Replication
===========

Physical replication is one of the strengths of PostgreSQL and one of
the reasons why some of the largest organizations in the world have
chosen it for the management of their data in business continuity
contexts. Primarily used to achieve high availability, physical
replication also allows scale-out of read-only workloads and offloading
of some work from the primary.

..  Important::
   This section is about replication within the same `Cluster`  resource managed in the same Kubernetes cluster. For information about how to replicate with another Postgres `Cluster`  resource, even across different Kubernetes clusters, please refer to the  :ref:`Replica clusters <Replica clusters>` 

section.

Application-level replication
-----------------------------

Having contributed throughout the years to the replication feature in
PostgreSQL, we have decided to build high availability in CloudNativePG
on top of the native physical replication technology, and integrate it
directly in the Kubernetes API.

In Kubernetes terms, this is referred to as **application-level
replication**, in contrast with *storage-level replication*.

A very mature technology
------------------------

PostgreSQL has a very robust and mature native framework for replicating
data from the primary instance to one or more replicas, built around the
concept of transactional changes continuously stored in the WAL (Write
Ahead Log).

Started as the evolution of crash recovery and point in time recovery
technologies, physical replication was first introduced in PostgreSQL
8.2 (2006) through WAL shipping from the primary to a warm standby in
continuous recovery.

PostgreSQL 9.0 (2010) enhanced it with WAL streaming and read-only
replicas via *hot standby*, while 9.1 (2011) introduced synchronous
replication at the transaction level (for RPO=0 clusters). Cascading
replication was released with PostgreSQL 9.2 (2012). The foundations of
logical replication were laid in PostgreSQL 9.4, while version 10 (2017)
introduced native support for the publisher/subscriber pattern to
replicate data from an origin to a destination.

Streaming replication support
-----------------------------

At the moment, CloudNativePG natively and transparently manages physical
streaming replicas within a cluster in a declarative way, based on the
number of provided ``instances`` in the ``spec`` :

::

   replicas = instances - 1 (where  instances > 0)

Immediately after the initialization of a cluster, the operator creates
a user called ``streaming_replica`` as follows:

.. code:: sql

   CREATE USER streaming_replica WITH REPLICATION;
      -- NOSUPERUSER INHERIT NOCREATEROLE NOCREATEDB NOBYPASSRLS

Out of the box, the operator automatically sets up streaming replication
within the cluster over an encrypted channel and enforces TLS client
certificate authentication for the ``streaming_replica`` user - as
highlighted by the following excerpt taken from ``pg_hba.conf`` :

::

   #  Require client certificate authentication for the streaming_replica user

   hostssl postgres streaming_replica all cert
   hostssl replication streaming_replica all cert

..  Seealso::
   For details on how CloudNativePG manages certificates, please refer to the  :ref:`Certificates <Certificates>` 

in the documentation.

If configured, the operator manages replication slots for all the
replicas in the HA cluster, ensuring that WAL files required by each
standby are retained on the primary’s storage, even after a failover or
switchover.

..  Seealso::
   For details on how CloudNativePG automatically manages replication slots for the High Availability replicas, please refer to the :ref:`Replication slots for High Availability <Replication slots for High Availability>` 

below.

Continuous backup integration
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

In case continuous backup is configured in the cluster, CloudNativePG
transparently configures replicas to take advantage of
``restore_command`` when in continuous recovery. As a result, PostgreSQL
can use the WAL archive as a fallback option whenever pulling WALs via
streaming replication fails.

Synchronous replication
-----------------------

CloudNativePG supports the configuration of **quorum-based synchronous
streaming replication** via two configuration options called
``minSyncReplicas`` and ``maxSyncReplicas`` , which are the minimum and
the maximum number of expected synchronous standby replicas available at
any time. For self-healing purposes, the operator always compares these
two values with the number of available replicas to determine the
quorum.

..  Important::
   By default, synchronous replication selects among all the available replicas indistinctively. You can limit on which nodes your synchronous replicas can be scheduled, by working on node labels through the `syncReplicaElectionConstraint`  option as described in the next section.

Synchronous replication is disabled by default (``minSyncReplicas`` and
``maxSyncReplicas`` are not defined). In case both ``minSyncReplicas``
and ``maxSyncReplicas`` are set, CloudNativePG automatically updates the
``synchronous_standby_names`` option in PostgreSQL to the following
value:

::

   ANY q (pod1, pod2, ...)

Where:

-  ``q`` is an integer automatically calculated by the operator to be:
   ``1 <= minSyncReplicas <= q <= maxSyncReplicas <= readyReplicas``

-  ``pod1, pod2, ...`` is the list of all PostgreSQL pods in the cluster

..  Warning::
   To provide self-healing capabilities, the operator can ignore `minSyncReplicas`  if such value is higher than the currently available number of replicas. Synchronous replication is automatically disabled when `readyReplicas`  is `0` .

As stated in the `PostgreSQL documentation <https://www.postgresql.org/docs/current/warm-standby.html#SYNCHRONOUS-REPLICATION>`_  , the *method ``ANY`` specifies a
quorum-based synchronous replication and makes transaction commits wait
until their WAL records are replicated to at least the requested number
of synchronous standbys in the list*.

..  Important::
   Even though the operator chooses self-healing over enforcement of synchronous replication settings, our recommendation is to plan for synchronous replication only in clusters with 3+ instances or, more generally, when `maxSyncReplicas < (instances - 1)` .

Select nodes for synchronous replication
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

CloudNativePG enables you to select which PostgreSQL instances are
eligible to participate in a quorum-based synchronous replication set
through anti-affinity rules based on the node labels where the PVC
holding the PGDATA and the Postgres pod are.

..  Seealso::
   For more information on the general pod affinity and anti-affinity rules, please check the  :ref:`Scheduling <Scheduling>`  .

As an example use-case for this feature: in a cluster with a single sync
replica, we would be able to ensure the sync replica will be in a
different availability zone from the primary instance, usually
identified by the ``topology.kubernetes.io/zone`` `label on a node <https://kubernetes.io/docs/reference/labels-annotations-taints/#topologykubernetesiozone>`_  . This
would increase the robustness of the cluster in case of an outage in a
single availability zone, especially in terms of recovery point
objective (RPO).

The idea of anti-affinity is to ensure that sync replicas that
participate in the quorum are chosen from pods running on nodes that
have different values for the selected labels (in this case, the
availability zone label) then the node where the primary is currently in
execution. If no node matches such criteria, the replicas are eligible
for synchronous replication.

..  Important::
   The self-healing enforcement still applies while defining additional constraints for synchronous replica election (see  :ref:`Zero-data-loss clusters through synchronous replication <Zero-data-loss clusters through synchronous replication>`  ).

The example below shows how this can be done through the
``syncReplicaElectionConstraint`` section within ``.spec.postgresql`` .
``nodeLabelsAntiAffinity`` allows you to specify those node labels that
need to be evaluated to make sure that synchronous replication will be
dynamically configured by the operator between the current primary and
the replicas which are located on nodes having a value of the
availability zone label different from that of the node where the
primary is:

.. code:: yaml

   spec:
     instances: 3
     postgresql:
       syncReplicaElectionConstraint:
         enabled: true
         nodeLabelsAntiAffinity:
         - topology.kubernetes.io/zone

As you can imagine, the availability zone is just an example, but you
could customize this behavior based on other labels that describe the
node, such as storage, CPU, or memory.

Replication slots
-----------------

 `Replication slots <https://www.postgresql.org/docs/current/warm-standby.html#STREAMING-REPLICATION-SLOTS>`_ 

are a native PostgreSQL feature introduced in 9.4 that provides an
automated way to ensure that the primary does not remove WAL segments
until all the attached streaming replication clients have received them,
and that the primary does not remove rows which could cause a recovery
conflict even when the standby is (temporarily) disconnected.

A replication slot exists solely on the instance that created it, and
PostgreSQL does not replicate it on the standby servers. As a result,
after a failover or a switchover, the new primary does not contain the
replication slot from the old primary. This can create problems for the
streaming replication clients that were connected to the old primary and
have lost their slot.

CloudNativePG provides a turn-key solution to synchronize the content of
physical replication slots from the primary to each standby, addressing
two use cases:

-  the replication slots automatically created for the High Availability
   of the Postgres cluster (see :ref:`Replication slots for High Availability <Replication slots for High Availability>`  for details)

-  :ref:`User-Defined Replication slots <User-Defined Replication slots>`  created on the primary

Replication slots for High Availability
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

CloudNativePG fills this gap by introducing the concept of
cluster-managed replication slots, starting with high availability
clusters. This feature automatically manages physical replication slots
for each hot standby replica in the High Availability cluster, both in
the primary and the standby.

In CloudNativePG, we use the terms:

-  **Primary HA slot**: a physical replication slot whose lifecycle is
   entirely managed by the current primary of the cluster and whose
   purpose is to map to a specific standby in streaming replication.
   Such a slot lives on the primary only.

-  **Standby HA slot**: a physical replication slot for a standby whose
   lifecycle is entirely managed by another standby in the cluster,
   based on the content of the ``pg_replication_slots`` view in the
   primary, and updated at regular intervals using
   ``pg_replication_slot_advance()`` .

This feature is enabled by default and can be disabled via
configuration. For details, please refer to the :ref:`ReplicationSlotsConfiguration {postgresql-cnpg-io-v1-ReplicationSlotsConfiguration} <ReplicationSlotsConfiguration {postgresql-cnpg-io-v1-ReplicationSlotsConfiguration}>`  . Here
follows a brief description of the main options:

``.spec.replicationSlots.highAvailability.enabled`` : if ``true`` , the
feature is enabled (``true`` is the default since 1.21)

``.spec.replicationSlots.highAvailability.slotPrefix`` : the prefix that
identifies replication slots managed by the operator for this feature
(default: ``_cnpg_`` )

``.spec.replicationSlots.updateInterval`` : how often the standby
synchronizes the position of the local copy of the replication slots
with the position on the current primary, expressed in seconds (default:
30)

..  Important::
   This capability requires PostgreSQL 11 or higher, as it relies on the `pg_replication_slot_advance() <https://www.postgresql.org/docs/current/functions-admin.html>`_ 

to directly manipulate the position of a replication slot.

..  Warning::
   In PostgreSQL 11, enabling replication slots if initially disabled, or conversely disabling them if initially enabled, will require a rolling update of the cluster (due to the presence of the `recovery.conf`  file that is only read at startup).

Although it is not recommended, if you desire a different behavior, you
can customize the above options.

For example, the following manifest will create a cluster with
replication slots disabled.

.. code:: yaml

   apiVersion: postgresql.cnpg.io/v1
   kind: Cluster
   metadata:
     name: cluster-example
   spec:
     instances: 3
     # Disable replication slots for HA in the cluster
     replicationSlots:
       highAvailability:
         enabled: false

     storage:
       size: 1Gi

User-Defined Replication slots
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Although CloudNativePG doesn’t support a way to declaratively define
physical replication slots, you can still `create your own slots via SQL <https://www.postgresql.org/docs/current/functions-admin.html#FUNCTIONS-REPLICATION>`_  .

..  Note::
   At the moment, we don't have any plans to manage replication slots in a declarative way, but it might change depending on the feedback we receive from users. The reason is that replication slots exist for a specific purpose and each should be managed by a specific application the oversees the entire lifecycle of the slot on the primary.

CloudNativePG can manage the synchronization of any user managed
physical replication slots between the primary and standbys, similarly
to what it does for the HA replication slots explained above (the only
difference is that you need to create the replication slot).

This feature is enabled by default (meaning that any replication slot is
synchronized), but you can disable it or further customize its behavior
(for example by excluding some slots using regular expressions) through
the ``synchronizeReplicas`` stanza. For example:

.. code:: yaml

   apiVersion: postgresql.cnpg.io/v1
   kind: Cluster
   metadata:
     name: cluster-example
   spec:
     instances: 3
     replicationSlots:
       synchronizeReplicas:
         enabled: true
         excludePatterns:
         - "^foo"

For details, please refer to the :ref:`ReplicationSlotsConfiguration {postgresql-cnpg-io-v1-ReplicationSlotsConfiguration} <ReplicationSlotsConfiguration {postgresql-cnpg-io-v1-ReplicationSlotsConfiguration}>`  . Here follows a brief
description of the main options:

``.spec.replicationSlots.synchronizeReplicas.enabled`` : When true or
not specified, every user-defined replication slot on the primary is
synchronized on each standby. If changed to false, the operator will
remove any replication slot previously created by itself on each
standby.

``.spec.replicationSlots.synchronizeReplicas.excludePatterns`` : A list
of regular expression patterns to match the names of user-defined
replication slots to be excluded from synchronization. This can be
useful to exclude specific slots based on naming conventions.

..  Warning::
   Users utilizing this feature should carefully monitor user-defined replication slots to ensure they align with their operational requirements and do not interfere with the failover process.

Synchronization frequency
^^^^^^^^^^^^^^^^^^^^^^^^^

You can also control the frequency with which a standby queries the
``pg_replication_slots`` view on the primary, and updates its local copy
of the replication slots, like in this example:

.. code:: yaml

   apiVersion: postgresql.cnpg.io/v1
   kind: Cluster
   metadata:
     name: cluster-example
   spec:
     instances: 3
     # Reduce the frequency of standby HA slots updates to once every 5 minutes
     replicationSlots:
       updateInterval: 300

     storage:
       size: 1Gi

Capping the WAL size retained for replication slots
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

When replication slots is enabled, you might end up running out of disk
space due to PostgreSQL trying to retain WAL files requested by a
replication slot. This might happen due to a standby that is
(temporarily?) down, or lagging, or simply an orphan replication slot.

Starting with PostgreSQL 13, you can take advantage of the
 :ref:`max_slot_wal_keep_size <Replication>` 

configuration option controlling the maximum size of WAL files that
replication slots are allowed to retain in the ``pg_wal`` directory at
checkpoint time. By default, in PostgreSQL ``max_slot_wal_keep_size`` is
set to ``-1`` , meaning that replication slots may retain an unlimited
amount of WAL files. As a result, our recommendation is to explicitly
set ``max_slot_wal_keep_size`` when replication slots support is
enabled. For example:

.. code:: ini

     # ...
     postgresql:
       parameters:
         max_slot_wal_keep_size: "10GB"
     # ...

Monitoring replication slots
^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Replication slots must be carefully monitored in your infrastructure. By
default, we provide the ``pg_replication_slots`` metric in our
Prometheus exporter with key information such as the name of the slot,
the type, whether it is active, the lag from the primary.

..  Seealso::
   Please refer to the  :ref:`MonitoringConfiguration {postgresql-cnpg-io-v1-MonitoringConfiguration} <MonitoringConfiguration {postgresql-cnpg-io-v1-MonitoringConfiguration}>`  for details on how to monitor a CloudNativePG deployment.