Add a CellZero

In order to maintain the API contract when using cells we need to store enough information to fulfill an instance show request even when the instance could not be scheduled to a cell.

Problem description

When an API request is made to build an instance there is a certain response contract that we need to honor. This means that we need to have stored certain information from the request such as image, flavor, name, uuid, etc… Under ideal circumstances the instance will have been scheduled to a host in a cell and the data will be in that cells database. In the event that no cell/host can hold the instance that data needs to be stored outside of any live cells.

Use Cases

  • Operators want to partition their deployments into cells for scaling, failure domain, and buildout reasons. When partitioned, we need to maintain the current API contract.

Proposed change

A cell0 will be added which consists of the database tables needed by an instance within a live cell. Instances, and necessary relations, can be stored here and included in API responses. The only available action for instances within this cell will be to delete them as they are in an error state. Instances that are stored here will have a regular instance_mapping set so that requests for those instances can follow the normal code path.

The boot process for instances will be updated to create instances in cell0 when the scheduler fails to pick a location for the instance.


An alternative would be to store the instance records, and relations, within the nova api database in a serialized format for easy retrieval for API responses. This was dismissed because it would require multiple upgrade paths when making changes to the instance table schema or related schemas.

Another alternative would be to maintain an instance table and related tables in the nova api database. This is very similar to what’s being proposed however having a cell0 construct is beneficial to avoid special casing lookups/deletes for instances here. They will have a normal instance_mapping set so operations will find them normally.

Data model impact

All database tables necessary for storing an instance and related objects, like security_groups or instance_extra, will be created and managed for this new cell. For now it would be best to just deploy a normal cell database for cell0 though it will contain unnecessary tables such as compute_nodes. Later work can trim this down if it’s deemed worthwhile.

REST API impact


Security impact


Notifications impact


Other end user impact


Performance Impact


Other deployer impact

Instances that were not schedulable will exist in this special cell. Deployers will need to be aware of this to aid proper debugging.

Developer impact




Primary assignee:


Other contributors:


Work Items

  • Devstack changes to setup a cell0.

  • Add logic to create instances in cell0 when they can not be scheduled.

  • Document deployment instructions for cell0.



Since this is designed to be an internal re-architecting of Nova with no user visible changes the current suite of Tempest or functional tests should suffice. At some point we will want to look at how to test multiple cells or potentially exposing the concept of a cell in the API and we will tackle testing requirements then.

Documentation Impact

Documentation will be written describing the flow of an instance build and how this affects it.