[ClusterLabs Developers] RA as a systemd wrapper -- the right way?

Adam Spiers aspiers at suse.com
Mon Sep 26 10:10:30 EDT 2016

Ken Gaillot <kgaillot at redhat.com> wrote:
> On 09/22/2016 10:39 AM, Adam Spiers wrote:
> > Ken Gaillot <kgaillot at redhat.com> wrote:
> >> On 09/22/2016 08:49 AM, Adam Spiers wrote:
> >>> Ken Gaillot <kgaillot at redhat.com> wrote:
> >>>> On 09/21/2016 03:25 PM, Adam Spiers wrote:
> >>>>> As a result I have been thinking about the idea of changing the
> >>>>> start/stop/status actions of these RAs so that they wrap around
> >>>>> service(8) (which would be even more portable across distros than
> >>>>> systemctl).
> >>>>>
> >>>>> The primary difference with your approach is that we probably wouldn't
> >>>>> need to make the RAs dynamically create any systemd configuration, since
> >>>>> that would already be provided by the packages which install the OpenStack
> >>>>> services.  But then AFAIK none of the OpenStack services use the
> >>>>> multi-instance feature of systemd (foo@{one,two,three,etc}.service).
> >>>>
> >>>> The main complication I see is that pacemaker expects OCF agents to
> >>>> return success only after an action is complete. For example, start
> >>>> should not return until the service is fully active. I believe systemctl
> >>>> does not behave this way, rather it initiates the action and returns
> >>>> immediately.
> >>>
> >>> But that's trivial to work around: polling via "service foo status"
> >>> after "service foo start" converts it back from an asynchronous
> >>> operation to a synchronous one.
> >>
> >> Yes, that's exactly what pacemaker does now: start/stop, then every two
> >> seconds, poll the status.
> >>
> >> However, I'm currently working on a project to change that, so that we
> >> use DBus signalling to be notified when the job completes, rather than
> >> (or in addition to) polling.
> >>
> >> The reason is twofold: the two-second wait can be an unnecessary
> >> recovery delay in some cases; and (at least from the DBus API, not sure
> >> about systemctl status) there's no reliable way to distinguish "service
> >> is inactive because the start didn't work properly" from "service is
> >> inactive because systemd has some slow-starting dependencies of its own
> >> to start first".
> > 
> > OK, that makes sense - thanks.

Although thinking about it more - why couldn't systemctl return
different exit codes for these two cases, or add an "is-starting"
subcommand, or similar?

> >>>> Pacemaker's native systemd integration has a lot of workarounds for
> >>>> quirks in systemd behavior (and more every release). I'm not sure
> >>>> moving/duplicating that logic to the RA is a good approach.
> >>>
> >>> What other quirks are there?
> >>
> >> When pacemaker starts a systemd service, it creates a unit override in
> >> /run/systemd/system/<agent>.service.d/50-pacemaker.conf, with these
> >> overrides (and removes the file when stopping the resource):
> >>
> >> * It prefixes the description with "Cluster Controlled" (e.g. "Postfix
> >> Mail Transport Agent" -> "Cluster Controlled Postfix Mail Transport
> >> Agent"). This gives a clear indicator in systemd messages in the syslog
> >> that it's a cluster resource.
> >>
> >> * "Before=pacemaker.service": This ensures that when someone shuts down
> >> the system via systemd, systemd doesn't stop pacemaker before pacemaker
> >> can stop the resource.
> >>
> >> * "Restart=no": This ensures that pacemaker stays in control of
> >> responding to service failures.
> > 
> > Yes, I was aware of that, and you're right that my approach of making
> > the RA wrap service(8) or systemctl(8) would need to duplicate this
> > functionality - *unless* the creation of the unit override could be
> > moved out of Pacemaker's C code into a shell script which both
> > Pacemaker and external RAs which want to adopt this wrapping technique
> > could call.
> > 
> >> Additionally:
> >>
> >> * Pacemaker uses intelligent timeout values (based on cluster
> >> configuration) when making systemd calls.
> > 
> > I guess I'd need more details to fully understand this, but couldn't
> > those intelligently chosen timeout values be passed to the RA if
> > necessary?  Although that does put a bit of a dampener on my hope of
> > using service(8) to remain agnostic to whichever pid-1 system happened
> > to be in use on the current machine.  Having said that, maybe everyone
> > in the OpenStack (HA) community has already moved to systemd by now
> > anyway.
> One pacemaker action (start/stop/whatever) may involve multiple
> interactions with systemd. At each step, pacemaker knows the remaining
> timeout for the whole action, so it can use an appropriate timeout with
> each systemd action.
> There's no way for the RA to know how much time is remaining.

Stupid question - why not?  Couldn't Pacemaker tell it?

> But I guess it's not important, since pacemaker will timeout the entire
> RA action if necessary.
> >> * Pacemaker interprets/remaps systemd return status as needed. For
> >> example, a stop followed by a status poll that returns "OK" means the
> >> service is still running. Fairly obvious, but there are a lot of cases
> >> that need to be handled.
> > 
> > Other than (obviously) start followed by status, what other cases are
> > there?
> It's just a matter of looking at all the possible return values of each
> systemd call, and then mapping that to something the cluster can
> interpret. Pacemaker uses the DBus API so the specifics will be
> different compared to systemctl. It's just important to get right.

I'm struggling to understand the specifics here, or find the bit of
Pacemaker code which corresponds to them.

> > All of this stuff sounds like generic problems which could be solved
> > once for all wrapper RAs via a simple shell library.  I'd happily
> > maintain this in openstack-resource-agents, although TBH it would
> > probably belong in resource-agents if anywhere.
> > 
> >> All of these were added gradually over the past few years, so I'd expect
> >> the list to grow over the next few years.
> > 
> > Well, hopefully they could be grown in a way which also supported
> > wrapper RAs :-)
> > 
> > Alternatively, if you think that there's a better solution than this
> > wrapper RA idea, I'm all ears.  The two main problems are essentially:
> > 
> >   1. RAs duplicate a whole bunch of logic / config already provided
> >      by vendor packages and systemd service units.
> > 
> >   2. RAs have a "monitor" action which can do proper application-level
> >      monitoring (e.g. HTTP pings), whereas apparently systemd has
> >      nothing equivalent.
> > 
> > So currently we are forced to choose between a) using systemd
> > Pacemaker resources, and b) having proper monitoring rather than just
> > naive pid-level monitoring, but having to duplicate a whole load of
> > stuff which systemd already does nicely.
> > 
> > If I'm missing something, or you can think of a better alternative
> > then please tell me!
> I don't see a clear answer.
> I suppose a resource-agents interface could minimize the problems.
> Something like ocf_start_via_systemd could create an override file,
> start a service, and poll until it has a status. Similarly for stop.


> The main drawbacks I see are that I'm not sure you can solve the
> problems with polling without the dbus interface

I still don't get why not - but that's most likely due to my
ignorance of the details.  Any pointers gratefully received if you
have time.

> and the override file is tailored to pacemaker (which
> resource-agents stays independent of).

Not sure what you mean by this, or why it would be a drawback?

> If you want to give it a try, here are some test cases:
> * A service that takes a long time to start, with another resource
> ordered after it (make sure the second resource doesn't start until the
> first is fully up)

I assume you mean the ordering would come from a Pacemaker order
constraint, not from systemd Before= or After= ?

In this case, the OCF RA "start" action would invoke the shared
ocf_start_via_systemd() library function, which would initiate
startup, and then poll until startup started, at which point the RA
action would complete and Pacemaker would continue by starting the
resource ordered after it.  I'm not sure I see the problem here.  Of
course there would be a Pacemaker start op timeout which would need to
be long enough, but that's no different regardless of which resource
agent is in use.

> * A service that has a Requires= dependency that takes a long time to
> start and is not managed by the cluster

Here you mean that if service A has Requires=B, then B (not A) takes a
long time to start and is not managed by the cluster - right?  Here
there are two cases: a) service A also has After=B, and b) it does

Again, I don't see a problem.  In case a), ocf_start_via_systemd()
would initiate startup of A and then poll until A completed startup,
and systemd would ensure that B started before A started.  In case b)
then systemd would initiate startup of A and B in parallel.  Again,
the polling would not return until A was fully started.  This could be
before B was fully started, but that would be OK because if it wasn't,
A would have had After=B.

The fact that I don't see any problems where you apparently do makes
me deeply suspicious of my own understanding ;-)  Please tell me what
I'm missing.

> * Use systemctl to shut down the host while the cluster is active, with
> resources that take a while to stop

The overrides tell systemd that it has no business shutting down
resources managed by Pacemaker.  So systemd only cares about shutting
down Pacemaker itself (which would not complete until all
Pacemaker-managed resources had been stopped by Pacemaker), and other
services not managed by Pacemaker.

Now, here I *do* see a potential problem.  If service B is managed by
Pacemaker, is configured with Requires=A and After=A, but service A is
*not* managed by Pacemaker, we would need to ensure that on system
shutdown, systemd would shutdown Pacemaker (and hence B) *before* it
(systemd) shuts down A, otherwise A could be stopped before B,
effectively pulling the rug from underneath B's feet.

But isn't that an issue even if Pacemaker only uses systemd resources?
I don't see how the currently used override files protect against this
issue.  Have I just "discovered" a bug, or more likely, is there again
a gap in my understanding?

More information about the Developers mailing list