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 ` section. Application-level replication ----------------------------- Having contributed throughout the years to the replication feature in PostgreSQL, we have decided to build high availability in EDB CloudNativePG Cluster 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 :ref:`Purpose of Release Candidates ` =0 clusters. Cascading replication was added in PostgreSQL 9.2 (2012). The foundations for :ref:`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. .. csv-table:: :header: Version,Year,Feature :widths: 12,10,25 :align: left :class: longtable 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, EDB CloudNativePG Cluster 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 EDB CloudNativePG Cluster manages certificates, please refer to the :ref:`CertificatesConfiguration ` 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 EDB CloudNativePG Cluster automatically manages replication slots for the High Availability replicas, please refer to the :ref:`Replication slots for High Availability ` below. Continuous backup integration ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ In case continuous backup is configured in the cluster, EDB CloudNativePG Cluster 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 ----------------------- EDB CloudNativePG Cluster supports both `quorum-based and priority-based synchronous replication for PostgreSQL `_ . .. Warning:: By default, synchronous replication pauses write operations if the required number of standby nodes for WAL replication during transaction commits is unavailable. This behavior prioritizes data durability and aligns with PostgreSQL DBA best practices. However, if self-healing is a higher priority than strict data durability in your setup, this setting can be adjusted. For details on managing this behavior, refer to the :ref:`Data Durability and Synchronous Replication ` section. Direct configuration of the ``synchronous_standby_names`` option is not permitted. However, EDB CloudNativePG Cluster automatically populates this option with the names of local pods, while also allowing customization to extend synchronous replication beyond the ``Cluster`` resource. This can be achieved through the :ref:`.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 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ In PostgreSQL, quorum-based synchronous replication ensures that transaction commits wait until their WAL records are replicated to a specified number of standbys. To enable this, set the ``method`` to ``any`` . This replication method is the most common setup for a EDB CloudNativePG Cluster cluster. Example ^^^^^^^ The example below, based on a typical ``cluster-example`` configuration with three instances, sets up quorum-based synchronous replication with at least one instance: .. code:: yaml postgresql: synchronous: method: any number: 1 With this configuration, EDB CloudNativePG Cluster automatically sets the content of ``synchronous_standby_names`` as follows: .. code:: console ANY 1 (cluster-example-2, cluster-example-3, cluster-example-1) Migrating from Deprecated Synchronous Replication Implementation ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ This section outlines how to migrate from the deprecated quorum-based synchronous replication format to the newer, more robust implementation in EDB CloudNativePG Cluster. Given the following manifest: .. code:: yaml apiVersion: postgresql.k8s.enterprisedb.io/v1 kind: Cluster metadata: name: angus spec: instances: 3 minSyncReplicas: 1 maxSyncReplicas: 1 storage: size: 1G You can update it to the new format as follows: .. code:: yaml apiVersion: postgresql.k8s.enterprisedb.io/v1 kind: Cluster metadata: name: angus spec: instances: 3 storage: size: 1G postgresql: synchronous: method: any number: 1 dataDurability: required To prioritize self-healing over strict data durability, set ``dataDurability`` to ``preferred`` instead. 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, EDB CloudNativePG Cluster 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` . EDB CloudNativePG Cluster 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: .. code:: yaml postgresql: synchronous: method: any number: 1 maxStandbyNamesFromCluster: 1 standbyNamesPre: - angus The content of ``synchronous_standby_names`` will be: .. code:: console ANY 1 (angus, cluster-example-2) If you set: .. code:: yaml postgresql: synchronous: method: any number: 1 maxStandbyNamesFromCluster: 0 standbyNamesPre: - angus - malcolm The content of ``synchronous_standby_names`` will be: .. code:: console ANY 1 (angus, malcolm) If you set: .. code:: yaml postgresql: synchronous: method: first number: 2 maxStandbyNamesFromCluster: 1 standbyNamesPre: - angus standbyNamesPost: - malcolm The ``synchronous_standby_names`` option will look like: .. code:: console FIRST 2 (angus, cluster-example-2, malcolm) Data Durability and Synchronous Replication ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ The ``dataDurability`` option in the ``.spec.postgresql.synchronous`` stanza controls the trade-off between data safety and availability for synchronous replication. It can be set to ``required`` or ``preferred`` , with the default being ``required`` if not specified. .. Important:: `preferred` can only be used when `standbyNamesPre` and `standbyNamesPost` are unset. Required Data Durability ^^^^^^^^^^^^^^^^^^^^^^^^ When ``dataDurability`` is set to ``required`` , PostgreSQL only considers transactions committed once WAL (Write-Ahead Log) records have been replicated to the specified number of synchronous standbys. This setting prioritizes data safety over availability, meaning write operations will pause if the required number of synchronous standbys is unavailable. This ensures zero data loss (RPO=0) but may reduce database availability during network disruptions or standby failures. Synchronous standbys are selected in this priority order: 1. Healthy instances 2. Unhealthy instances 3. Primary The list is then truncated based on ``maxStandbyNamesFromCluster`` if this value is set, prioritizing healthy instances and ensuring ``synchronous_standby_names`` is populated. .. _example-1: Example ''''''' Consider the following example: .. code:: yaml apiVersion: postgresql.k8s.enterprisedb.io/v1 kind: Cluster metadata: name: foo spec: instances: 3 postgresql: synchronous: method: any number: 1 dataDurability: required 1. Initial state. The content of ``synchronous_standby_names`` is: :: ANY 1 ("foo-2","foo-3","foo-1") 2. ``foo-2`` becomes unavailable. It gets pushed back in priority: :: ANY 1 ("foo-3","foo-2","foo-1") 3. ``foo-3`` also becomes unavailable. The list contains no healthy standbys: :: ANY 1 ("foo-2","foo-3","foo-1") At this point no write operations will be allowed until at least one of the standbys is available again. 4. When the standbys are available again, ``synchronous_standby_names`` will be back to the initial state. Preferred Data Durability ^^^^^^^^^^^^^^^^^^^^^^^^^ When ``dataDurability`` is set to ``preferred`` , the required number of synchronous instances adjusts based on the number of available standbys. PostgreSQL will attempt to replicate WAL records to the designated number of synchronous standbys, but write operations will continue even if fewer than the requested number of standbys are available. This setting balances data safety with availability, enabling applications to continue writing during temporary standby unavailability—hence, it’s also known as *self-healing mode*. .. Warning:: This mode may result in data loss if all standbys become unavailable. With ``preferred`` data durability, **only healthy replicas** are included in ``synchronous_standby_names`` . .. _example-2: Example ''''''' Consider the following example. For demonstration, we’ll use a cluster named ``bar`` with 5 instances and 2 synchronous standbys: .. code:: yaml apiVersion: postgresql.k8s.enterprisedb.io/v1 kind: Cluster metadata: name: bar spec: instances: 5 postgresql: synchronous: method: any number: 2 dataDurability: required 1. Initial state. The content of ``synchronous_standby_names`` is: :: ANY 2 ("bar-2","bar-3", "bar-4", "bar-5") 2. ``bar-2`` and ``bar-3`` become unavailable. They are removed from the list: :: ANY 2 ("bar-4", "bar-5") 3. ``bar-4`` also becomes unavailable. It gets removed from the list. Since the number of available standbys is less than the requested number, the requested amount gets reduced: :: ANY 1 ("bar-5") 4. ``bar-5`` also becomes unavailable. ``synchronous_standby_names`` becomes empty, disabling synchronous replication completely. Write operations will continue, but with the risk of potential data loss in case of a primary failure. 5. When the replicas are back, ``synchronous_standby_names`` will be back to the initial state. Synchronous Replication (Deprecated) ------------------------------------ .. Warning:: Prior to EDB CloudNativePG Cluster 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. EDB CloudNativePG Cluster 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, EDB CloudNativePG Cluster 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 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ EDB CloudNativePG Cluster 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 ` . .. Warning:: The `.spec.postgresql.syncReplicaElectionConstraint` option only applies to the legacy implementation of synchronous replication (see :ref:`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 ( :ref:`Purpose of Release Candidates ` ). 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 ` ). 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 `_ 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. EDB CloudNativePG Cluster 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 ` for details) - :: :ref:`User-Defined Replication slots ` created on the primary Replication slots for High Availability ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ EDB CloudNativePG Cluster 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 EDB CloudNativePG Cluster, 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 ` . 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: ``_cnp_`` ) ``.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) 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.k8s.enterprisedb.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 EDB CloudNativePG Cluster doesn’t support a way to declaratively define physical replication slots, you can still `create your own slots via SQL `_ . .. Note rmation:: 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. EDB CloudNativePG Cluster 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.k8s.enterprisedb.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 ` . 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.k8s.enterprisedb.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 ` 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:`Monitoring and metrics ` for details on how to monitor a EDB CloudNativePG Cluster deployment.