Blog: Kubernetes 1.25: PodHasNetwork condition for pods

Author:
Deep Debroy (Apple)

Kubernetes 1.25 introduces Alpha support for a new kubelet-managed pod condition
in the status field of a pod: PodHasNetwork. The kubelet, for a worker node,
will use the PodHasNetwork condition to accurately surface the initialization
state of a pod from the perspective of pod sandbox creation and network
configuration by a container runtime (typically in coordination with CNI
plugins). The kubelet starts to pull container images and start individual
containers (including init containers) after the status of the PodHasNetwork
condition is set to True. Metrics collection services that report latency of
pod initialization from a cluster infrastructural perspective (i.e. agnostic of
per container characteristics like image size or payload) can utilize the
PodHasNetwork condition to accurately generate Service Level Indicators
(SLIs). Certain operators or controllers that manage underlying pods may utilize
the PodHasNetwork condition to optimize the set of actions performed when pods
repeatedly fail to come up.

How is this different from the existing Initialized condition reported for pods?

The kubelet sets the status of the existing Initialized condition reported in
the status field of a pod depending on the presence of init containers in a pod.

If a pod specifies init containers, the status of the Initialized condition in
the pod status will not be set to True until all init containers for the pod
have succeeded. However, init containers, configured by users, may have errors
(payload crashing, invalid image, etc) and the number of init containers
configured in a pod may vary across different workloads. Therefore,
cluster-wide, infrastructural SLIs around pod initialization cannot depend on
the Initialized condition of pods.

If a pod does not specify init containers, the status of the Initialized
condition in the pod status is set to True very early in the lifecycle of the
pod. This occurs before the kubelet initiates any pod runtime sandbox creation
and network configuration steps. As a result, a pod without init containers will
report the status of the Initialized condition as True even if the container
runtime is not able to successfully initialize the pod sandbox environment.

Relative to either situation above, the PodHasNetwork condition surfaces more
accurate data around when the pod runtime sandbox was initialized with
networking configured so that the kubelet can proceed to launch user-configured
containers (including init containers) in the pod.

Note that a node agent may dynamically re-configure network interface(s) for a
pod by watching changes in pod annotations that specify additional networking
configuration (e.g. k8s.v1.cni.cncf.io/networks). Dynamic updates of pod
networking configuration after the pod sandbox is initialized by Kubelet (in
coordination with a container runtime) are not reflected by the PodHasNetwork
condition.

Try out the PodHasNetwork condition for pods

In order to have the kubelet report the PodHasNetwork condition in the status
field of a pod, please enable the PodHasNetworkCondition feature gate on the
kubelet.

For a pod whose runtime sandbox has been successfully created and has networking
configured, the kubelet will report the PodHasNetwork condition with status set to True:

$ kubectl describe pod nginx1
Name: nginx1
Namespace: default
...
Conditions:
Type Status
PodHasNetwork True
Initialized True
Ready True
ContainersReady True
PodScheduled True

For a pod whose runtime sandbox has not been created yet (and networking not
configured either), the kubelet will report the PodHasNetwork condition with
status set to False:

$ kubectl describe pod nginx2
Name: nginx2
Namespace: default
...
Conditions:
Type Status
PodHasNetwork False
Initialized True
Ready False
ContainersReady False
PodScheduled True

What’s next?

Depending on feedback and adoption, the Kubernetes team plans to push the
reporting of the PodHasNetwork condition to Beta in 1.26 or 1.27.

How can I learn more?

Please check out the
documentation for the
PodHasNetwork condition to learn more about it and how it fits in relation to
other pod conditions.

How to get involved?

This feature is driven by the SIG Node community. Please join us to connect with
the community and share your ideas and feedback around the above feature and
beyond. We look forward to hearing from you!

Acknowledgements

We want to thank the following people for their insightful and helpful reviews
of the KEP and PRs around this feature: Derek Carr (@derekwaynecarr), Mrunal
Patel (@mrunalp), Dawn Chen (@dchen1107), Qiutong Song (@qiutongs), Ruiwen Zhao
(@ruiwen-zhao), Tim Bannister (@sftim), Danielle Lancashire (@endocrimes) and
Agam Dua (@agamdua).