.. _concepts.scheduling:

Scheduling
##########

The scheduling of services spawned by Yaook is handled by the Kubernetes
Scheduler using Affinities and Tolerations. All services come with a set of
Affinities and Tolerations which allows fine-grained control over the placement
of the components.

To each scheduleable component a `Scheduling Key` is assigned. The Key is used as
key for both tolerations and labels. If a component has the key ``foo/bar``, it
will tolerate both ``NoExecute`` and ``NoSchedule`` taints *with that key* and
it will only run on nodes with that as label.

A notable exception is the Neutron Layer 2 Agent, which will schedule itself
based on other criteria (see :ref:`concepts.scheduling.neutron-l2-agent`
below).

In addition, a component may also listen to a `Scheduling Group Key`. In that
case, it will tolerate that key and it will also run on nodes which just have
a label matching that key.

.. note::

    This means that, on a fresh Kubernetes cluster, no services will be
    scheduled at all.

For details on the specific scheduling keys, please see
:class:`yaook.op.scheduling_keys.SchedulingKey`.

There is also a special downscale label used by YAOOK interally that should
never be directly assigned to any node (see
:ref:`concepts.scheduling.forced-downscale-label` below).

.. _concepts.scheduling.neutron-l2-agent:

Neutron OVN Agent
=================

Neutron's OVN Agent is a strict dependency of NovaComputeNode and OVNBGPAgent.

It will tolerate *all* the keys of the following services (in addition to
the :attr:`.SchedulingKey.NETWORK_NEUTRON_OVN_AGENT`) and
it will be spawned on all nodes matching any of their keys:

- Nova Compute: :attr:`.SchedulingKey.COMPUTE_HYPERVISOR`
- Neutron OVN BGP Agent: :attr:`.SchedulingKey.NETWORK_NEUTRON_BGP_AGENT`

Agents needing Neutron OVN (bgp, compute)
==================================================

Besides the `Scheduling Key` each component has (see
:class:`yaook.op.scheduling_keys.SchedulingKey`)
components that depend on OVN agent have another scheduling dependency.

The neutron_ovn operator sets an label to the
node, that no OVN maintenance is required. If this label is missing or set to
maintenance required, the agents/services won't be scheduled on or will be
deleted from the node.

More details about this:
.. _concepts.safe-eviction.implementation.neutron-ovn-agent:

Neutron OVN BGP Agent
=====================

This agent is optional, just like Neutron BGP DR Agent. To schedule it,
the neutron deployment needs to have the `spec.ovn.bgp` section defined with a
configname (will be used for naming the agent) and configTemplate (can be empty
nodeSelector).

.. _concepts.scheduling.forced-downscale-label:

Downscale label
===============

YAOOK uses a specially reserved label called
``state.yaook.cloud/forced-downscale`` to downscale DaemonSets and
ConfiguredDaemonSets. It is used as a ``nodeSelector`` which matches no nodes
and will lead to the removal of all Pods until removed.
This is used by YAOOK internally for cold upgrade procedures where entire
services need to be stopped temporarily during OpenStack release upgrades.

.. note::

    The ``state.yaook.cloud/forced-downscale`` label must never be attached
    as a node label directly to nodes! It should never match any node.