Cells host mapping


Since the scheduler will return a host rather than a cell we need to know which cell that host is in. A new table will be created which can store this mapping.

Problem description

When Nova is partitioned into cells, the compute api needs to know which cell to communicate with in order to build a scheduled instance, or for host API requests. There is currently no mapping of host to cell so given just a host there is no way to know how to pass information to that host.

Use Cases

  • Operators want to partition their deployments into cells for scaling, failure domain, and buildout reasons. When partitioned, we need a lookup table to know which partition a host is in.

Project Priority

Priorities have not been decided for Liberty.

Proposed change

The change being proposed is a new table in the ‘nova_api’ database for storing a mapping of host to cell and an object to interact with it. Migration of data into this table will be tackled in a separate spec.

The following diagram may help visualize it.:

                                 api/cell boundary
    scheduler returns a host             |
                |                        |
                v                        |
                +                        |
                +----+                   |
                     |                   |
                     v                   |
host_mapping (joined with) cell_mapping  |


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

Data model impact

A new ‘host_mapping’ table will be added to the ‘nova_api’ database.

The table will look like::

CREATE TABLE `host_mapping` (
  `created_at` datetime DEFAULT NULL,
  `updated_at` datetime DEFAULT NULL,
  `host` varchar(255) NOT NULL,
  `cell_uuid` varchar(36) NOT NULL)

And host will be an indexed column. Other indexes are possible as well and can be discussed in the code review.

It should be noted that there is no ‘deleted’ or ‘deleted_at’ column here. If a host exists it should be mapped, it should only be deleted if the host is being permanently removed in which case there is no reason to keep it here.

REST API impact


Security impact


Notifications impact


Other end user impact


Performance Impact

On its own this change does not introduce a performance impact. When it’s used by later specs it does introduce another database lookup for some actions within Nova.

Other deployer impact

This introduces a new table into the ‘nova_api’ database. And as described in the “Data model impact” section above it should be considered when running any cleanup of hosts. If hosts are removed from a deployment they can be removed from the host_mapping table as well.

Developer impact




Primary assignee:


Other contributors:


Work Items

  • Add database migration for ‘host_mapping’ table.

  • Add HostMapping object.




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 should be added about the new table and what its usage will be.