Merge pull request #47233 from p0lyn0mial/blog-feature-gates-client-go

Blog Post: Introducing Feature Gates to Client-Go: Enhancing Flexibility and Control

content/en/blog/_posts/2024-08-12-feature-gates_client-go.md

@@ -0,0 +1,118 @@
+---
+layout: blog
+title: 'Introducing Feature Gates to Client-Go: Enhancing Flexibility and Control'
+date: 2024-08-12
+slug: feature-gates-in-client-go
+author: >
+ Ben Luddy (Red Hat),
+ Lukasz Szaszkiewicz (Red Hat)
+---
+
+Kubernetes components use on-off switches called _feature gates_ to manage the risk of adding a new feature.
+The feature gate mechanism is what enables incremental graduation of a feature through the stages Alpha, Beta, and GA.
+
+Kubernetes components, such as kube-controller-manager and kube-scheduler, use the client-go library to interact with the API. 
+The same library is used across the Kubernetes ecosystem to build controllers, tools, webhooks, and more. client-go now includes 
+its own feature gating mechanism, giving developers and cluster administrators more control over how they adopt client features.
+
+To learn more about feature gates in Kubernetes, visit [Feature Gates](/docs/reference/command-line-tools-reference/feature-gates/).
+
+## Motivation
+
+In the absence of client-go feature gates, each new feature separated feature availability from enablement in its own way, if at all. 
+Some features were enabled by updating to a newer version of client-go. Others needed to be actively configured in each program that used them. 
+A few were configurable at runtime using environment variables. Consuming a feature-gated functionality exposed by the kube-apiserver sometimes 
+required a client-side fallback mechanism to remain compatible with servers that don’t support the functionality due to their age or configuration. 
+In cases where issues were discovered in these fallback mechanisms, mitigation required updating to a fixed version of client-go or rolling back.
+
+None of these approaches offer good support for enabling a feature by default in some, but not all, programs that consume client-go. 
+Instead of enabling a new feature at first only for a single component, a change in the default setting immediately affects the default 
+for all Kubernetes components, which broadens the blast radius significantly.
+
+## Feature gates in client-go
+
+To address these challenges, substantial client-go features will be phased in using the new feature gate mechanism. 
+It will allow developers and users to enable or disable features in a way that will be familiar to anyone who has experience 
+with feature gates  in the Kubernetes components.
+
+Out of the box, simply by using a recent version of client-go, this offers several benefits.
+
+For people who use software built with client-go:
+
+
+* Early adopters can enable a default-off client-go feature on a per-process basis.
+* Misbehaving features can be disabled without building a new binary.
+* The state of all known client-go feature gates is logged, allowing users to inspect it.
+
+For people who develop software built with client-go:
+
+* By default, client-go feature gate overrides are read from environment variables. 
+  If a bug is found in a client-go feature, users will be able to disable it without waiting for a new release.
+* Developers can replace the default environment-variable-based overrides in a program to change defaults, 
+  read overrides from another source, or disable runtime overrides completely. 
+  The Kubernetes components use this customizability to integrate client-go feature gates with 
+  the existing `--feature-gates` command-line flag, feature enablement metrics, and logging.
+
+## Overriding client-go feature gates
+
+**Note**: This describes the default method for overriding client-go feature gates at runtime. 
+It can be disabled or customized by the developer of a particular program. 
+In Kubernetes components, client-go feature gate overrides are controlled by the `--feature-gates` flag.
+
+Features of client-go can be enabled or disabled by setting environment variables prefixed with `KUBE_FEATURE`. 
+For example, to enable a feature named `MyFeature`, set the environment variable as follows:
+
+```
+ KUBE_FEATURE_MyFeature=true
+```
+
+To disable the feature, set the environment variable to `false`:
+
+```
+ KUBE_FEATURE_MyFeature=false
+```
+
+**Note**: Environment variables are case-sensitive on some operating systems. 
+Therefore, `KUBE_FEATURE_MyFeature` and `KUBE_FEATURE_MYFEATURE` would be considered two different variables.
+
+## Customizing client-go feature gates
+
+The default environment-variable based mechanism for feature gate overrides can be sufficient for many programs in the Kubernetes ecosystem, 
+and requires no special integration. Programs that require different behavior can replace it with their own custom feature gate provider. 
+This allows a program to do things like force-disable a feature that is known to work poorly, 
+read feature gates directly from a remote configuration service, or accept feature gate overrides through command-line options.
+
+The Kubernetes components replace client-go’s default feature gate provider with a shim to the existing Kubernetes feature gate provider. 
+For all practical purposes, client-go feature gates are treated the same as other Kubernetes 
+feature gates: they are wired to the `--feature-gates` command-line flag, included in feature enablement metrics, and logged on startup.
+
+To replace the default feature gate provider, implement the Gates interface and call ReplaceFeatureGates 
+at package initialization time, as in this simple example:
+
+```go
+import (
+ “k8s.io/client-go/features”
+)
+
+type AlwaysEnabledGates struct{}
+
+func (AlwaysEnabledGates) Enabled(features.Feature) bool {
+ return true
+}
+
+func init() {
+ features.ReplaceFeatureGates(AlwaysEnabledGates{})
+}
+```
+
+Implementations that need the complete list of defined client-go features can get it by implementing the Registry interface 
+and calling `AddFeaturesToExistingFeatureGates`. 
+For a complete example, refer to [the usage within Kubernetes](https://github.com/kubernetes/kubernetes/blob/64ba17c605a41700f7f4c4e27dca3684b593b2b9/pkg/features/kube_features.go#L990-L997).
+
+## Summary
+
+With the introduction of feature gates in client-go v1.30, rolling out a new client-go feature has become safer and easier. 
+Users and developers can control the pace of their own adoption of client-go features. 
+The work of Kubernetes contributors is streamlined by having a common mechanism for graduating features that span both sides of the Kubernetes API boundary.
+
+Special shoutout to [@sttts](https://github.com/sttts) and [@deads2k](https://github.com/deads2k) for their help in shaping this feature.