Merge pull request #36197 from ddebroy/podHasNetworkBlog2

Add blog post for PodHasNetwork condition

Author: k8s-ci-robot

content/en/blog/_posts/2022-09-14-pod-has-network-condition.md

@@ -0,0 +1,123 @@
+---
+layout: blog
+title: 'Kubernetes 1.25: PodHasNetwork condition for pods'
+date: 2022-09-14
+slug: pod-has-network-condition
+---
+
+**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](/docs/concepts/workloads/pods/pod-lifecycle/) 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).