Compute Manager Objects Support (Juno Work)

Compute Manager Objects Support (Juno Work)

https://blueprints.launchpad.net/nova/+spec/compute-manager-objects-juno

This blueprint represents the remaining work to be done in Juno around moving the compute manager (and associated modules, like nova.compute.utils) to using objects instead of raw conductor methods. This is important because objects provide versioning of the actual data, which supports our upgrade goals.

Problem description

The nova compute manager still sends unversioned bundles of data using conductor and compute RPC methods, which is problematic during an upgrade where the format of the data has changed across releases. This is especially important for compute manager, because it is likely that it will be speaking to a newer conductor and compute node at times. During an upgrade, migrate and live-migrate operations are expected, and by nature will involve compute nodes running different versions of the code to communicate.

Proposed change

Migrate uses of raw condutor methods in the compute manager to objects. For example consider this:

service_ref = self.conductor_api.service_get_by_compute_host(
        context, self.host)
self.conductor_api.compute_node_delete(context, service_ref['compute_node'])

would become:

service = service_obj.Service.get_by_compute_host(context,
                                                  self.host)
service.compute_node.destroy()

Alternatives

This is the accepted direction of the project to solve this problem. However, alternatives would be:

  1. Don’t solve the problem and continue using unversioned data

  2. Attempt to enforce version bumps of individual methods when any data (including nested downstream data) has changed

Data model impact

The low-level data model (i.e. the SQLAlchemy models) will not need to change. However, additional high-level objects may be added where necessary to provide versioned wrappers around the low-level models.

REST API impact

None.

Security impact

None.

Notifications impact

In general, conversion of code to use objects does not affect notifications. However, at times, emission of notifications is embedded into an object method to achieve higher consistency about when and how the notifications are sent. No such changes are antitipated in this work, but it’s always a possibility.

Other end user impact

None.

Performance Impact

None.

Other deployer impact

Moving to objects enhances the ability for deployers to incrementally roll out new code. It is, however, largely transparent for them.

Developer impact

This is normal refactoring, so the impact is minimal. In general, objects-based code is easier to work with, so long-term it is a win for the developers.

Implementation

Assignee(s)

Primary assignee:

danms

Work Items

  • check_can_live_migrate_destination

  • check_can_live_migrate_source

  • live_migration

  • _post_live_migration

  • _rollback_live_migration

  • _rollback_live_migration_at_destination

  • refresh_instance_security_rules

  • run_instance

  • detach_volume

  • Remaining uses of instance[attr] in compute/manager.py

Dependencies

There is a cross-dependency between this blueprint and the following:

At times, a virt driver will need to be modified to accept an object from the compute manager before the manager method can be fully converted.

Testing

In general, unit tests require minimal change when this happens, depending on how the tests are structured. Ideally, they are already mocking out database calls, which means the change to objects is a transparent one. In reality, this usually means minor tweaking to the tests to return whole data models, etc.

Documentation Impact

None.

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