[openstack-dev] [placement] The "intended purpose" of traits
soulxu at gmail.com
Sat Sep 29 01:56:43 UTC 2018
Chris Dent <cdent+os at anticdent.org> 于2018年9月29日周六 上午1:19写道：
> On Fri, 28 Sep 2018, Jay Pipes wrote:
> > On 09/28/2018 09:25 AM, Eric Fried wrote:
> >> It's time somebody said this.
> Yes, a useful topic, I think.
++, I'm interesting this topic also, since it confuses me for a long time...
> >> Every time we turn a corner or look under a rug, we find another use
> >> case for provider traits in placement. But every time we have to have
> >> the argument about whether that use case satisfies the original
> >> "intended purpose" of traits.
> >> That's only reason I've ever been able to glean: that it (whatever "it"
> >> is) wasn't what the architects had in mind when they came up with the
> >> idea of traits.
> > Don't pussyfoot around things. It's me you're talking about, Eric. You
> > just ask me instead of passive-aggressively posting to the list like
> It's not just you. Ed and I have also expressed some fairly strong
> statement about how traits are "supposed" to be used and I would
> guess that from Eric's perspective all three of us (amongst others)
> have some form of architectural influence. Since it takes a village
> and all that.
> > They aren't arbitrary. They are there for a reason: a trait is a boolean
> > capability. It describes something that either a provider is capable of
> > supporting or it isn't.
> This is somewhat (maybe even only slightly) different from what I
> think the definition of a trait is, and that nuance may be relevant.
> I describe a trait as a "quality that a resource provider has" (the
> car is blue). This contrasts with a resource class which is a
> "quantity that a resource provider has" (the car has 4 doors).
Yes, this is what I'm thinking when I propose the Trait. Basically, I'm
trying to match two points in the proposal: #1 we need qualitative of
resource, #2 we don't want another metadata API, since metadata API isn't
discoverable and wild place, people put anything to it. Nobody knows what
metadata available in the code except deep into the code.
For #1, just as Chris said.
For #2, You have to create Trait before using it, and we have API to query
traits, make it discoverable in the API. And standard trait make its naming
has rule, then as Jay suggested, we have os-traits library to store all the
standard traits. But we have to have custom trait, since there have
use-case for managing resource out of OpenStack.
> Our implementation is pretty much exactly that ^. We allow
> clients to ask "give me things that have qualities x, y, z, not
> qualities a, b, c, and quanities of G of 5 and H of 7".
> Add in aggregates and we have exactly what you say:
> > * Does the provider have *capacity* for the requested resources?
> > * Does the provider have the required (or forbidden) *capabilities*?
> > * Does the provider belong to some group?
> The nuance of difference is that your description of *capabilities*
> seems more narrow than my description of *qualities* (aka
> characteristics). You've got something fairly specific in mind, as a
> way of constraining the profusion of noise that has happened with
> how various kinds of information about resources of all sorts is
> managed in OpenStack, as you describe in your message.
> I do not think it should be placement's job to control that noise.
> It should be placement's job to provide a very strict contract about
> what you can do with a trait:
> * create it, if necessary
> * assign it to one or more resource providers
> * ask for providers that either have it
> * ... or do not have it
> That's all. Placement _code_ should _never_ be aware of the value of
> a trait (except for the magical MISC_SHARES...). It should never
> become possible to regex on traits or do comparisons
> (required=<CUSTOM_TEMP_85). Just "yes" or "no" to presence of quality.
> > If we want to add further constraints to the placement allocation
> > request that ask things like:
> > * Does the provider have version 1.22.61821 of BIOS firmware from
> > installed on it?
> That's a quality of the provider in a moment.
> > * Does the provider support an FPGA that has had an OVS program flashed
> to it
> > in the last 20 days?
> If you squint, so is this.
> > * Does the provider belong to physical network "corpnet" and also
> > creation of virtual NICs of type either "DIRECT" or "NORMAL"?
> And these.
> But at least some of them are dynamic rather than some kind of
> platonic ideal associated with the resource provider.
> I don't think placement should be concerned about temporal aspects
> of traits. If we can't write a web service that can handle setting
> lots of traits every second of every day, we should go home. If
> clients of placement want to set weird traits, more power to them.
> However, if clients of placement (such as nova) which are being the
> orchestrator of resource providers manipulated by multiple systems
> (neutron, cinder, ironic, cyborg, etc) wish to set some constraints
> on how and what traits can do and mean, then that is up to them.
> nova-scheduler is the thing that is doing `GET
> /allocation_candidates` for those multiple system. It presumably
> should have some say in what traits it is willing to express and
> But the placement service doesn't and shouldn't care.
> > Then we should add a data model that allow providers to be decorated
> > key/value (or more complex than key/value) information where we can
> query for
> > those kinds of constraints without needing to encode all sorts of
> > bits of information into a capability string.
> Let's never do this, please. The three capabilities (ha!) of
> placement that you listed above ("Does the...") are very powerful as
> is and have a conceptual integrity that's really quite awesome. I
> think keeping it contained and constrained in very "simple" concepts
> like that was stroke of genius you (Jay) made and I'd hope we can
> keep it clean like that.
> If we weren't a multiple-service oriented system, and instead had
> some kind of k8s-like etcd-like
> keeper-of-all-the-info-about-everything, then sure, having what we
> currently model as resource providers be a giant blob of metadata
> (with quantities, qualitiies, and key-values) that is an authority
> for the entire system might make some kind of sense.
> But we don't. If we wanted to migrate to having something like that,
> using placement as the trojan horse for such a change, either with
> intent or by accident, would be unfortunate.
> > Propose such a thing and I'll gladly support it. But I won't support
> > bastardizing the simple concept of a boolean capability just because we
> > want to change the API or database schema.
> For me, it is not a matter of not wanting to change the API or the
> database schema. It's about not wanting to expand the concepts, and
> thus the purpose, of the system. It's about wanting to keep focus
> and functionality narrow so we can have a target which is "maturity"
> and know when we're there.
> My summary: Traits are symbols that are 255 characters long that are
> associated with a resource provider. It's possible to query for
> resource providers that have or do not have a specific trait. This
> has the effect of making the meaning of a trait a descriptor of the
> resource provider. What the descriptor signifies is up to the thing
> creating and using the resource provider, not placement. We need to
> harden that contract and stick to it. Placement is like a common
> carrier, it doesn't care what's in the box.
> /me cues brad pitt
> Chris Dent ٩◔̯◔۶ https://anticdent.org/
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