Cells v2 mapping


In order for compute api nodes to communicate with cells they will need to have knowledge on how to connect to each cell. A new database and table will be created which can store this information for use by the database and RPC layers of Nova.

Problem description

When Nova is partitioned into cells, the compute api needs to be able to communicate with each one via a message bus and a database connection. There is currently no mapping of a cell identifier to a message queue and database connection. And there is no mechanism to dispatch RPC message or database queries to different endpoints on a per call basis.

Use Cases

  • Developers who want to make database queries or send RPC messages to a specific cell.

Project Priority

Cells v2 has been made a project priority for Kilo.

Proposed change

The change being proposed is a new database and table for storing a mapping of cell to database and message queue. A new database is being proposed because the new tables that will be added to it belong with the compute api layer of Nova, as compared to some of the current tables which belong in a cells database. The exact split of which information belongs where is an in-progress effort.

The new database will require a separate line of migrations to be applied to it. Since there have been discussions for a while now around potential benefits of using alembic to handle db migrations this might be a good opportunity to do so. I think it should be researched and used if there’s consensus that it would be beneficial and continue to use sqlaclhemy-migrate if not.

Nova will need a connection string to connect to this new database and at the same time continue to connect to the current ‘nova’ database until we can fully migrate away from that. A new config option will be introduced to store the connection info for the ‘nova_api’ database.

Additionally the database and rpc abstractions in Nova need to be capable of communicating with an arbitrary endpoint for every call/query.

There is nothing in place to use this yet so the scope of work ends at just having the capability to do it.

The following diagram may help visualize it. When a request comes in for an instance in a cell nova-api will query the cell_mapping table to get the necessary information for interacting with the cell database or message queue. The instance to cell mapping is described in another spec.:

        |       |
        +       |
        |       |
        |       |
        v       |
cell_mapping    |


We could continue to use the nova-cells model in place today.

Data model impact

A new ‘cell_mapping’ table will be added. And it will be added outside of the current ‘nova’ database in a new ‘nova_api’ database. This new database will have deployment ramifications as described below

The table will look like::

CREATE TABLE `cell_mapping` (
  `created_at` datetime DEFAULT NULL,
  `updated_at` datetime DEFAULT NULL,
  `deleted_at` datetime DEFAULT NULL,
  `uuid` varchar(36) NOT NULL,
  `name` varchar(255) DEFAULT NULL,
  `deleted` int(11) DEFAULT NULL,
  `transport_url` mediumtext NOT NULL,
  `database_connection` mediumtext NOT NULL)

REST API impact


Security impact

The transport_url and database_connection fields in the database could contain sensitive data.

Notifications impact


Other end user impact


Performance Impact

On its own this change does not introduce a performance impact. When it is used by later specs it does introduce another database lookup for many actions within Nova. For example a ‘nova show <uuid>’ will require Nova to look up the database that an instance is in before it can query it for instance data. This data will remain relatively stable and could be cached quite easily to help offset any performance penalty.

Other deployer impact

This blueprint introduces the concept of a database that is conceptually distinct from the current nova database. Deployers will need to consider how they want to manage a second database, whether it resides on the same host as their current nova database or not. It will be used primarily by the nova-api service so that should be considered when considering how to deploy it.

Developer impact

This change means that developers should understand that RPC messages or database queries may hit one of many endpoints. At this point it should not affect developers work within Nova. Developers adding future database migrations will need to consider whether it goes at the API or cell level and add it to the appropriate set of migrations.



Primary assignee:


Other contributors:


Work Items

  • Ensure Nova database API can communicate with an arbitrary database on each call.

  • Add config option for connecting to the new database.

  • Research how to have a separate migration path within Nova for a new database.

  • Setup separate database migration path for migrations on a new database.

  • Add database migration for ‘cell_mapping’ table.




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

The existence and management of a new database will need to be documented. It is not required that the database be deployed at this time but deployers should be prepped on how to start managing it.