Open Stack

One of the big goals for the Kilo cycle by users and developers of the 
cells functionality within Nova is to get it to a point where it can be 
considered a first class citizen of Nova.  Ultimately I think this comes 
down to getting it tested by default in Nova jobs, and making it easy 
for developers to work with.  But there's a lot of work to get there.  
In order to raise awareness of this effort, and get the conversation 
started on a few things, I've summarized a little bit about cells and 
this effort below.


Goals:

Testing of a single cell setup in the gate.
Feature parity.
Make cells the default implementation.  Developers write code once and 
it works for  cells.

Ultimately the goal is to improve maintainability of a large feature 
within the Nova code base.


Feature gaps:

Host aggregates
Security groups
Server groups


Shortcomings:

Flavor syncing
     This needs to be addressed now.

Cells scheduling/rescheduling
Instances can not currently move between cells
     These two won't affect the default one cell setup so they will be 
addressed later.


What does cells do:

Schedule an instance to a cell based on flavor slots available.
Proxy API requests to the proper cell.
Keep a copy of instance data at the global level for quick retrieval.
Sync data up from a child cell to keep the global level up to date.


Simplifying assumptions:

Cells will be treated as a two level tree structure.


Plan:

Fix flavor breakage in child cell which causes boot tests to fail. 
Currently the libvirt driver needs flavor.extra_specs which is not 
synced to the child cell.  Some options are to sync flavor and extra 
specs to child cell db, or pass full data with the request. 
https://review.openstack.org/#/c/126620/1 offers a means of passing full 
data with the request.

Determine proper switches to turn off Tempest tests for features that 
don't work with the goal of getting a voting job.  Once this is in place 
we can move towards feature parity and work on internal refactorings.

Work towards adding parity for host aggregates, security groups, and 
server groups.  They should be made to work in a single cell setup, but 
the solution should not preclude them from being used in multiple 
cells.  There needs to be some discussion as to whether a host aggregate 
or server group is a global concept or per cell concept.

Work towards merging compute/api.py and compute/cells_api.py so that 
developers only need to make changes/additions in once place.  The goal 
is for as much as possible to be hidden by the RPC layer, which will 
determine whether a call goes to a compute/conductor/cell.

For syncing data between cells, look at using objects to handle the 
logic of writing data to the cell/parent and then syncing the data to 
the other.

A potential migration scenario is to consider a non cells setup to be a 
child cell and converting to cells will mean setting up a parent cell 
and linking them.  There are periodic tasks in place to sync data up 
from a child already, but a manual kick off mechanism will need to be added.


Future plans:

Something that has been considered, but is out of scope for now, is that 
the parent/api cell doesn't need the same data model as the child cell.  
Since the majority of what it does is act as a cache for API requests, 
it does not need all the data that a cell needs and what data it does 
need could be stored in a form that's optimized for reads.


Thoughts?