Make checks before live-migration async

The existing nova live-migration operation is complex: verification of the environment is required before accepting and triggering the live-migration. This verification is done in a synchronous manner with several RPC calls that must all be successful before the process can begin and the REST API layer can respond. It would be better to make the checks asynchronous and have the REST API respond with an immediate 202 - Accepted.

Problem description

The existing workflow for the environment verification required before a live-migration involves several steps: an RPC call is made to the conductor which triggers two more RPC calls to the relevant compute nodes. Only once those are okay is there an RPC cast to trigger the migration and a response to the HTTP request.

Use Cases

A user doing a live migration would like to have a fast response to a live-migration request and use the instance-actions service to get valid status.

Proposed change

Instead of doing all the checks in a block way, this spec proposes to record the conductor’s method with wrap_instance_event so it will be possible to check live-migration status. Upon receiving a request for a live-migration the Nova API will send an RPC cast to the conductor and then respond immediately with the HTTP response. To separate blocking and non-blocking live-migration types new method will be introduced to conductor’s RPC API: * migrate_server - existing general method for cold migration, resize and live-migration. This will continue providing existing blocking behaviour, by sending RPC call from Nova API to conductor. * live_migrate_server - new method for live-migration only. This method will implement non-blocking checks by using rpc cast from the Nova API layer. Both methods should be decorated with wrap_instance_event, so events will be recorded to database for blocking and non-blocking paths too.


Leave things as they are. Another alternative would be to switch to non-blocking rpc cast without maintaining previous behavior and bumping Nova REST API microversion.

Data model impact


REST API impact

As this change will cause verification checks to be done in the background, the number of cases in which the live-migration API will respond with badRequest(400) will be limited to receipt of bad input. REST API microversion should be bumped, to retain possibility of switching to previous behaviour.

Security impact


Notifications impact


Other end user impact

Users should check live-migration status, using instance-actions operation.

Performance Impact

Nova REST API method becomes non-blocking, which should increase overall throughput of the API layer. At the same time more writes to database should be added(writing instance actions details on conductor’s side).

Other deployer impact


Developer impact




Primary assignee:


Other contributors:


Work Items

  • Add wrap_instance_event decorator to conductor’s methods

  • Make live-migration checks nonblocking

  • Bump Nova REST API version.




Test for checking that conductor’s part of live-migration is recorded by instance-actions should be added to tempest. Also code should be covered by unit and functional tests too.

Documentation Impact

As this change changes workflow, docs should be updated. It’s needed to highlight to the end user that it’s required to check live-migration process over instance-actions.




Release Name