Kube API Linter (KAL) is a Golang based linter for Kubernetes API types. It checks for common mistakes and enforces best practices. The rules implemented by the Kube API Linter, are based on the Kubernetes API Conventions.
Kube API Linter is aimed at being an assistant to API review, by catching the mechanical elements of API review, and allowing reviewers to focus on the more complex aspects of API design.
Kube API Linter ships as a golangci-lint plugin.
To install the golangci-lint plugin, first you must have golangci-lint installed.
If you do not have golangci-lint installed, review the golangci-lint install guide.
You will need to create a .custom-gcl.yml file to describe the custom linters you want to run. The following is an example of a .custom-gcl.yml file:
version: v1.62.0
name: golangci-kube-api-linter
destination: ./bin
plugins:
- module: 'sigs.k8s.io/kube-api-linter'
version: 'v0.0.0' # Replace with the latest versionOnce you have created the custom configuration file, you can run the following command to build the custom golangci-kal binary:
golangci-lint customThe output binary will be a combination of the initial golangci-lint binary and the Kube API linter plugin.
This means that you can use any of the standard golangci-lint configuration or flags to run the binary, but may also include the Kube API Linter rules.
If you wish to only use the Kube API Linter rules, you can configure your .golangci.yml file to only run the Kube API Linter:
linters-settings:
custom:
kubeapilinter:
type: "module"
description: Kube API LInter lints Kube like APIs based on API conventions and best practices.
settings:
linters: {}
lintersConfig: {}
linters:
disable-all: true
enable:
- kubeapilinter
# To only run Kube API Linter on specific path
issues:
exclude-rules:
- path-except: "api/*"
linters:
- kubeapilinterThe settings for Kube API Linter are based on the GolangCIConfig struct and allow for finer control over the linter rules.
If you wish to use the Kube API Linter in conjunction with other linters, you can enable the Kube API Linter in the .golangci.yml file by ensuring that kubeapilinter is in the linters.enabled list.
To provide further configuration, add the custom.kubeapilinter section to your linter-settings as per the example above.
Where fixes are available within a rule, these can be applied automatically with the --fix flag.
golangci-kube-api-linter run path/to/api/types --fixSince VSCode already integrates with golangci-lint via the Go extension, you can use the golangci-kal binary as a linter in VSCode.
If your project authors are already using VSCode and have the configuration to lint their code when saving, this can be a seamless integration.
Ensure that your project setup includes building the golangci-kube-api-linter binary, and then configure the go.lintTool and go.alternateTools settings in your project .vscode/settings.json file.
{
"go.lintTool": "golangci-lint",
"go.alternateTools": {
"golangci-lint": "${workspaceFolder}/bin/golangci-kube-api-linter",
}
}Alternatively, you can also replace the binary with a script that runs the golangci-kube-api-linter binary,
allowing for customisation or automatic copmilation of the project should it not already exist.
{
"go.lintTool": "golangci-lint",
"go.alternateTools": {
"golangci-lint": "${workspaceFolder}/hack/golangci-lint.sh",
}
}The conditions linter checks that Conditions fields in the API types are correctly formatted.
The Conditions field should be a slice of metav1.Condition with the following tags and markers:
// +listType=map
// +listMapKey=type
// +patchStrategy=merge
// +patchMergeKey=type
// +optional
Conditions []metav1.Condition `json:"conditions,omitempty" patchStrategy:"merge" patchMergeKey:"type" protobuf:"bytes,1,opt,name=conditions"`Conditions are idiomatically the first field within the status struct, and the linter will highlight when the Conditions are not the first field.
Protobuf tags and patch strategy are required for in-tree API types, but not for CRDs.
When linting CRD based types, set the useProtobuf and usePatchStrategy config option to Ignore or Forbid.
lintersConfig:
conditions:
isFirstField: Warn | Ignore # The policy for the Conditions field being the first field. Defaults to `Warn`.
useProtobuf: SuggestFix | Warn | Ignore | Forbid # The policy for the protobuf tag on the Conditions field. Defaults to `SuggestFix`.
usePatchStrategy: SuggestFix | Warn | Ignore | Forbid # The policy for the patchStrategy tag on the Conditions field. Defaults to `SuggestFix`.The conditions linter can automatically fix the tags on the Conditions field.
When they do not match the expected format, the linter will suggest to update the tags to match the expected format.
For CRDs, protobuf tags and patch strategy are not expected.
By setting the useProtobuf/usePatchStrategy configuration to Ignore, the linter will not suggest to add the protobuf/patch strategy tag to the Conditions field tags.
By setting the useProtobuf/usePatchStrategy configuration to Forbid, the linter will suggest to remove the protobuf/patch strategy tag from the Conditions field tags.
The linter will also suggest to add missing markers. If any of the 5 markers in the example above are missing, the linter will suggest to add them directly above the field.
When usePatchStrategy is set to Ignore, the linter will not suggest to add the patchStrategy and patchMergeKey tags to the Conditions field markers.
When usePatchStrategy is set to Forbid, the linter will suggest to remove the patchStrategy and patchMergeKey tags from the Conditions field markers.
The commentstart linter checks that all comments in the API types start with the serialized form of the type they are commenting on.
This helps to ensure that generated documentation reflects the most common usage of the field, the serialized YAML form.
The commentstart linter can automatically fix comments that do not start with the serialized form of the type.
When the json tag is present, and matches the first word of the field comment in all but casing, the linter will suggest that the comment be updated to match the json tag.
The integers linter checks for usage of unsupported integer types.
Only int32 and int64 types should be used in APIs, and other integer types, including unsigned integers are forbidden.
The jsontags linter checks that all fields in the API types have a json tag, and that those tags are correctly formatted.
The json tag for a field within a Kubernetes API type should use a camel case version of the field name.
The jsontags linter checks the tag name against the regex "^[a-z][a-z0-9]*(?:[A-Z][a-z0-9]*)*$" which allows consecutive upper case characters, to allow for acronyms, e.g. requestTTL.
lintersConfig:
jsonTags:
jsonTagRegex: "^[a-z][a-z0-9]*(?:[A-Z][a-z0-9]*)*$" # Provide a custom regex, which the json tag must match.The maxlength linter checks that string and array fields in the API are bounded by a maximum length.
For strings, this means they have a +kubebuilder:validation:MaxLength marker.
For arrays, this means they have a +kubebuilder:validation:MaxItems marker.
For arrays of strings, the array element should also have a +kubebuilder:validation:MaxLength marker if the array element is a type alias,
or +kubebuilder:validation:items:MaxLenth if the array is an element of the built-in string type.
Adding maximum lengths to strings and arrays not only ensures that the API is not abused (used to store overly large data, reduces DDOS etc.), but also allows CEL validation cost estimations to be kept within reasonable bounds.
The nobools linter checks that fields in the API types do not contain a bool type.
Booleans are limited and do not evolve well over time. It is recommended instead to create a string alias with meaningful values, as an enum.
The nofloats linter checks that fields in the API types do not contain a float32 or float64 type.
Floating-point values cannot be reliably round-tripped without changing and have varying precision and representation across languages and architectures. Their use should be avoided as much as possible. They should never be used in spec.
The nomaps linter checks the usage of map types.
Maps are discouraged apart from map[string]string which is used for labels and annotations in Kubernetes APIs since it's hard to distinguish between structs and maps in spec. Instead of plain map, lists of named subobjects are preferred.
lintersConfig:
nomaps:
policy: Enforce | AllowStringToStringMaps | Ignore # Determines how the linter should handle maps of simple types. Defaults to AllowStringToStringMaps.The nophase linter checks that the fields in the API types don't contain a 'Phase', or any field which contains 'Phase' as a substring, e.g MachinePhase.
The optionalorrequired linter checks that all fields in the API types are either optional or required, and are marked explicitly as such.
The linter expects to find a comment marker // +optional or // +required within the comment for the field.
It also supports the // +kubebuilder:validation:Optional and // +kubebuilder:validation:Required markers, but will suggest to use the // +optional and // +required markers instead.
If you prefer to use the Kubebuilder markers instead, you can change the preference in the configuration.
The optionalorrequired linter also checks for the presence of optional or required markers on type declarations, and forbids this pattern.
lintersConfig:
optionalOrRequired:
preferredOptionalMarker: optional | kubebuilder:validation:Optional # The preferred optional marker to use, fixes will suggest to use this marker. Defaults to `optional`.
preferredRequiredMarker: required | kubebuilder:validation:Required # The preferred required marker to use, fixes will suggest to use this marker. Defaults to `required`.The optionalorrequired linter can automatically fix fields that are using the incorrect form of either the optional or required marker.
It will also remove the secondary marker where both the preferred and secondary marker are present on a field.
The requiredfields linter checks that fields that are marked as required, follow the convention of not being pointers,
and not having an omitempty value in their json tag.
lintersConfig:
requiredFields:
pointerPolicy: Warn | SuggestFix # The policy for pointers in required fields. Defaults to `SuggestFix`.The requiredfields linter can automatically fix fields that are marked as required, but are pointers.
It will suggest to remove the pointer from the field, and update the json tag to remove the omitempty value.
If you prefer not to suggest fixes for pointers in required fields, you can change the pointerPolicy to Warn.
The linter will then only suggest to remove the omitempty value from the json tag.
The statussubresource linter checks that the status subresource is configured correctly for
structs marked with the kubebuilder:object:root:=true marker. Correct configuration is that
when there is a status field the kubebuilder:subresource:status marker is present on the struct
OR when the kubebuilder:subresource:status marker is present on the struct there is a status field.
This linter is not enabled by default as it is only applicable to CustomResourceDefinitions.
In the case where there is a status field present but no kubebuilder:subresource:status marker, the
linter will suggest adding the comment // +kubebuilder:subresource:status above the struct.
New linters can be added by following the New Linter guide.
KAL is licensed under the Apache License, Version 2.0. See LICENSE for the full license text.