Thank you both for your feedback - that's really helpful.
Let me say a few more words about what we're trying to accomplish here
overall so that maybe we can figure out what the right way forward is.
(it may be keeping the glance api servers setting, but let me at least
make the case real quick)
From a 10,000 foot view, the thing we're trying to do is to get nova's
consumption of all of the OpenStack services it uses to be less special.
The clouds have catalogs which list information about the services -
public, admin and internal endpoints and whatnot - and then we're asking
admins to not only register that information with the catalog, but to
also put it into the nova.conf. That means that any updating of that
info needs to be an API call to keystone and also a change to nova.conf.
If we, on the other hand, use the catalog, then nova can pick up changes
in real time as they're rolled out to the cloud - and there is hopefully
a sane set of defaults we could choose (based on operator feedback like
what you've given) so that in most cases you don't have to tell nova
where to find glance _at_all_ becuase the cloud already knows where it
is. (nova would know to look in the catalog for the interal interface of
the image service - for instance - there's no need to ask an operator to
add to the config "what is the service_type of the image service we
should talk to" :) )
Now - glance, and the thing you like that we don't - is especially hairy
because of the api_servers list. The list, as you know, is just a list
of servers, not even of URLs. This means it's not possible to configure
nova to talk to glance over SSL (which I know you said works for you,
but we'd like for people to be able to choose to SSL all their things)
We could add that, but it would be an additional pile of special config.
Because of all of that, we also have to attempt to make working URLs
from what is usually a list of IP addresses. This is also clunky and
prone to failure.
The implementation on the underside of the api_servers code is the
world's dumbest load balancer. It picks a server from the list at
random and uses it. There is no facility for dealing with a server in
the list that stops working or for allowing rolling upgrades like there
would with a real load-balancer across the set. If one of the API
servers goes away, we have no context to know that, so just some of your
internal calls to glance fail.
Those are the issues - basically:
- current config is special and fragile
- impossible to SSL
- unflexible/unpowerful de-facto software loadbalancer
Now - as is often the case - it turns out the combo of those things is
working very well for you -so we need to adjust our thinking on the
topic a bit. Let me toss out some alternatives and see what you think:
Alternative One - Do Both things
We add the new "consume from catalog" and make it default. (and make it
default to consuming the internal interface by default) We have to do
that in parallel with the current glance api_servers setting anyway,
because of deprecation periods, so the code to support both approaches
will exist. Instead of then deprecating the api_servers list, we keep
it- but add a big doc warning listing the gotchas and limitations - but
for those folks for whom they are not an issue, you've got an out.
Alternative Two - Hybrid Approach - optional list of URLs
We go ahead and move to service config being the standard way one lists
how to consume a service from the catalog. One of the standard options
for consuming services is "endpoint_override" - which is a way an API
user can say "hi, please to ignore the catalog and use this endpoint
I've given you instead". The endpoint in question is a full URL, so
https/http and ports and whatnot are all handled properly.
We add, in addition, an additional option "endpoint_override_list" which
allows you to provide a list of URLs (not API servers) and if you
provide that option, we'll keep the logic of choosing one at random at
API call time. It's still a poor load balancer, and we'll still put
warnings in the docs about it not being a featureful load balancing
solution, but again would be available if needed.
Alternative Three - We ignore you and give you docs
I'm only including this because in the name of completeness. But we
could write a bunch of docs about a recommended way of putting your
internal endpoints in a load balancer and registering that with the
internal endpoint in keystone. (I would prefer to make the operators
happy, so let's say whatever vote I have is not for this option)
Alternative Four - We update client libs to understand multiple values
from keystone for endpoints
I _really_ don't like this one - as I think us doing dumb software
loadbalancing client side is prone to a ton of failures. BUT - right now
the assumption when consuming endpoints from the catalog is that one and
only one endpoint will be returned for a given
service_type/service_name/interface. Rather than special-casing the
url roundrobin in nova, we could move that round-robin to be in the base
client library, update api consumption docs with round-robin
recommendations and then have you register the list of endpoints with
keystone.
I know the keystone team has long been _very_ against using keystone as
a list of all the endpoints, and I agree with them. Putting it here for
sake of argument.
Alternative Five - We update keystone to round-robin lists of endpoints
Potentially even worse than four and even more unlikely given the
keystone team's feelings, but we could have keystone continue to only
return one endpoint, but have it do the round-robin selection at catalog
generation time.
Sorry - you caught me in early morning brainstorm mode.
I am neither nova core nor keystone core. BUT:
I think honestly if adding a load balancer in front of your internal
endpoints is an undue burden and/or the usefulness of the lists
outweighs the limitations they have, we should go with One or Two. (I
think three through five are all terrible)
My personal preference would be for Two - the round-robin code winds up
being the same logic in both cases, but at least in Two folks who want
to SSL all the way _can_, and it shouldn't be an undue extra burden on
those of you using the api_servers now. We also don't have to do the
funky things we currently have to do to turn the api_severs list into
workable URLs.
On 04/27/2017 11:50 PM, Blair Bethwaite wrote:
We at Nectar are in the same boat as Mike. Our use-case is a little
bit more about geo-distributed operations though - our Cells are in
different States around the country, so the local glance-apis are
particularly important for caching popular images close to the
nova-computes. We consider these glance-apis as part of the underlying
cloud infra rather than user-facing, so I think we'd prefer not to see
them in the service-catalog returned to users either... is there going
to be a (standard) way to hide them?
On 28 April 2017 at 09:15, Mike Dorman <mdor...@godaddy.com> wrote:
We make extensive use of the [glance]/api_servers list. We configure that on
hypervisors to direct them to Glance servers which are more “local”
network-wise (in order to reduce network traffic across security
zones/firewalls/etc.) This way nova-compute can fail over in case one of the
Glance servers in the list is down, without putting them behind a load
balancer. We also don’t run https for these “internal” Glance calls, to save
the overhead when transferring images.
End-user calls to Glance DO go through a real load balancer and then are
distributed out to the Glance servers on the backend. From the end-user’s
perspective, I totally agree there should be one, and only one URL.
However, we would be disappointed to see the change you’re suggesting
implemented. We would lose the redundancy we get now by providing a list. Or
we would have to shunt all the calls through the user-facing endpoint, which
would generate a lot of extra traffic (in places where we don’t want it) for
image transfers.
Thanks,
Mike
On 4/27/17, 4:02 PM, "Matt Riedemann" <mriede...@gmail.com> wrote:
On 4/27/2017 4:52 PM, Eric Fried wrote:
> Y'all-
>
> TL;DR: Does glance ever really need/use multiple endpoint URLs?
>
> I'm working on bp use-service-catalog-for-endpoints[1], which intends
> to deprecate disparate conf options in various groups, and centralize
> acquisition of service endpoint URLs. The idea is to introduce
> nova.utils.get_service_url(group) -- note singular 'url'.
>
> One affected conf option is [glance]api_servers[2], which currently
> accepts a *list* of endpoint URLs. The new API will only ever return
*one*.
>
> Thus, as planned, this blueprint will have the side effect of
> deprecating support for multiple glance endpoint URLs in Pike, and
> removing said support in Queens.
>
> Some have asserted that there should only ever be one endpoint URL for
> a given service_type/interface combo[3]. I'm fine with that - it
> simplifies things quite a bit for the bp impl - but wanted to make sure
> there were no loudly-dissenting opinions before we get too far down this
> path.
>
> [1]
>
https://blueprints.launchpad.net/nova/+spec/use-service-catalog-for-endpoints
> [2]
>
https://github.com/openstack/nova/blob/7e7bdb198ed6412273e22dea72e37a6371fce8bd/nova/conf/glance.py#L27-L37
> [3]
>
http://eavesdrop.openstack.org/irclogs/%23openstack-keystone/%23openstack-keystone.2017-04-27.log.html#t2017-04-27T20:38:29
>
> Thanks,
> Eric Fried (efried)
> .
>
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Thanks,
Matt
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