kube-cel

Kubernetes CEL extension functions for Rust, built on top of cel.

Implements the Kubernetes-specific CEL libraries defined in k8s.io/apiserver/pkg/cel/library and cel-go/ext, enabling client-side evaluation of CRD validation rules.

Installation

[dependencies]
kube-cel = "0.8"

kube-cel re-exports the cel crate it was built against as kube_cel::cel. Import cel types through that re-export rather than declaring a separate cel dependency, otherwise a version mismatch surfaces as a cryptic Context type error.

Usage

use kube_cel::KubeCelExt;
use kube_cel::cel::{Context, Program};

let ctx = Context::default().with_all();

// String functions
let result = Program::compile("'hello'.upperAscii()")
    .unwrap().execute(&ctx).unwrap();

// Quantity comparison
let result = Program::compile("quantity('1Gi').isGreaterThan(quantity('500Mi'))")
    .unwrap().execute(&ctx).unwrap();

// Semver
let result = Program::compile("semver('1.2.3').isLessThan(semver('2.0.0'))")
    .unwrap().execute(&ctx).unwrap();

CRD Validation Pipeline

With the validation feature, you can compile and evaluate x-kubernetes-validations CEL rules client-side — no API server required.

[dependencies]
kube-cel = { version = "0.8", features = ["validation"] }

use kube_cel::Validator;
use serde_json::json;

let schema = json!({
    "type": "object",
    "properties": {
        "spec": {
            "type": "object",
            "properties": {
                "replicas": {
                    "type": "integer",
                    "x-kubernetes-validations": [
                        {"rule": "self >= 0", "message": "replicas must be non-negative"}
                    ]
                }
            },
            "x-kubernetes-validations": [
                {"rule": "self.replicas >= 1", "message": "at least one replica"}
            ]
        }
    }
});

let object = json!({"spec": {"replicas": -1}});

let validator = Validator::new();
let errors = validator.validate(&schema, &object, None).unwrap_err();

assert_eq!(errors.len(), 2);
assert_eq!(errors[0].field_path, "spec");
assert_eq!(errors[1].field_path, "spec.replicas");

Validation entry points return Result<(), ValidationErrors>: Ok(()) when every rule passes (so a caller can ?-propagate failure), and Err(ValidationErrors) otherwise. ValidationErrors derefs to [ValidationError] and is iterable, so every individual failure stays inspectable; into_vec() recovers the owned Vec.

The validator walks the schema tree, compiles rules at each node, and evaluates them with self bound to the corresponding object value. Transition rules (referencing oldSelf) are supported by passing old_object.

Schema-aware `format` support

Fields with format: "date-time" or format: "duration" in the schema are automatically converted to CEL Timestamp / Duration values, matching K8s API server behavior:

let schema = json!({
    "type": "object",
    "properties": {
        "expiresAt": { "type": "string", "format": "date-time" },
        "timeout":   { "type": "string", "format": "duration" }
    },
    "x-kubernetes-validations": [
        {"rule": "self.expiresAt > timestamp('2024-01-01T00:00:00Z')", "message": "expired"},
        {"rule": "self.timeout <= duration('1h')", "message": "too long"}
    ]
});

Invalid strings gracefully fall back to Value::String.

Field name escaping

JSON field names that are CEL reserved words or contain special characters are automatically escaped when converting to CEL map keys, matching K8s API server behavior:

JSON field name	CEL access
`namespace`	`self.__namespace__`
`foo-bar`	`self.foo__dash__bar`
`a.b`	`self.a__dot__b`
`x/y`	`self.x__slash__y`
`my_field`	`self.my__field`

Schema defaults

Apply schema default values before validation, matching K8s API server behavior:

use kube_cel::apply_defaults;

let schema = json!({
    "type": "object",
    "properties": {
        "replicas": {"type": "integer", "default": 1},
        "strategy": {"type": "string", "default": "RollingUpdate"}
    }
});

let object = json!({"replicas": 3});
let defaulted = apply_defaults(&schema, &object);
// defaulted = {"replicas": 3, "strategy": "RollingUpdate"}

Or use the convenience method:

let result = Validator::new().validate_with_defaults(&schema, &object, None);

Root-level variables

Bind CRD-level apiVersion, apiGroup, kind variables for root-level rules:

use kube_cel::{Validator, RootContext};

let root_ctx = RootContext {
    api_version: "apps/v1".into(),
    api_group: "apps".into(),
    kind: "Deployment".into(),
};
let result = Validator::new().validate_with_context(&schema, &object, None, Some(&root_ctx));

ValidatingAdmissionPolicy (VAP)

Evaluate ValidatingAdmissionPolicy CEL expressions client-side — no API server required. Supports all VAP variables except authorizer.

use kube_cel::{VapEvaluator, VapExpression, AdmissionRequest};

let evaluator = VapEvaluator::builder()
    .object(json!({"spec": {"replicas": 3}}))
    .request(AdmissionRequest {
        operation: "CREATE".into(),
        namespace: "production".into(),
        ..Default::default()
    })
    .params(json!({"maxReplicas": 5}))
    .build();

let results = evaluator.evaluate(&[VapExpression {
    expression: "object.spec.replicas <= params.maxReplicas".into(),
    message: Some("too many replicas".into()),
    message_expression: None,
}]);

assert!(results[0].passed);

For repeated evaluation, pre-compile expressions:

let compiled = evaluator.compile_expressions(&expressions);
let results = evaluator.evaluate_compiled(&compiled); // no re-parsing

Static Analysis

Catch CEL rule issues before deployment — variable scope violations and cost budget warnings:

use kube_cel::{analyze_rule, ScopeContext};
use kube_cel::compile_schema;

let compiled = compile_schema(&schema);

let warnings = analyze_rule(
    "self.items.all(item, item.size() > 0)",
    &compiled,
    ScopeContext::CrdValidation,
);
// Warns: "list field has no maxItems bound" (may exceed K8s 1M cost budget)

ScopeContext::CrdValidation catches admission-only variables (request, object, etc.) used in CRD rules. ScopeContext::AdmissionPolicy allows the full VAP variable set.

Supported Functions

Strings

charAt, indexOf, lastIndexOf, lowerAscii, upperAscii, replace, split, substring, trim, join, reverse, strings.quote

Lists

isSorted, sum, min, max, indexOf, lastIndexOf, slice, sort, flatten, reverse, distinct, first, last, lists.range

Sets

sets.contains, sets.equivalent, sets.intersects

Regex

find, findAll

URLs

url, isURL, getScheme, getHost, getHostname, getPort, getEscapedPath, getQuery

IP / CIDR

ip, isIP, isIPv4, isIPv6, ip.isCanonical, family, isLoopback, isUnspecified, isLinkLocalMulticast, isLinkLocalUnicast, isGlobalUnicast, <IP>.string(), cidr, isCIDR, isCIDRv4, isCIDRv6, containsIP, containsCIDR, prefixLength, masked, <CIDR>.ip(), <CIDR>.string()

Semver

semver, isSemver, major, minor, patch, isGreaterThan, isLessThan, compareTo

Quantity

quantity, isQuantity, isInteger, asInteger, asApproximateFloat, sign, add, sub, isGreaterThan, isLessThan, compareTo

Format

<string>.format(<list>) with verbs: %s, %d, %f, %e, %b, %o, %x, %X

Named Format Validation

format.dns1123Label, format.dns1123Subdomain, format.dns1035Label, format.dns1035LabelPrefix, format.dns1123LabelPrefix, format.dns1123SubdomainPrefix, format.qualifiedName, format.labelValue, format.uri, format.uuid, format.byte, format.date, format.datetime, format.named, validate

// Returns optional: none = valid, of([...errors]) = invalid
// K8s pattern: !format.<name>().validate(value).hasValue()
let result = Program::compile("!format.dns1123Label().validate('my-name').hasValue()")
    .unwrap().execute(&ctx).unwrap();
// Value::Bool(true)

// Dynamic format lookup
let result = Program::compile("!format.named('uuid').validate('550e8400-e29b-41d4-a716-446655440000').hasValue()")
    .unwrap().execute(&ctx).unwrap();
// Value::Bool(true)

Math

math.ceil, math.floor, math.round, math.trunc, math.abs, math.sign, math.sqrt, math.isInf, math.isNaN, math.isFinite, math.bitAnd, math.bitOr, math.bitXor, math.bitNot, math.bitShiftLeft, math.bitShiftRight, math.greatest, math.least

Encoders

base64.decode, base64.encode

JSONPatch

jsonpatch.escapeKey

// RFC 6901: ~ → ~0, / → ~1
let result = Program::compile("jsonpatch.escapeKey('k8s.io/my~label')")
    .unwrap().execute(&ctx).unwrap();
// Value::String("k8s.io~1my~0label")

Feature Flags

Cargo features are the only granularity axis — there is no runtime per-library registration. All extension-function features are enabled by default. To narrow the set you must disable the defaults first:

# Correct: only string + list helpers.
kube-cel = { version = "0.8", default-features = false, features = ["strings", "lists"] }

# No-op narrowing: without `default-features = false` the list is ADDED to the
# already-complete default set, so you still get everything.
kube-cel = { version = "0.8", features = ["strings", "lists"] }

# Restore the whole surface (all 13 function groups + the validation engine)
# in one flag, e.g. after narrowing for a downstream build profile.
kube-cel = { version = "0.8", default-features = false, features = ["full"] }

Feature	Dependencies	Description
`strings`	-	String extension functions
`lists`	-	List extension functions
`sets`	-	Set operations
`regex_funcs`	`regex`	Regex find/findAll
`urls`	`url`	URL parsing and accessors
`ip`	`ipnet`	IP/CIDR parsing and operations
`semver_funcs`	`semver`	Semantic versioning
`format`	-	String formatting
`quantity`	-	Kubernetes resource quantities
`jsonpatch`	-	JSONPatch key escaping (RFC 6901)
`named_format`	-	Named format validation (`format.dns1123Label()`, etc.)
`math`	-	Math functions (`math.ceil`, `math.abs`, bitwise, etc.)
`encoders`	`base64`	Base64 encode/decode
`validation`	`serde_json`, `serde`, `chrono`	CRD validation pipeline, VAP evaluation, static analysis, schema defaults
`full`	(all of the above)	Umbrella: every extension-function group plus `validation`. Not in `default` (which is functions-only)

Versioning and stability

kube-cel is pre-1.0 and cannot reach 1.0 until the cel crate does. Its public signatures expose cel::Context and cel::Value, and a crate cannot be stable while its public dependencies are not (Rust API Guidelines, C-STABLE). Once cel reaches 1.0, kube-cel 1.x will track cel 1.y; a cel major bump forces a kube-cel major bump. While below 1.0, minor releases may contain breaking changes (the Cargo convention for 0.x).

Two stability tiers, by surface:

Tier 1 — registration (committed). The KubeCelExt trait (with_all/register_all), the pub use cel re-export, and the set of registered Kubernetes CEL functions. This is the crate's core identity and changes only deliberately.
Tier 2 — validation engine (evolving, validation feature). Validator, the VAP evaluator, static analysis, schema defaults, and the compilation/validation/vap/analysis types. Still maturing; expect its surface to change across pre-1.0 minors as the validation pipeline hardens.

apiserver divergence

The validation pipeline aims to return the same verdict the Kubernetes API server would, client-side. Where it cannot today, it diverges fail-closed — it reports an error rather than silently accepting — so a passing result is always at least as strict as the API server, never less. These divergences are pinned by tests/apiserver_divergence.rs.

Feature	API server	kube-cel	Direction
`<list>.sortBy(var, expr)`	evaluates	`UnsupportedReference`	fail-closed
`cel.bind(var, init, expr)`	evaluates	`UnsupportedReference`	fail-closed
Two-arg comprehensions (`all(i, v, …)`, `transformList`, `transformMap`; K8s 1.33+)	evaluates	`UnsupportedReference`	fail-closed
Schema nesting deeper than 64 levels	enforces (rejects over-limit schemas at registration)	`SchemaTooDeep` error	fail-closed
Rule whose `messageExpression` does not compile	rejects the CRD at registration	`CompilationFailure` (the rule is rejected, not just its message)	fail-closed
Authz library (`authorizer.*`)	evaluates against the cluster	not available	out of scope

The unsupported CEL macros above parse successfully but error at evaluation (cel 0.13 has no such reference), so they surface as the dedicated ErrorKind::UnsupportedReference — distinct from EvaluationError (reserved for genuine runtime errors in a supported rule), so a consumer can tell a kube-cel coverage gap apart from an actual rule failure. Single-argument comprehensions (list.all(x, …), map(x, …), etc.) are fully supported and do not diverge. The macro gaps lift once the cel crate gains compiler-macro support; the authz library requires a live API server and is out of scope for a client library.

License

Apache-2.0

Name		Name	Last commit message	Last commit date
Latest commit History 134 Commits
.github		.github
examples		examples
src		src
tests		tests
.gitignore		.gitignore
CHANGELOG.md		CHANGELOG.md
Cargo.toml		Cargo.toml
LICENSE		LICENSE
README.md		README.md
justfile		justfile
rustfmt.toml		rustfmt.toml

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kube-cel

Installation

Usage

CRD Validation Pipeline

Schema-aware format support

Field name escaping

Schema defaults

Root-level variables

ValidatingAdmissionPolicy (VAP)

Static Analysis

Supported Functions

Strings

Lists

Sets

Regex

URLs

IP / CIDR

Semver

Quantity

Format

Named Format Validation

Math

Encoders

JSONPatch

Feature Flags

Versioning and stability

apiserver divergence

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Schema-aware `format` support

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