Comment 3 for bug 1807219

Reviewed: https://review.openstack.org/623246
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=66e44c64297e77395195d77104017fb6fcea58d2
Submitter: Zuul
Branch: master

commit 66e44c64297e77395195d77104017fb6fcea58d2
Author: Matt Riedemann <email address hidden>
Date: Thu Dec 6 11:17:15 2018 -0500

    Only construct SchedulerReportClient on first access from API

    With commit 5d1a50018510b2b281ad33895ae2d9555f5d5b05 each
    API/AggregateAPI class instance constructs a SchedulerReportClient
    which holds an in-memory lock during its initialization.

    With at least 34 API extensions constructing at least
    one of those two API classes, the accumulated affect of the
    SchedulerReportClient construction can slow down nova-api startup
    times, especially when running with multiple API workers, like
    in our tempest-full CI job (there are 2 workers, so 68 inits).

    This change simply defers constructing the SchedulerReportClient
    until it is used, which is only in a few spots in the API code,
    which should help with nova-api start times.

    The AggregateAPI also has to construct the SchedulerQueryClient
    separately because SchedulerClient creates both the query and
    report clients.

    Long-term we could consider making it a singleton in nova.compute.api
    if that is safe (the aggregate code might be relying on some caching
    aspects in the SchedulerReportClient).

    Change-Id: Idf6e548d725db0181629a451f46b6a3a5850d186
    Closes-Bug: #1807219