[Freeipa-devel] idempotent installer [from LinuxAlt 2013]

[Jan Pazdziora]

On Thu, Nov 14, 2013 at 03:40:52PM +0100, Petr Spacek wrote:
> 
> In reality, it means that you can re-run OpenStack installer on the
> same machine/set of machines (with the same configuration, of
> course!) and it will re-do everything again. You can re-run

The point is that it should *not* redo everything again.

> installer again and again without any harm!
> 
> This solves case where something went wrong during the installation,
> the installation was aborted and the machine was left in some
> inconsistent state. Think about e.g. network failure during
> installation, improper configuration which prevented installation
> from finishing (crap in DNS), some intermittent and mysterious
> errors in Dogtag installer and so on.
> 
> It does mean that you don't need to recycle whole machine if
> something went wrong during installation ...

I am not sure idempotence focuses on failures, really. It says that
if your previous installer run passed, the subsequent run should
detect that individual pieces of configuration are already in place
and it should not re-run the steps to re-create them. The implication
being that it should only configure the pieces missing.

But if your installation is borked beyond repair for whatever reason,
it does not mean that the setup will be self-healing.

I think that our installers do a pretty decent job of cleaning after
themselves if things fails (rolling back the changes) which I find
much more important than leaving mess around with the intent of fixing
it upon the next run. Leaving things in consistent state is higher
value than idempotence.

-- 
Jan Pazdziora
Principal Software Engineer, Identity Management Engineering, Red Hat