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Alex Spinov
Alex Spinov

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CUE Has a Free API: The Configuration Language That Validates, Generates, and Queries Your Data Like a Type System for JSON

#cue #programming #config #devops

Your Kubernetes YAML has 500 lines and a typo in a resource limit crashes production. CUE adds types, constraints, and validation to configuration — catch errors before they deploy.

What Is CUE?

CUE is a data validation language that makes JSON and YAML safe. It combines data definition, validation, code generation, and querying in one tool. Originally designed by a Go team member, it brings the rigor of a type system to configuration files.

Why CUE Is Different

  • Types as values: Define schemas and data in the same language
  • Validation: Constraint-based validation with clear error messages
  • Code generation: Generate Go, JSON Schema, OpenAPI from CUE
  • Querying: Query and transform data
  • Kubernetes: First-class support for K8s manifests
  • Hermetic: Deterministic evaluation, no side effects
  • Completely free and open source

Quick Start

Install CUE:

brew install cue-lang/tap/cue
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Define a schema with constraints:

// schema.cue
package config

#Service: {
  name:     string & =~"^[a-z][a-z0-9-]*$"
  port:     int & >=1 & <=65535
  replicas: int & >=1 & <=100
  image:    string & =~"^[a-z0-9/:-]+$"
  resources: {
    cpu:    string & =~"^[0-9]+m$"
    memory: string & =~"^[0-9]+(Mi|Gi)$"
  }
  env: [...{name: string, value: string}]
}
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Use the schema:

// prod.cue
package config

api: #Service & {
  name:     "api-server"
  port:     8080
  replicas: 3
  image:    "myapp/api:v1.2.3"
  resources: {
    cpu:    "500m"
    memory: "256Mi"
  }
  env: [{
    name:  "DATABASE_URL"
    value: "postgres://db:5432/prod"
  }]
}
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Validate:

# This succeeds
cue vet prod.cue schema.cue

# This fails with clear error:
cue vet bad.cue schema.cue
# api.port: invalid value 99999 (out of bound <=65535)
# api.replicas: invalid value -1 (out of bound >=1)
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Export to JSON/YAML:

cue export prod.cue --out json
cue export prod.cue --out yaml
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Why Teams Choose CUE

A platform team managed 200+ Kubernetes manifests in YAML. A developer set memory limits to "256" instead of "256Mi", causing OOM kills in production. After migrating to CUE with typed schemas, every manifest is validated before applying. Config-related incidents dropped from monthly to never.

Who Is This For?

  • Platform engineers managing Kubernetes configurations
  • DevOps teams wanting validated, typed infrastructure configs
  • API developers needing schema validation for OpenAPI specs
  • Anyone tired of YAML bugs causing production issues

Start Validating

CUE brings type safety to configuration. Define schemas once, validate everywhere.

Need help with configuration management or Kubernetes? I build custom DevOps solutions — reach out to discuss your project.

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