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+---
+title: "Eschewed Features"
+linkTitle: "Eschewed Features"
+type: docs
+weight: 99
+description: >
+    Eschewed Features
+---
+
+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].
+
+## Removal directives
+
+`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.
+
+## Unstructured edits
+
+_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].
+Those can be verbose, so common patches,
+like adding labels or annotatations, get dedicated 
+transformer plugins - `LabelTransformer`,
+`AnnotationsTransformer`, etc.
+These accept relatively simple YAML configuration
+allowing easy targeting of any number of resources.
+
+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 reflection-style edits
+are called _replacements_.
+
+The above edits create valid output given valid input,
+and can provide syntactically and semantically
+informed error messages if inputs are invalid.
+
+_Unstructured edits_, edits that don't limit
+themselves to a syntax or object structure,
+come in many forms.  A common one in the 
+configuration domain is the template or
+parameterization approach.
+
+In this technique, the source
+material is sprinkled with strings of the
+form `${VAR}`.  A scanner replaces them
+with a value taken from a map using `VAR`
+as the map key. It's trivial to implement.
+
+kustomize eschews parameterization, because
+
+- The source yaml gets polluted with `$VARs`
+  and can no longed be applied as is
+  to the cluster (it _must_ be processed).
+- The source material is no longer structured,
+  making it unusable with any YAML processor.
+  It's no longer _data_, it's now logic that
+  must be compiled.
+- Errors in the output are disconnected from
+  the edit that caused it.
+- The input becomes [unintelligible] as the project
+  scales in any number of dimensions (resource
+  count, cluster count, environment count, etc.)
+    
+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.
+We've used such systems, and never want to again.
+
+Other tools (sed, jinja, erb, envsubst, kafka, 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.  If you want to
+go all-in on _configuration as a language_, consider [cue].
+
+kustomize is going to stick to YAML in / YAML out.
+
+## Build-time side effects from CLI args or env variables
+
+`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`.
+
+
+## Globs in kustomization files
+
+`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.
+
+[base]: /kustomize/api-reference/glossary#base
+[DAM]: /kustomize/api-reference/glossary#declarative-application-management
+[java import]: https://www.codebyamir.com/blog/pitfalls-java-import-wildcards
+[JSON patches]: /kustomize/api-reference/glossary#patchjson6902
+[kustomization]: /kustomize/api-reference/glossary#kustomization
+[OTS workflow]: /kustomize/api-reference/glossary#off-the-shelf-configuration
+[SMP patches]: /kustomize/api-reference/glossary#patchstrategicmerge
+[parameterization pitfall discussion]: https://github.com/kubernetes/community/blob/master/contributors/design-proposals/architecture/declarative-application-management.md#parameterization-pitfalls
+[unintelligible]: https://github.com/helm/charts/blob/e002378c13e91bef4a3b0ba718c191ec791ce3f9/stable/artifactory/templates/artifactory-deployment.yaml
+[cue]: https://cuelang.org/