Re: convergence and undoing changes

[Mark Burgess]

On Fri, 2005-11-18 at 11:45 -0500, christian pearce wrote:
> On 11/12/05, Viraj Alankar <valankar@gmail.com> wrote:
> > I'm a little confused on how to deal with undoing changes in a
> > convergence methodology. Let's say I decide to push a file
> > /usr/local/bin/foo to all of my servers. Then I decide not to do this,
> > and take it out of cfengine. This will be correct for new servers
> > 'converging,' but not the current ones which still have the file. Is
> > the normal procedure to have cfengine remove the file if it is there?
> > But then that removal is unnecessary on new servers.
> >
> > Or is a better method to make cfengine undo the changes temporarily,
> > and remove this 'undo' configuration once all of the old systems are
> > fixed?
> >
> Alva Couch gave a presentation about this:
> 
> http://homepages.informatics.ed.ac.uk/group/lssconf/config2005e/Slides/cfengine.pdf

I have not seen this before. In fact it annoys me a little because it is
factually incorrect and seems just to be an unnecessary slur on
cfengine. I would not expect that from Alva. There is plenty to
criticize in cfengine 2, but it does not help to misrepresent its
behaviour. This network level divergence example is highly misleading.
Of course there can be periods of divergence, but the talk seems to
imply that some other tool could improve on the problem. In fact I would
suggest that no other tool provides more consistency than cfengine
today.

Cfengine does not have a rollback function, because that would require
it to remember everything about the previous states (which state is
that?). It requirs exponentially growing memory and has no obvious
versioning control. There is currently no fully predictable solution to
this problem except to reprogram the state you want as if it were a
forward change.

M

-- 

Mark Burgess

Professor of Network and System Administration
Oslo University College

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Work: +47 22453272            Email:  Mark.Burgess@iu.hio.no
Fax : +47 22453205            WWW  :  http://www.iu.hio.no/~mark
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~