The maintainers established this list to place bounds on the kustomize feature set. The bounds can be changed with a consensus on the risks.
For a bigger picture about why kustomize does some things and not others, see the glossary entry for DAM.
kustomize supports configurations that can be reasoned about as
compositions or mixins - concepts that are widely accepted as
a best practice in various programming languages.
To this end, kustomize offers various addition directives.
One may add labels, annotations, patches, resources, bases, etc.
Corresponding removal directives are not offered.
Removal semantics would introduce many possibilities for inconsistency, and the need to add code to detect, report and reject it. It would also allow, and possibly encourage, unnecessarily complex configuration layouts.
When faced with a situation where removal is desirable, it's always possible to remove things from a base like labels and annotations, and/or split multi-resource manifests into individual resource files - then add things back as desired via the kustomization.
If the underlying base is outside of one's control, an OTS workflow is the recommended best practice. Fork the base, remove what you don't want and commit it to your private fork, then use kustomize on your fork. As often as desired, use git rebase to capture improvements from the upstream base.
Structured edits are changes controlled by knowledge of the k8s API, and YAML or JSON syntax.
Most edits performed by kustomize can be expressed as JSON patches or SMP patches. Common edits, like adding labels or adding a name prefix, get dedicated shorthand commands. Another class of edits take data from one specific object's field and use it in another (e.g. a service object's name found and copied into a container's command line).
These edits are designed to create valid output given valid input, and can provide syntactically and semantically informed error messages if inputs are invalid.
Unstructured edits, e.g. a templating approach, or a command to replace any target string in the character stream with some other string, aren't limited by any syntax or object structure.
Such powerful techniques are eschewed because
- There would be no way to say that a kustomization was correct without running it and checking the output.
- Errors in the output would be disconnected from the edit that caused it.
- They are toil to maintain by a rotating staff of operators.
Kustomizations are meant to be sharable and stackable. Imagine tracing down a problem rooted in a clever set of stacked regexp replacements performed by various overlays on some remote base.
Other tools (sed, jinja, erb, envsubst, helm, ksonnet, etc.) provide varying degrees of unstructured editting and/or embedded languages, and can be used instead of, or in a pipe with, kustomize.
kustomize supports the best practice of storing one's
entire configuration in a version control system.
Changing kustomize build configuration output as a result
of additional arguments or flags to build, or by
consulting shell environment variable values in build
code, would frustrate that goal.
kustomize insteads offers kustomization file edit
commands. Like any shell command, they can accept
environment variable arguments.
For example, to set the tag used on an image to match an environment variable, run
kustomize edit set image nginx:$MY_NGINX_VERSION
as part of some encapsulating work flow executed before
kustomize build.
kustomize supports the best practice of storing one's
entire configuration in a version control system.
Globbing the local file system for files not explicitly
declared in the kustomization file at kustomize build time
would violate that goal.
Allowing globbing in a kustomization file would also introduce the same problems as allowing globbing in java import declarations or BUILD/Makefile dependency rules.
kustomize will instead provide kustomization file editting
commands that accept globbed arguments, expand them at edit
time relative to the local file system, and store the resulting
explicit names into the kustomization file.