:_mod-docs-content-type: CONCEPT

// This module is included in the following assemblies:
//
// upgrading/assembly-upgrade.adoc

[id='con-upgrade-cluster-{context}']
= Upgrading Kubernetes with minimal downtime

[role="_abstract"]
If you are upgrading Kubernetes, refer to the Kubernetes upgrade documentation to check the upgrade path and the steps to upgrade your nodes correctly.
Before upgrading Kubernetes, link:{supported-configurations}[check the supported versions for your version of Strimzi^].

When performing your upgrade, ensure the availability of your Kafka clusters by following these steps:

. Configure pod disruption budgets
. Roll pods using one of these methods:
.. Use the Strimzi Drain Cleaner (recommended)
.. Apply an annotation to your pods to roll them manually

For Kafka to stay operational, topics must also be replicated for high availability.
This requires topic configuration that specifies a replication factor of at least 3 and a minimum number of in-sync replicas to 1 less than the replication factor.

.Kafka topic replicated for high availability
[source,yaml,subs="attributes+"]
----
apiVersion: {KafkaTopicApiVersion}
kind: KafkaTopic
metadata:
  name: my-topic
  labels:
    strimzi.io/cluster: my-cluster
spec:
  partitions: 1
  replicas: 3
  config:
    # ...
    min.insync.replicas: 2
    # ...
----

In a highly available environment, the Cluster Operator maintains a minimum number of in-sync replicas for topics during the upgrade process so that there is no downtime.

== Rolling pods using Drain Cleaner

When using the Strimzi Drain Cleaner to evict nodes during Kubernetes upgrade, it annotates pods with a manual rolling update annotation to inform the Cluster Operator to perform a rolling update of the pod that should be evicted and have it moved away from the Kubernetes node that is being upgraded.

For more information, see xref:assembly-drain-cleaner-{context}[].

== Rolling pods manually (alternative to Drain Cleaner)

As an alternative to using the Drain Cleaner to roll pods, you can trigger a manual rolling update of pods through the Cluster Operator.
Using `Pod` resources, rolling updates restart the pods of resources with new pods.
To replicate the operation of the Drain Cleaner by keeping topics available, you must also set the `maxUnavailable` value to zero for the pod disruption budget.
Reducing the pod disruption budget to zero prevents Kubernetes from evicting pods automatically.

.Specifying a pod disruption budget
[source,yaml,subs=attributes+]
----
apiVersion: {KafkaApiVersion}
kind: Kafka
metadata:
  name: my-cluster
  namespace: myproject
spec:
  kafka:
    # ...
    template:
      podDisruptionBudget:
        maxUnavailable: 0
# ...
----

The `PodDisruptionBudget` resource created for Kafka clusters covers all associated node pool pods. 

Watch the node pool pods that need to be drained and add a pod annotation to make the update.
Here, the annotation updates a Kafka pod named `my-cluster-pool-a-1`.

.Performing a manual rolling update on a Kafka pod
[source,shell,subs="+quotes"]
----
kubectl annotate pod my-cluster-pool-a-1 strimzi.io/manual-rolling-update="true"
----

[role="_additional-resources"]
.Additional resources
* xref:assembly-drain-cleaner-str[Draining pods using the Strimzi Drain Cleaner]
* xref:proc-manual-rolling-update-pods-str[Performing a rolling update using a pod annotation]
* link:{BookURLConfiguring}#type-PodDisruptionBudgetTemplate-reference[`PodDisruptionBudgetTemplate` schema reference^]
* {kubernetes-docs}[Kubernetes documentation^]
