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


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.

Best,
-jay

[1] this was a mistake for which I take full responsibility.