Kubernetes 1.27: Safer, More Performant Pruning in kubectl apply

Authors: Katrina Verey (independent) 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:

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 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 3659: 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. To provide feedback on the feature, please file a bug or request an enhancement on the kubernetes/kubectl repository.

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