Fencing

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:

##  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:

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:

##  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 .