.. _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`.

.. _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).