Merge pull request #46909 from ardaguclu/kep-4292-blog

Custom profiling in kubectl debug feature blog

content/en/blog/_posts/2024-06-21-custom-profiling-kubectl-debug.md

@@ -0,0 +1,103 @@
+---
+layout: blog
+title: "Kubernetes 1.31: Custom Profiling in Kubectl Debug Graduates to Beta"
+date: 2024-08-22
+slug: kubernetes-1-31-custom-profiling-kubectl-debug
+author: >
+  Arda Güçlü (Red Hat)
+---
+
+There are many ways of troubleshooting the pods and nodes in the cluster. However, `kubectl debug` is one of the easiest, highly used and most prominent ones. It
+provides a set of static profiles and each profile serves for a different kind of role. For instance, from the network administrator's point of view, 
+debugging the node should be as easy as this:
+
+```shell
+$ kubectl debug node/mynode -it --image=busybox --profile=netadmin
+```
+
+On the other hand, static profiles also bring about inherent rigidity, which has some implications for some pods contrary to their ease of use.
+Because there are various kinds of pods (or nodes) that all have their specific
+necessities, and unfortunately, some can't be debugged by only using the static profiles. 
+
+Take an instance of a simple pod consisting of a container whose healthiness relies on an environment variable:
+
+```yaml
+apiVersion: v1
+kind: Pod
+metadata:
+  name: example-pod
+spec:
+  containers:
+  - name: example-container
+    image: customapp:latest
+    env:
+    - name: REQUIRED_ENV_VAR
+      value: "value1"
+```
+
+Currently, copying the pod is the sole mechanism that supports debugging this pod in kubectl debug. Furthermore, what if user needs to modify the `REQUIRED_ENV_VAR` to something different
+for advanced troubleshooting?. There is no mechanism to achieve this.
+
+## Custom Profiling
+
+Custom profiling is a new functionality available under `--custom` flag, introduced in kubectl debug to provide extensibility. It expects partial `Container` spec in either YAML or JSON format. 
+In order to debug the example-container above by creating an ephemeral container, we simply have to define this YAML:
+
+```yaml
+# partial_container.yaml
+env:
+  - name: REQUIRED_ENV_VAR
+    value: value2
+```
+
+and execute:
+
+```shell
+kubectl debug example-pod -it --image=customapp --custom=partial_container.yaml
+```
+
+Here is another example that modifies multiple fields at once (change port number, add resource limits, modify environment variable) in JSON:
+
+```json
+{
+  "ports": [
+    {
+      "containerPort": 80
+    }
+  ],
+  "resources": {
+    "limits": {
+      "cpu": "0.5",
+      "memory": "512Mi"
+    },
+    "requests": {
+      "cpu": "0.2",
+      "memory": "256Mi"
+    }
+  },
+  "env": [
+    {
+      "name": "REQUIRED_ENV_VAR",
+      "value": "value2"
+    }
+  ]
+}
+```
+
+## Constraints
+
+Uncontrolled extensibility hurts the usability. So that, custom profiling is not allowed for certain fields such as command, image, lifecycle, volume devices and container name.
+In the future, more fields can be added to the disallowed list if required.
+
+## Limitations
+
+The `kubectl debug` command has 3 aspects: Debugging with ephemeral containers, pod copying, and node debugging. The largest intersection set of these aspects is the container spec within a Pod
+That's why, custom profiling only supports the modification of the fields that are defined with `containers`. This leads to a limitation that if user needs to modify the other fields in the Pod spec, it is not supported.
+
+## Acknowledgments
+
+Special thanks to all the contributors who reviewed and commented on this feature, from the initial conception to its actual implementation (alphabetical order):
+
+- [Eddie Zaneski](https://github.com/eddiezane)
+- [Maciej Szulik](https://github.com/soltysh)
+- [Lee Verberne](https://github.com/verb)