types" slist => { "fast_server", "slow_server" };

        classes:

                "server" or => { "@(server_types)" };

                "workstation" expression => "!server";

                "$(server_types)" expression => "any";

        reports:

                server.!workstation::

                "$(this.bundle) AM a server";

                workstation.!server::

                "$(this.bundle) NOT a server";

                server.workstation::

                "$(this.bundle) BOTH server and workstation";

}

bundle agent basic_classes_b

{

        vars:

                "server_types" slist => { "fast_server", "slow_server" };

        classes:

                "$(server_types)" expression => "any";

                "server" or => { "@(server_types)" };

                "workstation" expression => "!server";

        reports:

                server.!workstation::

                "$(this.bundle) AM a server";

                workstation.!server::

                "$(this.bundle) NOT a server";

                server.workstation::

                "$(this.bundle) BOTH server and workstation";

}

cf-agent -K -f /tmp/basic_classes.cf 

2014-12-23T16:49:59-0500   notice: R: basic_classes_a NOT a server

2014-12-23T16:49:59-0500   notice: R: basic_classes_a BOTH server and workstation

2014-12-23T16:49:59-0500   notice: R: basic_classes_b AM a server