[placement][ptg] Enabling other projects to continue with placement or get started

Dmitry Tantsur dtantsur at redhat.com
Wed Apr 10 11:11:04 UTC 2019

On 4/10/19 12:14 PM, Jay Pipes wrote:
> On 04/10/2019 05:47 AM, Dmitry Tantsur wrote:
>> On 4/9/19 7:20 PM, Jay Pipes wrote:
>>> On 04/09/2019 12:51 PM, Dmitry Tantsur wrote:
>>>>  From ironic perspective there is no issue, but there is a critical question 
>>>> to decide: when Ironic+Placement is used, which of them acts as the final 
>>>> authority? If Ironic, then we need to teach Placement to talk to its 
>>>> Allocation API when allocating a bare metal node. If Placement, then we need 
>>>> to support Allocation API talking to Placement. I suspect the latter is 
>>>> saner, but I'd like to hear more opinions.
>>> Ironic (scheduler?) would request candidates from the placement service using 
>>> the GET /allocation_candidates API. Ironic (scheduler?) would then claim the 
>>> resources on a provider (a baremetal node) by calling the POST /allocations API.
>> Okay, this matches my expectation.
>> My concern will be with Blazar and reservations. If reservations happen 
>> through Placement only, how will ironic know about them? I guess we need to 
>> teach Blazar to talk to Ironic, which in turn will talk to Placement.
> Hmm. So, here's the problem: placement has no concept of time. [1]
> Placement only knows about one period of time: now. Placement doesn't have any 
> concept of an allocation or an inventory existing at some point in the future or 
> in the past.
> Therefore, Blazar must unfortunately keep all of the temporal state about 
> reservations in its own data store. So, Ironic would actually have to talk to 
> Blazar to create a reservation of some amount of resources and Blazar will need 
> to call Placement to manage the state of resource inventory and allocations over 
> time; as reservations are activated, Blazar will either create or swap 
> allocation records in Placement to consume the Ironic resources for a tenant 
> that made the reservation.

I imagined it a bit differently. I assumed that
1. a user (operator?) talks to Blazar to create a bare metal node reservation.
1.1. some kind of quota is checked? does Blazar have any notion of quota?
2. Blazar talks to Ironic to create an allocation (yes, we also call it 
allocation, feel free to blame me)
3. Ironic gets a reservation from Placement and updates its internal records
4. Ironic returns an allocation to Blazar
5. Blazar returns a reservation to a user

And when a reservation in Blazar expires or is returned:
6. Blazar talks to Ironic to undeploy the node
6.1. Ironic deletes its allocation automatically
6.2. And thus notifies Placement to remove its allocation

Am I missing something?

> Best,
> -jay
> [1] this was a mistake for which I take full responsibility.

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