Add blog post introducing ApplySet-based pruning

content/en/blog/_posts/2023-05-09-Safer-More-Performant-Pruning-in-kubectl-apply.md

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+---
+layout: blog
+title: "Kubernetes 1.27: Safer, More Performant Pruning in kubectl apply"
+date: 2023-05-09
+slug: introducing-kubectl-applyset-pruning
+---
+
+**Authors:** Katrina Verey (Shopify) and Justin Santa Barbara (Google)
+
+Declarative configuration management with the `kubectl apply` command is the gold standard approach
+to creating or modifying Kubernetes resources. However, one challenge it presents is the deletion
+of resources that are no longer needed. In Kubernetes version 1.5, the `--prune` flag was
+introduced to address this issue, allowing kubectl apply to automatically clean up previously
+applied resources removed from the current configuration.
+
+Unfortunately, that existing implementation of `--prune` has design flaws that diminish its
+performance and can result in unexpected behaviors. The main issue stems from the lack of explicit
+encoding of the previously applied set by the preceding `apply` operation, necessitating
+error-prone dynamic discovery. Object leakage, inadvertent over-selection of resources, and limited
+compatibility with custom resources are a few notable drawbacks of this implementation. Moreover,
+its coupling to client-side apply hinders user upgrades to the superior server-side apply
+mechanism.
+
+Version 1.27 of `kubectl` introduces an alpha version of a revamped pruning implementation that
+addresses these issues. This new implementation, based on a concept called _ApplySet_, promises
+better performance and safety.
+
+An _ApplySet_ is a group of resources associated with a _parent_ object on the cluster, as
+identified and configured through standardized labels and annotations. Additional standardized
+metadata allows for accurate identification of ApplySet _member_ objects within the cluster,
+simplifying operations like pruning.
+
+To leverage ApplySet-based pruning, set the `KUBECTL_APPLYSET=true` environment variable and include
+the flags `--prune` and `--applyset` in your `kubectl apply` invocation:
+
+```shell
+KUBECTL_APPLYSET=true kubectl apply -f <directory/> --prune --applyset=<name>
+```
+
+By default, ApplySet uses a Secret as the parent object. However, you can also use
+a ConfigMap with the format `--applyset=configmaps/<name>`. If your desired Secret or
+ConfigMap object does not yet exist, `kubectl` will create it for you. Furthermore, custom
+resources can be enabled for use as ApplySet parent objects.
+
+The ApplySet implementation is based on a new low-level specification that can support higher-level
+ecosystem tools by improving their interoperability. The lightweight nature of this specification
+enables these tools to continue to use existing object grouping systems while opting in to
+ApplySet's metadata conventions to prevent inadvertent changes by other tools (such as `kubectl`).
+
+ApplySet-based pruning offers a promising solution to the shortcomings of the previous `--prune`
+implementation in `kubectl` and can help streamline your Kubernetes resource management. Please
+give this new feature a try and share your experiences with the community—ApplySet is under active
+development, and your feedback is invaluable!
+
+
+### Additional resources
+
+- For more information how to use ApplySet-based pruning, read
+  [Declarative Management of Kubernetes Objects Using Configuration Files](/docs/tasks/manage-kubernetes-objects/declarative-config/) in the Kubernetes documentation.
+- For a deeper dive into the technical design of this feature or to learn how to implement the
+  ApplySet specification in your own tools, refer to [KEP&nbsp;3659](https://git.k8s.io/enhancements/keps/sig-cli/3659-kubectl-apply-prune/README.md):
+  _ApplySet: `kubectl apply --prune` redesign and graduation strategy_.
+
+
+### How do I get involved?
+
+If you want to get involved in ApplySet development, you can get in touch with the developers at
+[SIG CLI](https://git.k8s.io/community/sig-cli). To provide feedback on the feature, please
+[file a bug](https://github.com/kubernetes/kubectl/issues/new?assignees=knverey,justinsb&labels=kind%2Fbug&template=bug-report.md)
+or [request an enhancement](https://github.com/kubernetes/kubectl/issues/new?assignees=knverey,justinsb&labels=kind%2Fbug&template=enhancement.md)
+on the `kubernetes/kubectl` repository.