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

[Petr Spacek]

On 15.11.2013 02:14, Jan Pazdziora wrote:
> 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.
Okay, may be that "re-do" is not the best description ever ;-)

>> 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
No, it says that you will reach the same state again and again - if the 
conditions are the same. It does not apply only to successful installations.

> 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.
I agree, "re-do" wasn't the best wording.

> But if your installation is borked beyond repair for whatever reason,
> it does not mean that the setup will be self-healing.
Sure, but you describe case when installation failed for whatever reason and 
you ran it again without any change. In that case it will fail again, 
idempotently :-)

The point is that you have a chance to fix the problem (reconfigure firewall, 
DNS etc.), run the installer again and it will finish the installation or fail 
later on some other problem. It means that you don't need to start from 
scratch. (This is exactly what was described at the conference by the speaker.)

Also, it allows you to reproduce the failure again and again (without any 
change to existing system) and dig towards the cause. This can be major 
advantage if you don't know why it failed.

> 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.

I think that this goes hand-in-hand. IMHO it is much harder to write and 
maintain uninstaller if you have just imperative installer, because you have 
to read it's code and manually write the uninstaller.

I think you have much better position with declarative installer, because you 
can extract a lot of information from the 'declarations' and generate 
uninstaller from that. (I don't say that it will be 100% bullet proof, but I'm 
pretty sure that this will lower the chance that you added something to 
installer but forgot to add the same step to uninstaller.)

-- 
Petr^2 Spacek