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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.

> 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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