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 From Replica Clusters to Distributed Topology

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) introduced WAL streaming and read-only replicas through hot standby. In 2011, PostgreSQL 9.1 brought synchronous replication at the transaction level, supporting RPO=0 clusters. Cascading replication was added in PostgreSQL 9.2 (2012). The foundations for logical replication were established in PostgreSQL 9.4 (2014), and version 10 (2017) introduced native support for the publisher/subscriber pattern to replicate data from an origin to a destination. The table below summarizes these milestones.

Version	Year	Feature
8.2	2006	Warm Standby with WAL shipping
9.0	2010	Hot Standby and physical streaming replication
9.1	2011	Synchronous replication (priority-based)
9.2	2012	Cascading replication
9.4	2014	Foundations of logical replication
10	2017	Logical publisher/subscriber and quorum-based synchronous replication

This table highlights key PostgreSQL replication features and their respective versions.

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:

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

See also

For details on how CloudNativePG manages certificates, please refer to the 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.

See also

For details on how CloudNativePG automatically manages replication slots for the High Availability replicas, please refer to the 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 both quorum-based and priority-based synchronous replication for PostgreSQL .

Warning

Please be aware that synchronous replication will halt your write operations if the required number of standby nodes to replicate WAL data for transaction commits is unavailable. In such cases, write operations for your applications will hang. This behavior differs from the previous implementation in CloudNativePG but aligns with the expectations of a PostgreSQL DBA for this capability.

While direct configuration of the synchronous_standby_names option is prohibited, CloudNativePG allows you to customize its content and extend synchronous replication beyond the Cluster resource through the .spec.postgresql.synchronous .

Synchronous replication is disabled by default (the synchronous stanza is not defined). When defined, two options are mandatory:

method : either any (quorum) or first (priority)
number : the number of synchronous standby servers that transactions must wait for responses from

Quorum-based Synchronous Replication

PostgreSQL’s quorum-based synchronous replication makes transaction commits wait until their WAL records are replicated to at least a certain number of standbys. To use this method, set method to any .

Migrating from the Deprecated Synchronous Replication Implementation

This section provides instructions on migrating your existing quorum-based synchronous replication, defined using the deprecated form, to the new and more robust capability in CloudNativePG.

Suppose you have the following manifest:

apiVersion: postgresql.cnpg.io/v1
kind: Cluster
metadata:
  name: angus
spec:
  instances: 3

  minSyncReplicas: 1
  maxSyncReplicas: 1

  storage:
    size: 1G

You can convert it to the new quorum-based format as follows:

apiVersion: postgresql.cnpg.io/v1
kind: Cluster
metadata:
  name: angus
spec:
  instances: 3

  storage:
    size: 1G

  postgresql:
    synchronous:
      method: any
      number: 1

Important

The primary difference with the new capability is that PostgreSQL will always prioritize data durability over high availability. Consequently, if no replica is available, write operations on the primary will be blocked. However, this behavior is consistent with the expectations of a PostgreSQL DBA for this capability.

Priority-based Synchronous Replication

PostgreSQL’s priority-based synchronous replication makes transaction commits wait until their WAL records are replicated to the requested number of synchronous standbys chosen based on their priorities. Standbys listed earlier in the synchronous_standby_names option are given higher priority and considered synchronous. If a current synchronous standby disconnects, it is immediately replaced by the next-highest-priority standby. To use this method, set method to first .

Important

Currently, this method is most useful when extending synchronous replication beyond the current cluster using the maxStandbyNamesFromCluster , standbyNamesPre , and standbyNamesPost options explained below.

Controlling `synchronous_standby_names` Content

By default, CloudNativePG populates synchronous_standby_names with the names of local pods in a Cluster resource, ensuring synchronous replication within the PostgreSQL cluster. You can customize the content of synchronous_standby_names based on your requirements and replication method (quorum or priority) using the following optional parameters in the .spec.postgresql.synchronous stanza:

maxStandbyNamesFromCluster : the maximum number of pod names from the local Cluster object that can be automatically included in the synchronous_standby_names option in PostgreSQL.
standbyNamesPre : a list of standby names (specifically application_name ) to be prepended to the list of local pod names automatically listed by the operator.
standbyNamesPost : a list of standby names (specifically application_name ) to be appended to the list of local pod names automatically listed by the operator.

Warning

You are responsible for ensuring the correct names in standbyNamesPre and standbyNamesPost . CloudNativePG expects that you manage any standby with an application_name listed here, ensuring their high availability. Incorrect entries can jeopardize your PostgreSQL database uptime.

Examples

Here are some examples, all based on a cluster-example with three instances:

If you set:

postgresql:
  synchronous:
    method: any
    number: 1

The content of synchronous_standby_names will be:

ANY 1 (cluster-example-2, cluster-example-3)

If you set:

postgresql:
  synchronous:
    method: any
    number: 1
    maxStandbyNamesFromCluster: 1
    standbyNamesPre:
      - angus

The content of synchronous_standby_names will be:

ANY 1 (angus, cluster-example-2)

If you set:

postgresql:
  synchronous:
    method: any
    number: 1
    maxStandbyNamesFromCluster: 0
    standbyNamesPre:
      - angus
      - malcolm

The content of synchronous_standby_names will be:

ANY 1 (angus, malcolm)

If you set:

postgresql:
  synchronous:
    method: first
    number: 2
    maxStandbyNamesFromCluster: 1
    standbyNamesPre:
      - angus
    standbyNamesPost:
      - malcolm

The synchronous_standby_names option will look like:

FIRST 2 (angus, cluster-example-2, malcolm)

Synchronous Replication (Deprecated)

Warning

Prior to CloudNativePG 1.24, only the quorum-based synchronous replication implementation was supported. Although this method is now deprecated, it will not be removed anytime soon. The new method prioritizes data durability over self-healing and offers more robust features, including priority-based synchronous replication and full control over the synchronous_standby_names option. It is recommended to gradually migrate to the new configuration method for synchronous replication, as explained in the previous paragraph.

Important

The deprecated method and the new method are mutually exclusive.

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 , 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.

See also

For more information on the general pod affinity and anti-affinity rules, please check the Scheduling .

Warning

The .spec.postgresql.syncReplicaElectionConstraint option only applies to the legacy implementation of synchronous replication (see Synchronous Replication (Deprecated) ).

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 . 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 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:

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

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 Replication slots for High Availability for details)
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 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)

.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()

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.

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 .

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:

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 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:

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 max_slot_wal_keep_size

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:

# ...
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.

See also

Please refer to the ClusterMonitoringTLSConfiguration {postgresql-cnpg-io-v1-ClusterMonitoringTLSConfiguration} for details on how to monitor a CloudNativePG deployment.

Replication

Application-level replication

A very mature technology

Streaming replication support

Continuous backup integration

Synchronous Replication

Quorum-based Synchronous Replication

Migrating from the Deprecated Synchronous Replication Implementation

Priority-based Synchronous Replication

Controlling synchronous_standby_names Content

Examples

Synchronous Replication (Deprecated)

Select nodes for synchronous replication

Replication slots

Replication slots for High Availability

User-Defined Replication slots

Synchronization frequency

Capping the WAL size retained for replication slots

Monitoring replication slots

Controlling `synchronous_standby_names` Content