Replica clusters v1
A replica cluster is an independent EDB Postgres for Kubernetes Cluster
resource that has
the main characteristic to be in replica from another Postgres instance,
ideally also managed by EDB Postgres for Kubernetes. Normally, a replica cluster is in another
Kubernetes cluster in another region. Replica clusters can be cascading too,
and they can solely rely on object stores for replication of the data from
the source, as described further down.
The diagram below - taken from the "Architecture" section containing more information about this capability - shows just an example of architecture that you can implement with replica clusters.
Basic concepts
EDB Postgres for Kubernetes relies on the foundations of the PostgreSQL replication framework even when a PostgreSQL cluster is created from an existing one (source) and kept synchronized through the replica cluster feature. The source can be a primary cluster or another replica cluster (cascading replica cluster).
The first step is to bootstrap the replica cluster, choosing among one of the available methods:
- streaming replication, via
pg_basebackup
- recovery from a volume snapshot
- recovery from a Barman Cloud backup in an object store
Please refer to the "Bootstrap" section
for information on how to clone a PostgreSQL server using either
pg_basebackup
(streaming) or recovery
(volume snapshot or object store).
Once the replica cluster's base backup is available, you need to define how changes are replicated from the origin, through PostgreSQL continuous recovery. There are two options:
- use streaming replication between the replica cluster and the source (this will certainly require some administrative and security related work to be done to make sure that the network connection between the two clusters are correctly setup)
- use the WAL archive (on an object store) to fetch the WAL files that are
regularly shipped from the source to the object store and pulled by
barman-cloud-wal-restore
in the replica cluster - any of the two
All you have to do is actually define an external cluster.
If the external cluster contains a barmanObjectStore
section:
- you'll be able to use the WAL archive, and EDB Postgres for Kubernetes will automatically
set the
restore_command
in the designated primary instance - you'll be able to bootstrap the replica cluster from an object store
using the
recovery
section, in case you cannot take advantage of volume snapshots
If the external cluster contains a connectionParameters
section:
- you'll be able to bootstrap the replica cluster via streaming replication
using the
pg_basebackup
section - EDB Postgres for Kubernetes will automatically set the
primary_conninfo
option in the designated primary instance, so that a WAL receiver process is started to connect to the source cluster and receive data
The created replica cluster can perform backups in a reserved object store from the designated primary, enabling symmetric architectures in a distributed fashion.
You have full flexibility and freedom to decide your favorite distributed architecture for a PostgreSQL database by choosing:
- a private cloud spanning over multiple Kubernetes clusters in different data centers
- a public cloud spanning over multiple Kubernetes clusters in different regions
- a mix of the previous two (hybrid)
- a public cloud spanning over multiple Kubernetes clusters in different regions and on different Cloud Service Providers
Setting up a replica cluster
To set up a replica cluster from a source cluster, we need to create a cluster YAML file and define the following parts accordingly:
- define the
externalClusters
section in the replica cluster - define the bootstrap part for the replica cluster. We can either bootstrap via
streaming using the
pg_basebackup
section, or bootstrap from a volume snapshot or an object store using therecovery
section - define the continuous recovery part (
.spec.replica
) in the replica cluster. All we need to do is to enable the replica mode through option.spec.replica.enabled
and set theexternalClusters
name in option.spec.replica.source
Example using pg_basebackup
This first example defines a replica cluster using streaming replication in both bootstrap and continuous recovery. The replica cluster connects to the source cluster using TLS authentication.
You can check the sample YAML
in the samples/
subdirectory.
Note the bootstrap
and replica
sections pointing to the source cluster.
In the externalClusters
section, remember to use the right namespace for the
host in the connectionParameters
sub-section.
The -replication
and -ca
secrets should have been copied over if necessary,
in case the replica cluster is in a separate namespace.
Example using a Backup from an object store
The second example defines a replica cluster that bootstraps from an object
store using the recovery
section and continuous recovery using both streaming
replication and the given object store. For streaming replication, the replica
cluster connects to the source cluster using basic authentication.
You can check the sample YAML
for it in the samples/
subdirectory.
Note the bootstrap
and replica
sections pointing to the source cluster.
In the externalClusters
section, take care to use the right namespace in the
endpointURL
and the connectionParameters.host
.
And do ensure that the necessary secrets have been copied if necessary, and that
a backup of the source cluster has been created already.
Note
To use streaming replication between the source cluster and the replica cluster, we need to make sure there is network connectivity between the two clusters, and that all the necessary secrets which hold passwords or certificates are properly created in advance.
Example using a Volume Snapshot
If you use volume snapshots and your storage class provides snapshots cross-cluster availability, you can leverage that to bootstrap a replica cluster through a volume snapshot of the source cluster.
The third example defines a replica cluster that bootstraps
from a volume snapshot using the recovery
section. It uses
streaming replication (via basic authentication) and the object
store to fetch the WAL files.
You can check the sample YAML
for it in the samples/
subdirectory.
Demoting a Primary to a Replica Cluster
EDB Postgres for Kubernetes provides the functionality to demote a primary cluster to a
replica cluster. This action is typically planned when transitioning the
primary role from one data center to another. The process involves demoting the
current primary cluster (e.g., cluster-eu-south) to a replica cluster and
subsequently promoting the designated replica cluster (e.g.,
cluster-eu-central
) to primary when fully synchronized.
Provided you have defined an external cluster in the current primary Cluster
resource that points to the replica cluster that's been selected to become the
new primary, all you need to do is to enable replica mode and define the source
as follows:
Promoting the designated primary in the replica cluster
To promote a replica cluster (e.g. cluster-eu-central
) to a primary cluster
and make the designated primary a real primary, all you need to do is to
disable the replica mode in the replica cluster through the option
.spec.replica.enabled
:
If you have first demoted the cluster-eu-south
and waited for
cluster-eu-central
to be in sync, once cluster-eu-central
starts as
primary, the cluster-eu-south
cluster will seamlessly start as a replica
cluster, without the need of re-cloning.
If you disable replica mode without prior demotion, the replica cluster and the source cluster will become two separate clusters.
When replica mode is disabled, the designated primary in the replica cluster will be promoted to be that cluster's primary.
You can verify the role change using the cnp
plugin, checking the status of
the cluster which was previously the replica:
Note
Disabling replication is an irreversible operation. Once replication is disabled and the designated primary is promoted to primary, the replica cluster and the source cluster become two independent clusters definitively. Ensure to follow the demotion procedure correctly to avoid unintended consequences.
Delayed replicas
In addition to standard replica clusters, our system supports the creation of
delayed replicas through the utilization of PostgreSQL's
recovery_min_apply_delay
option.
Delayed replicas intentionally lag behind the primary database by a specified
amount of time. This delay is configurable using the recovery_min_apply_delay
option in PostgreSQL. The primary objective of introducing delayed replicas is
to mitigate the impact of unintentional executions of SQL statements on the
primary database. This is particularly useful in scenarios where an incorrect
or missing WHERE
clause is used in operations such as UPDATE
or DELETE
.
To introduce a delay in a replica cluster, adjust the
recovery_min_apply_delay
option. This parameter determines the time by which
replicas lag behind the primary. For example:
Monitor and adjust the delay as needed based on your recovery time objectives and the potential impact of unintended primary database operations.
The main use cases of delayed replicas can be summarized into:
mitigating human errors: reduce the risk of data corruption or loss resulting from unintentional SQL operations on the primary database
recovery time optimization: facilitate quicker recovery from unintended changes by having a delayed replica that allows you to identify and rectify issues before changes are applied to other replicas.
enhanced data protection: safeguard critical data by introducing a time buffer that provides an opportunity to intervene and prevent the propagation of undesirable changes.
By integrating delayed replicas into your replication strategy, you can enhance the resilience and data protection capabilities of your PostgreSQL environment. Adjust the delay duration based on your specific needs and the criticality of your data.
Important
Always measure your goals. Depending on your environment, it might be more efficient to rely on volume snapshot-based recovery for faster outcomes. Evaluate and choose the approach that best aligns with your unique requirements and infrastructure.