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Kubernetes 1.21 is officially released, and this is also the first version of 2021. This release contains 51 enhancements: 13 enhancements have been phased into stable release, 16 enhancements have been converted into Beta release, 20 enhancements have been moved into Alpha release, and 2 features have been deprecated. The new version updates revolve around the following themes:

CronJobs Graduated to Stable

CronJobs has been a beta feature since Kubernetes 1.8, and in 1.21, the widely used API stabilized. CronJobs is used to perform scheduled operations on a regular basis, such as backups, report generation, etc. Each task should be configured to repeat indefinitely, and you can define the point in time at which the job should start within that interval.

ImmutableSecrets and ConfigMap

Immutable Secrets and ConfigMap add a new field for resource types that, if set, will reject changes to these objects. Secrets and ConfigMaps are mutable by default, which is beneficial for pods that can use changes. Variable Secrets and ConfigMaps can also cause problems if the wrong configuration is pushed to the pod that uses them.

By marking Secrets and ConfigMaps immutable, you can ensure that your application configuration does not change. To change this, create a new, uniquely named Secret or ConfigMap and deploy a new pod to use the resource. Immutable resources also have the scaling advantage that the controller does not need to poll API servers to observe changes.

IPv4/IPv6 dual stack support

IP addresses are consumable resources, and cluster operators and administrators need to ensure that they are not exhausted. In particular, public IPv4 addresses are now scarce, with dual-stack support that routes native IPv6 to pods and services while still allowing your cluster to use IPv4 where it is needed. Dual-stack clustering also improves the potential scaling limits of workloads.

Dual stack support in Kubernetes means that pods, services, and nodes can acquire IPv4 and IPv6 addresses. In Kubernetes 1.21, dual-stack networking has been upgraded from alpha to beta and enabled by default, allowing both IPv4 and IPv6 addresses to be assigned.

IPv4 / IPv6 dual stack provides the following features

Dual-stack Pod Network (each Pod is assigned an IPv4 and IPv6 address)

IPv4 and IPv6 support services

Out-of-cluster egress routing (e.g. Internet) over both IPv4 and IPv6 interfaces

Graceful Node Shutdown

GracefulNode Shutdown has been upgraded to Beta in Kubernetes 1.21, which allows kubelet to know that a node is down and gracefully terminate pods scheduled to that node.

Currently, pods do not follow the expected end-of-life cycle when nodes are shut down, nor do they shut down properly, which can cause problems under many different workloads. Later, kubelet will be able to detect an impending system shutdown via systemd and then notify the running pods so they can terminate as normally as possible.

PersistentVolume Health Monitor

PersistentVolume is typically used to obtain local, file-based storage within an application. They can be used in many different ways and can help users migrate applications without having to rewrite the storage backend.

Kubernetes 1.21 has a new alpha feature that monitors PV health and flags volumes when they become unhealthy. Workloads will be able to react to health conditions to protect data from being written to or read from unhealthy volumes.

Deprecated PodSecurityPolicy

PodSecurityPolicy, as a commonly used security policy management and authorization mechanism in Kubernetes, provides a lot of convenience in some scenarios that restrict the use of storage and network privileges. PodSecurityPolicy was deprecated in Kubernetes 1.21 due to version incompatibility issues. As with all deprecated features of Kubernetes, PodSecurityPolicy will continue to be available and provide full functionality in several other versions. PodSecurityPolicy, previously in beta, is scheduled to be removed in Kubernetes 1.25.

Discard TopologyKeys

In Kubernetes 1.21, the service field topologyKeys is deprecated; all component functionality that uses this field, formerly alpha, is deprecated. We replaced topologyKeys with a way to implement topology-aware routing, a topology-aware hint that was an alpha feature in Kubernetes 1.21.

Reduce build maintenance for Kubernetes

Previously, Kubernetes maintained multiple build systems, which was often the source of friction and complexity between new and existing contributors. During the last release cycle, we put a lot of effort into simplifying the build process and standardizing the native Golang build tools.

This release cycle also saw a significant shift in ownership around the release process: from a synchronous communication model that regularly requested input from the community to a community-selected model that included features or blogs in the release. This further enhances community collaboration and teamwork.

Introduction of suspended job feature

Jobs are an important part of Kubernetes API. While other types of workloads (such as Deployments, ReplicaSets, StatefulSets, and DaemonSets) address use cases that require Pods to run forever, Jobs are useful when Pods need to run to completion. Jobs are often used for parallel batch processing and can be used for a variety of applications such as video rendering, database maintenance, scientific computing, and more.

While parallelism and Job completion conditions are configurable, the Kubernetes API lacks the ability to suspend and resume jobs. When cluster resources are limited and a higher priority Job needs to be performed in the place of another Job, it is often necessary to delete the lower priority Job, which is a poor solution because the Pod completion history and other job-related metrics will be lost.

In Kubernetes version 1.21, you can suspend jobs by updating their specifications. This feature is currently in alpha and requires you to enable the suspend Job feature gate on the API Server and Controller Manager to use it.

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