Excerpts from Jay Pipes's message of 2016-10-14 16:55:39 -0400:
> Alex, so sorry for the long delayed response! :( This just crept to
> the back of my inbox unfortunately. Answer inline...
> On 09/14/2016 07:24 PM, Bashmakov, Alexander wrote:
> >> Glance and Keystone do not participate in a rolling upgrade,
> >> because Keystone and Glance do not have a distributed component
> >> architecture. Online data migrations will reduce total downtime
> >> experienced during an *overall upgrade procedure* for an OpenStack
> >> cloud, but Nova, Neutron and Cinder are the only parts of OpenStack
> >> that are going to participate in a rolling upgrade because they are
> >> the services that are distributed across all the many compute
> >> nodes.
> >
> > Hi Jay, I'd like to better understand why your definition of rolling
> > upgrades excludes Glance and Keystone? Granted they don't run
> > multiple disparate components over distributed systems, however, they
> > can still run the same service on multiple distributed nodes. So a
> > rolling upgrade can still be applied on a large cloud that has, for
> > instance 50 Glance nodes.
> If you've seen a cloud with 50 Glance nodes, I would be astonished :) 
> That said, the number 50 doesn't really have to do with my definition of 
> rolling... lemme explain.
> The primary thing that, to me at least, differentiates rolling upgrades 
> of distributed software is that different nodes can contain multiple 
> versions of the software and continue to communicate with other nodes in 
> the system without issue.

Database are often (mis)used to communicate.

> In the case of Glance, you cannot have different versions of the Glance 
> service running simultaneously within an environment, because those 
> Glance services each directly interface with the Glance database and 
> therefore expect the Glance DB schema to look a particular way for a 
> specific version of the Glance service software.

That's not a constraint of Glance, but a constraint of the way Glance
has been interfacing with the database. The argument of the thread was
that one can make schema changes in such a way that one can have
multiple versions of the same component running during an update.

> In contrast, Nova's distributed service nodes -- the nova-compute 
> services and (mostly) the nova-api services do *not* talk directly to 
> the Nova database. If those services need to get or set data in the 
> database, they communicate with the nova-conductor services which are 
> responsible for translating (called back-versioning) the most updated 
> object model schema that matches the Nova database to the schema that 
> the calling node understands. This means that Nova deployers can update 
> the Nova database schema and not have to at the same time update the 
> software on the distributed compute nodes. In this way deployers can 
> "roll out" an upgrade of the Nova software across many hundreds of 
> compute nodes over an extended period of time without needing to 
> restart/upgrade services all at once.
> Hope this clarifies things.

It clarifies your thinking, so thanks for that. However, I'm not so sure
there's any difference between components that are the same software,
and components that are different software, if they end up interacting
anyway because one version can write and read data that another version

What I think is important is understanding the interfaces, and how they
can be tested to ensure that rolling/partial/0-downtime updates can be
done safely.

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