Make checks before live-migration async

APIImpact

bp: async-live-migration-rest-check

Change-Id: I07d0581cc7db9b5fdc34073625a548d1f42f8633

specs/newton/approved/async-live-migration-rest-check.rst

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+..
+ This work is licensed under a Creative Commons Attribution 3.0 Unported
+ License.
+
+ http://creativecommons.org/licenses/by/3.0/legalcode
+
+=======================================
+Make checks before live-migration async
+=======================================
+
+https://blueprints.launchpad.net/nova/+spec/async-live-migration-rest-check
+
+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.
+
+
+Alternatives
+------------
+
+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
+-----------------
+
+None
+
+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
+---------------
+
+None
+
+Notifications impact
+--------------------
+
+None
+
+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
+---------------------
+
+None
+
+Developer impact
+----------------
+
+None
+
+
+Implementation
+==============
+
+Assignee(s)
+-----------
+
+Primary assignee:
+  tdurakov
+
+Other contributors:
+  rpodolyaka
+
+Work Items
+----------
+
+* Add `wrap_instance_event` decorator to conductor's methods
+* Make live-migration checks nonblocking
+* Bump Nova REST API version.
+
+
+Dependencies
+============
+
+None
+
+
+Testing
+=======
+
+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`.
+
+References
+==========
+
+* https://review.openstack.org/#/c/314932/ - PoC
+
+
+History
+=======
+
+.. list-table:: Revisions
+   :header-rows: 1
+
+   * - Release Name
+     - Description
+   * - Newton
+     - Introduced