Open Stack

Stackers,

Today, a few of us had a chat to discuss changes to the Placement REST 
API [1] that will allow multiple clients to safely update a single 
consumer's set of resource allocations. This email is to summarize the 
decisions coming out of that chat.

Note that Ed is currently updating the following nova-spec:

https://review.openstack.org/#/c/556971/

The decisions made were as follows:

1) The GET /allocations/{consumer_uuid} REST API endpoint will now have 
a required consumer_generation field in the response. This will be an 
integer value.

2) The PUT /allocations/{consumer_uuid} REST API endpoint will have a 
new consumer_generation required field in the request payload.

3) Callers to PUT /allocations/{consumer_uuid} that believe they are the 
first caller to set allocations for the consumer will set 
consumer_generation to None.

4) If consumer_generation is None in the request to PUT 
/allocations/{consumer_uuid} and the placement service notes that 
allocations already exist for that consumer, a 409 conflict will be 
returned. The caller will need to then GET /allocations/{consumer_uuid} 
to retrieve the consumer's current generation and allocations, merge its 
new resources into those allocations and retry PUT 
/allocations/{consumer_uuid}, passing the merged allocation set and 
consumer generation.

5) The POST /allocations REST API endpoint is currently only used by 
nova when performing migrate or resize operations for a virtual machine. 
The POST /allocations REST API request payload will contain a new 
required consumer_generation field in each top-level dict element 
corresponding to the allocations to overwrite for one or more consumers. 
(the migrate/resize code paths use multiple consumer UUIDs to identify 
the resources that are allocated to the source and destination hosts)

6) The HTTP response codes for both PUT /allocations/{consumer_uuid} and 
POST /allocations will continue to be 204 No Content.

Thanks,
-jay

[1] https://docs.openstack.org/nova/latest/user/placement.html