Stop using the scheduler run_instance method

Currently the scheduler is used to both pick a host for an instance to be built on and to handle some setup and failure conditions for booting an instance. The scheduler should be responsible for placement logic and everything else should be moved elsewhere. This will make efforts to introduce new scheduler drivers or split the scheduler out of Nova easier to tackle by keeping a clean interface with a clear responsibility.

Problem description

The flow of execution for spawning an instance is complicated and highly distributed. Some amount of distribution is necessary but there is work happening and decisions being made in unexpected parts of the code. This makes it very difficult to look at separating the scheduler out, and means that it will need intimate integration with Nova that should be unnecessary. It is also unecessarily difficult to reason about what is happening at which point in the code which makes it challenging to improve those parts of the code.

Proposed change

In Havana it became possible to query the scheduler for a list of hosts to provision an instance to. The conductor service also emerged as a place to help orchestrate tasks that don’t logically belong in either the api or compute nodes. There has already been work to move some of the spawn instance workflow into the conductor and the final part of that effort is to have the conductor communicate with compute nodes rather than the scheduler.

There is a new, currently unused, build_and_run_instance method in the compute manager which mimics the currently used run_instance method, but handles a failed build by sending an RPC cast to the conductor service rather than the scheduler. The proposed change is to have the conductor query the scheduler and send a message to a compute which invokes the new build_and_run_instance method. Because the new method is unused and therefore untested by Tempest there will likely be some work required to achieve full compatibility with the current run_instance method.


An alternative would be to rework the run_instance method to cast back to conductor rather than use the new build_and_run_instance method. This was decided against because the amount of refactoring that would need to happen there meant it was easier to rebuild that method from scratch.

Data model impact


REST API impact


Security impact


Notifications impact

None. The notifications being sent by the scheduler will be ported over to conductor to maintain the same behaviour.

Other end user impact


Performance Impact

Some database updates that were occuring within the scheduler will be moved out to less performance critical sections of code. This should speed up the scheduler.

There may be a decrease in the amount of time to boot an instance if it needs to be rescheduled. The new build_and_run_instance performs some pre-build checks earlier and doesn’t generally deallocate and reallocate networks for a rescheduled instance. It will deallocate/reallocate if the baremetal driver is in use as the networking there is host specific.

Other deployer impact


Developer impact

Developers will need to be aware of the new code path being used.



Primary assignee:


Other contributors:


Work Items

  • Change the conductor to query the scheduler and cast to a compute.

    • Move notifications from the scheduler into the conductor.




This is essentially a refactoring of the current spawn process. So the current Tempest tests will act as good integration tests for this change since the new method will be used on every instance boot.

Documentation Impact