Rolling-upgrade: lack of indirection API

[Grzegorz Grasza]

Because the indirection_api of IronicObject is not implemented, when trying to perform a rolling upgrade, IncompatibleObjectVersion is raised when an object is sent to a service with an older version.

https://github.com/openstack/oslo.versionedobjects/blob/master/oslo_versionedobjects/base.py#L823

As a result, a rolling-upgrade without downtime (Ironic services running at different versions) is not possible. Implemeting the
indirection_api gets Ironic one step closer to supporting rolling-upgrades.

[Grzegorz Grasza (xek)]

Example implementation: https://review.openstack.org/184791

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/224079

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/224079
Committed: https://git.openstack.org/cgit/openstack/ironic/commit/?id=db9ddd39d3f3d854d38e4b05655ebddd98f1a727
Submitter: Jenkins
Branch: master

commit db9ddd39d3f3d854d38e4b05655ebddd98f1a727
Author: Grzegorz Grasza <email address hidden>
Date: Wed Sep 16 15:12:42 2015 +0200

    Implement indirection_api

    During a rolling upgrade, ironic conductor and api services are running
    with different versions. When an object is received in an incompatible
    version, IncompatibleObjectVersion is raised. Implementation of the
    indirection API allows the object to be backported to a supported version
    by the conductor (conductor has to be the first service to be upgraded).

    This change enables backporting of objects from Mitaka. This lays the
    foundation to be able to do rolling upgrades from Liberty (or from this
    patch onwards) to Mitaka. There may still be other issues that will need
    fixing in Mitaka before we will be able to do a full rolling upgrade.

    Enabling the indirection_api causes all object methods decorated with
    the remotable or remotable_classmethod to do RPC from ironic-api to
    ironic-conductor service. To keep the current behavior, I'm removing all
    remotable decorators on object methods and thus not enabling object RPC
    calls in this patch. These calls caused random timeouts on the CI gates
    (probably due to a race in which Nova calls the ironic-api service before
    ironic-conductor is up). RPC calls made via the indirection_api should
    be enabled one-by-one, when the implications are fully understood.

    Change-Id: Ia381348da93f95d764c83f3ec2a2ed5a1db5ad6d
    Closes-Bug: 1493816
    Depends-On: I81400305f166d62aa4612aab54602abb8178b64c