Open Stack

On 12/23/2015 08:35 PM, Morgan Fainberg wrote:
> On Wed, Dec 23, 2015 at 10:32 AM, Jay Pipes <jaypipes at gmail.com
> <mailto:jaypipes at gmail.com>> wrote:
>
>     On 12/23/2015 12:27 PM, Lars Kellogg-Stedman wrote:
>
>         I've been looking into the startup constraints involved when
>         launching
>         Nova services with systemd using Type=notify (which causes
>         systemd to
>         wait for an explicit notification from the service before
>         considering
>         it to be "started".  Some services (e.g., nova-conductor) will
>         happily
>         "start" even if the backing database is currently unavailable (and
>         will enter a retry loop waiting for the database).
>
>         Other services -- specifically, nova-scheduler -- will block waiting
>         for the database *before* providing systemd with the necessary
>         notification.
>
>         nova-scheduler blocks because it wants to initialize a list of
>         available aggregates (in
>         scheduler.host_manager.HostManager.__init__),
>         which it gets by calling objects.AggregateList.get_all.
>
>         Does it make sense to block service startup at this stage?  The
>         database disappearing during runtime isn't a hard error -- we will
>         retry and reconnect when it comes back -- so should the same
>         situation
>         at startup be a hard error?  As an operator, I am more interested in
>         "did my configuration files parse correctly?" at startup, and would
>         generally prefer the service to start (and permit any dependent
>         services to start) even when the database isn't up (because that's
>         probably a situation of which I am already aware).
>
>
>     If your configuration file parsed correctly but has the wrong
>     database connection URI, what good is the service in an active
>     state? It won't be able to do anything at all.
>
>     This is why I think it's better to have hard checks like for
>     connections on startup and not have services active if they won't be
>     able to do anything useful.
>
>
> Are you advocating that scheduler bails out and ceases to run or that it
> doesn't mark itself as active? I am in favour of the second scenario but
> not the first. There are cases where it would be nice to start the
> scheduler and have it at least report "hey I can't contact the DB" but
> not mark itself active, but continue to run and on <interval> report/try
> to reconnect.

I am in favor of the service not starting at all if the database cannot 
be connected to in a "test connection" scenario.

> It isn't clear which level of "hard check" you're advocating in your
> response and I want to clarify for the sake of conversation.

If the scheduler cannot contact the database, it cannot do anything 
useful at all. I don't see the point of having the service daemon "up" 
if it cannot do anything useful.

Most monitoring tooling (Nagios or nginx for simple load balancing) and 
distributed service management (Zookeeper) look at whether a service is 
responding on some port to determine if the service is up. If the 
service responds on said port, but cannot do anything useful, the 
information is less than useful...it's harmful, IMHO.

For errors that are recoverable, sure keep the service up and running 
and retry the condition that is recoverable. But in the case of bad 
configuration, it's not a recoverable error, and I don't think the 
service should be started at all.

Hope that clears things up.

Best,
-jay

>         It would be relatively easy to have the scheduler lazy-load the list
>         of aggregates on first references, rather than at __init__.
>
>
>     Sure, but if the root cause of the issue is a problem due to
>     misconfigured connection string, then that lazy-load will just bomb
>     out and the scheduler will be useless anyway. I'd rather have a
>     fail-early/fast occur here than a fail-late.
>
>     Best,
>     -jay
>
>      > I'm not
>
>         familiar enough with the nova code to know if there would be any
>         undesirable implications of this behavior.  We're already punting
>         initializing the list of instances to an asynchronous task in
>         order to
>         avoid blocking service startup.
>
>         Does it make sense to permit nova-scheduler to complete service
>         startup in the absence of the database (and then retry the
>         connection
>         in the background)?
>
>
>
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