System Host Management

Currently, neutron does not provide an independent system host management
ability. Instead, the host segments functionality is based on the state
reported by the agent according to the deployment level configurations, a table
is updated to track which hosts have access to which provider networks. This is
being used to make DHCP scheduling decisions but bugs have been identified in
how they handle multiple mechanism drivers (see [1] and [2]). In addition,
there are also performance costs with accessing the information for complex
queries which may impact larger deployments during scheduling operations, and
therefore require further improvements.

This spec proposes an host extension which adds host management to the neutron
API. It is expected that another entity (e.g., NFV-VIM or Bare Metal and Node
Management) is sending host update notifications to let neutron know when hosts
are added, deleted or when they change state. It also allows for that external
entity to tell neutron which provider networks are associated to which host.
This enables neutron to make scheduling decisions about where DHCP and router
services can be scheduled. This also enables neutron to generate, report
host-level issues when something goes wrong. Otherwise, the system will operate
based on agent availability only, if the extension is not loaded and host state
is not being managed.

[1] https://bugs.launchpad.net/neutron/+bug/1732448
[2] https://bugs.launchpad.net/neutron/+bug/1732445