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Re: Executing a file only on later passes


From: Mark . Burgess
Subject: Re: Executing a file only on later passes
Date: Mon, 3 Nov 2003 20:53:18 +0100 (MET)


On  3 Nov, Eric Sorenson wrote:
> On Mon, 3 Nov 2003, Chip Seraphine wrote:
> 
>> Had I some sort of 'laterpasses' thingie, I could ensure that cf.finish 
>> always executed last.  I agree that this could cause some danger in that 
>> the contents of cf.finish would not benefit from a multiple-pass 
>> approach, but that should be a risk that the user should be willing to 
>> shoulder if he goes out of his way to do so.  I.e., it is my foot so it 
>> is my own fault if I shoot it on purpose; cfengine should only be 
>> worried about preventing me from shooting it accidentally :-)
> 
> Hi Chip. 
> 
> I thought this was exactly what the "AddInstallable" control: definition
> was for -- if C was in AddInstallable, and your cf.finish stanza referenced
> C, cfengine would know that although C might not be defined at first parse,
> it might be defined by the end of the run, and would defer the execution
> of the relevant actions until the second pass, after those actions with
> 'define=C' parameters have had a chance to run.
> 
> Unless I'm totally wrong about that... Mark?

That's right!

>> Hmm.  Is it just me, or is all this talk of ordering starting tomake me 
>> sound like Steve Traugott? :-)
> 
> By the end of last week, even Steve came to agree that order mattering was
> a sign of some weakness in the toolset, not an intrisic property of 
> all repeatable configuration actions. It's hard to read Luke's paper 
> objectively (setting aside 'religious' bias) and not come to the same
> conclusion.

Yes, this whole thing is really well understood now. Order only
matters when operations are not independent. Cfengine ties such
operations together with classes and then iterates until all the
orderings are resolved. Hence the need for the second pass.

Steve's need for ordering is a result of the use of tools that are
more complex than are necessary in his scripts. Thus he cannot
separate things that are indepedent.

Mark

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