Blog: Kubernetes 1.26: Eviction policy for unhealthy pods guarded by PodDisruptionBudgets

Authors: Filip Křepinský (Red Hat), Morten Torkildsen (Google), Ravi Gudimetla (Apple)

Ensuring the disruptions to your applications do not affect its availability isn’t a simple
task. Last month’s release of Kubernetes v1.26 lets you specify an unhealthy pod eviction policy
for PodDisruptionBudgets (PDBs)
to help you maintain that availability during node management operations.
In this article, we will dive deeper into what modifications were introduced for PDBs to
give application owners greater flexibility in managing disruptions.

What problems does this solve?

API-initiated eviction of pods respects PodDisruptionBudgets (PDBs). This means that a requested voluntary disruption
via an eviction to a Pod, should not disrupt a guarded application and .status.currentHealthy of a PDB should not fall
below .status.desiredHealthy. Running pods that are Unhealthy
do not count towards the PDB status, but eviction of these is only possible in case the application
is not disrupted. This helps disrupted or not yet started application to achieve availability
as soon as possible without additional downtime that would be caused by evictions.

Unfortunately, this poses a problem for cluster administrators that would like to drain nodes
without any manual interventions. Misbehaving applications with pods in CrashLoopBackOff
state (due to a bug or misconfiguration) or pods that are simply failing to become ready
make this task much harder. Any eviction request will fail due to violation of a PDB,
when all pods of an application are unhealthy. Draining of a node cannot make any progress
in that case.

On the other hand there are users that depend on the existing behavior, in order to:

• prevent data-loss that would be caused by deleting pods that are guarding an underlying resource or storage
• achieve the best availability possible for their application

Kubernetes 1.26 introduced a new experimental field to the PodDisruptionBudget API: .spec.unhealthyPodEvictionPolicy.
When enabled, this field lets you support both of those requirements.

How does it work?

API-initiated eviction is the process that triggers graceful pod termination.
The process can be initiated either by calling the API directly,
by using a kubectl drain command, or other actors in the cluster.
During this process every pod removal is consulted with appropriate PDBs,
to ensure that a sufficient number of pods is always running in the cluster.

The following policies allow PDB authors to have a greater control how the process deals with unhealthy pods.

There are two policies IfHealthyBudget and AlwaysAllow to choose from.

The former, IfHealthyBudget, follows the existing behavior to achieve the best availability
that you get by default. Unhealthy pods can be disrupted only if their application
has a minimum available .status.desiredHealthy number of pods.

By setting the spec.unhealthyPodEvictionPolicy field of your PDB to AlwaysAllow,
you are choosing the best effort availability for your application.
With this policy it is always possible to evict unhealthy pods.
This will make it easier to maintain and upgrade your clusters.

We think that AlwaysAllow will often be a better choice, but for some critical workloads you may
still prefer to protect even unhealthy Pods from node drains or other forms of API-initiated
eviction.

How do I use it?

This is an alpha feature, which means you have to enable the PDBUnhealthyPodEvictionPolicy
feature gate,
with the command line argument --feature-gates=PDBUnhealthyPodEvictionPolicy=true
to the kube-apiserver.

Here’s an example. Assume that you’ve enabled the feature gate in your cluster, and that you
already defined a Deployment that runs a plain webserver. You labelled the Pods for that
Deployment with app: nginx.
You want to limit avoidable disruption, and you know that best effort availability is
sufficient for this app.
You decide to allow evictions even if those webserver pods are unhealthy.
You create a PDB to guard this application, with the AlwaysAllow policy for evicting
unhealthy pods:

apiVersion: policy/v1
kind: PodDisruptionBudget
metadata:
 name: nginx-pdb
spec:
 selector:
 matchLabels:
 app: nginx
 maxUnavailable: 1
 unhealthyPodEvictionPolicy: AlwaysAllow

How can I learn more?

• Read the KEP: Unhealthy Pod Eviction Policy for PDBs
• Read the documentation: Unhealthy Pod Eviction Policy for PodDisruptionBudgets
• Review the Kubernetes documentation for PodDisruptionBudgets, draining of Nodes and evictions

How do I get involved?

If you have any feedback, please reach out to us in the #sig-apps channel on Slack (visit https://slack.k8s.io/ for an invitation if you need one), or on the SIG Apps mailing list: [email protected]