Fencing
=======

.. raw:: html

   <!-- SPDX-License-Identifier: CC-BY-4.0 -->

Fencing in CloudNativePG is the ultimate process of protecting the data
in one, more, or even all instances of a PostgreSQL cluster when they
appear to be malfunctioning. When an instance is fenced, the PostgreSQL
server process (``postmaster`` ) is guaranteed to be shut down, while
the pod is kept running. This makes sure that, until the fence is
lifted, data on the pod is not modified by PostgreSQL and that the file
system can be investigated for debugging and troubleshooting purposes.

How to fence instances
----------------------

In CloudNativePG you can fence:

- a specific instance

- a list of instances

- an entire Postgres ``Cluster``

Fencing is controlled through the content of the
``cnpg.io/fencedInstances`` annotation, which expects a JSON formatted
list of instance names. If the annotation is set to ``'["*"]'`` , a
singleton list with a wildcard, the whole cluster is fenced. If the
annotation is set to an empty JSON list, the operator behaves as if the
annotation was not set.

For example:

- ``cnpg.io/fencedInstances: '["cluster-example-1"]'`` will fence just
  the ``cluster-example-1`` instance

- ``cnpg.io/fencedInstances: '["cluster-example-1","cluster-example-2"]'``
  will fence the ``cluster-example-1`` and ``cluster-example-2``
  instances

- ``cnpg.io/fencedInstances: '["*"]'`` will fence every instance in the
  cluster.

The annotation can be manually set on the Kubernetes object, for example
via the ``kubectl annotate`` command, or in a transparent way using the
``kubectl cnpg fencing on`` subcommand:

.. code:: shell


   ##  to fence only one instance

   kubectl cnpg fencing on cluster-example 1

   ##  to fence all the instances in a Cluster

   kubectl cnpg fencing on cluster-example "*"

Here is an example of a ``Cluster`` with an instance that was previously
fenced:

.. code:: yaml

   apiVersion: postgresql.cnpg.io/v1
   kind: Cluster
   metadata:
       annotations:
         cnpg.io/fencedInstances: ["cluster-example-1"]
   [...]

How to lift fencing
-------------------

Fencing can be lifted by clearing the annotation, or set it to a
different value.

As for fencing, this can be done either manually with
``kubectl annotate`` , or using the ``kubectl cnpg fencing`` subcommand
as follows:

.. code:: shell


   ##  to lift the fencing only for one instance

   ##  N.B.: at the moment this wont work if the whole cluster was fenced previously,

   ##  in that case you will have to manually set the annotation as explained above

   kubectl cnpg fencing off cluster-example 1

   ##  to lift the fencing for all the instances in a Cluster

   kubectl cnpg fencing off cluster-example "*"

How fencing works
-----------------

Once an instance is set for fencing, the procedure to shut down the
``postmaster`` process is initiated, identical to the one of the
switchover. This consists of an initial fast shutdown with a timeout set
to ``.spec.switchoverDelay`` , followed by an immediate shutdown. Then:

- the Pod will be kept alive

- the Pod won’t be marked as *Ready*

- all the changes that don’t require the Postgres instance to be up will
  be reconciled, including:

  - configuration files
  - certificates and all the cryptographic material

- metrics will not be collected, except ``cnpg_collector_fencing_on``
  which will be set to 1

..  Warning::
   If a **primary instance** is fenced, its postmaster process is shut down but no failover is performed, interrupting the operativity of the applications. When the fence will be lifted, the primary instance will be started up again without performing a failover.

Given that, we advise users to fence primary instances only if strictly
required. 　

If a fenced instance is deleted, the pod will be recreated normally, but
the postmaster won’t be started. This can be extremely helpful when
instances are ``Crashlooping`` .