[openstack-dev] [TripleO] os-refresh-config run frequency

Clint Byrum clint at fewbar.com
Fri Jun 27 16:00:40 UTC 2014

Excerpts from Macdonald-Wallace, Matthew's message of 2014-06-27 00:14:49 -0700:
> Hi Clint,
> > -----Original Message-----
> > From: Clint Byrum [mailto:clint at fewbar.com]
> > Sent: 26 June 2014 20:21
> > To: openstack-dev
> > Subject: Re: [openstack-dev] [TripleO] os-refresh-config run frequency
> >
> > So I see two problems highlighted above.
> > 
> > 1) We don't re-assert ephemeral state set by o-r-c scripts. You're right, and
> > we've been talking about it for a while. The right thing to do is have os-collect-
> > config re-run its command on boot. I don't think a cron job is the right way to
> > go, we should just have a file in /var/run that is placed there only on a successful
> > run of the command. If that file does not exist, then we run the command.
> > 
> > I've just opened this bug in response:
> > 
> > https://bugs.launchpad.net/os-collect-config/+bug/1334804
> Cool, I'm more than happy for this to be done elsewhere, I'm glad that people are in agreement with me on the concept and that work has already started on this.
> I'll add some notes to the bug if needed later on today.
> > 2) We don't re-assert any state on a regular basis.
> > 
> > So one reason we haven't focused on this, is that we have a stretch goal of
> > running with a readonly root partition. It's gotten lost in a lot of the craziness of
> > "just get it working", but with rebuilds blowing away root now, leading to
> > anything not on the state drive (/mnt currently), there's a good chance that this
> > will work relatively well.
> > 
> > Now, since people get root, they can always override the readonly root and
> > make changes. <golem>we hates thiss!</golem>.
> > 
> > I'm open to ideas, however, os-refresh-config is definitely not the place to solve
> > this. It is intended as a non-resident command to be called when it is time to
> > assert state. os-collect-config is intended to gather configurations, and expose
> > them to a command that it runs, and thus should be the mechanism by which os-
> > refresh-config is run.
> > 
> > I'd like to keep this conversation separate from one in which we discuss more
> > mechanisms to make os-refresh-config robust. There are a bunch of things we
> > can do, but I think we should focus just on "how do we re-assert state?".
> OK, that's fair enough.
> > Because we're able to say right now that it is only for running when config
> > changes, we can wave our hands and say it's ok that we restart everything on
> > every run. As Jan alluded to, that won't work so well if we run it every 20
> > minutes.
> Agreed, and chatting with Jan and a couple of others yesterday we came to the conclusion that whatever we do here it will require "tweaking" of a number of elements to safely restart services.
> > So, I wonder if we can introduce a config version into os-collect-config.
> > 
> > Basically os-collect-config would keep a version along with its cache.
> > Whenever a new version is detected, os-collect-config would set a value in the
> > environment that informs the command "this is a new version of config". From
> > that, scripts can do things like this:
> > 
> > if [ -n "$OS_CONFIG_NEW_VERSION" ] ; then
> >   service X restart
> > else
> >   if !service X status ; then service X start fi
> > 
> > This would lay the groundwork for future abilities to compare old/new so we
> > can take shortcuts by diffing the two config versions. For instance if we look at
> > old vs. new and we don't see any of the keys we care about changed, we can
> > skip restarting.
> I like this approach - does this require a new spec? If so, I'll start an etherpad to collect thoughts on it before writing it up for approval.

I think this should be a tripleo spec. If you're volunteering write it,
hooray \o/. It will require several work items. Off the top of my head:

- Add version awareness to os-collect-config
- Add version awareness to all os-refresh-config scripts that do
  disruptive things.
- Add periodic command run to os-collect-config

Let's call it 're-assert-system-state'. Sound good?

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