Rolling Updates
===============

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The operator allows changing the PostgreSQL version used in a cluster
while applications are running against it.

..  Important::
   Only upgrades for PostgreSQL minor releases are supported.

Rolling upgrades are started when:

- the user changes the ``imageName`` attribute of the cluster
  specification;

- the :ref:`Image Catalog <Image Catalog>`  is updated with a new image for the major used by
  the cluster;

- a change in the PostgreSQL configuration requires a restart to be
  applied;

- a change on the ``Cluster`` ``.spec.resources`` values

- a change in size of the persistent volume claim on AKS

- after the operator is updated, to ensure the Pods run the latest
  instance manager (unless :ref:`in-place updates are enabled <Installation and upgrades>`  ).

The operator starts upgrading all the replicas, one Pod at a time, and
begins from the one with the highest serial.

The primary is the last node to be upgraded.

Rolling updates are configurable and can be either entirely automated
(``unsupervised`` ) or requiring human intervention (``supervised`` ).

The upgrade keeps the CloudNativePG identity, without re-cloning the
data. Pods will be deleted and created again with the same PVCs and a
new image, if required.

During the rolling update procedure, each service endpoints move to
reflect the cluster’s status, so that applications can ignore the node
that is being updated.

Automated updates (``unsupervised`` )
-------------------------------------

When ``primaryUpdateStrategy`` is set to ``unsupervised`` , the rolling
update process is managed by Kubernetes and is entirely automated. Once
the replicas have been upgraded, the selected ``primaryUpdateMethod``
operation will initiate on the primary. This is the default behavior.

The ``primaryUpdateMethod`` option accepts one of the following values:

- ``restart`` : if possible, perform an automated restart of the pod
  where the primary instance is running. Otherwise, the restart request
  is ignored and a switchover issued. This is the default behavior.

- ``switchover`` : a switchover operation is automatically performed,
  setting the most aligned replica as the new target primary, and
  shutting down the former primary pod.

There’s no one-size-fits-all configuration for the update method, as
that depends on several factors like the actual workload of your
database, the requirements in terms of :ref:`PgBouncerPoolMode {postgresql-cnpg-io-v1-PgBouncerPoolMode} <PgBouncerPoolMode {postgresql-cnpg-io-v1-PgBouncerPoolMode}>`  and :ref:`RTO and RPO impact <RTO and RPO impact>`  ,
whether your PostgreSQL architecture is shared or shared nothing, and so
on.

Indeed, being PostgreSQL a primary/standby architecture database
management system, the update process inevitably generates a downtime
for your applications. One important aspect to consider for your context
is the time it takes for your pod to download the new PostgreSQL
container image, as that depends on your Kubernetes cluster settings and
specifications. The ``switchover`` method makes sure that the promoted
instance already runs the target image version of the container. The
``restart`` method instead might require to download the image from the
origin registry after the primary pod has been shut down. It is up to
you to determine whether, for your database, it is best to use
``restart`` or ``switchover`` as part of the rolling update procedure.

Manual updates (``supervised`` )
--------------------------------

When ``primaryUpdateStrategy`` is set to ``supervised`` , the rolling
update process is suspended immediately after all replicas have been
upgraded.

This phase can only be completed with either a manual switchover or an
in-place restart. Keep in mind that image upgrades can not be applied
with an in-place restart, so a switchover is required in such cases.

You can trigger a switchover with:

.. code:: bash

   kubectl cnpg promote [cluster] [new_primary]

You can trigger a restart with:

.. code:: bash

   kubectl cnpg restart [cluster] [current_primary]

You can find more information in the :ref:`Backup {postgresql-cnpg-io-v1-Backup} <Backup {postgresql-cnpg-io-v1-Backup}>`  .