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docs/concept: how CUE works with OpenAPI
This adds a concept guide demonstrating how CUE works with OpenAPI. Much of the comparative discussion from https://cuelang.org/docs/concept/schema-definition-use-case/#json-schema--openapi is omitted, keeping the document focused on the pragmatic and possible. cue-lang/cue#3133 was opened as part of this change, which asks the project to support full round-tripping from OpenAPI to CUE and back again; or to decide and document which OpenAPI elements aren't supported. For cue-lang/docs-and-content#72 Preview-Path: /docs/concept/how-cue-works-with-openapi/ Signed-off-by: Jonathan Matthews <[email protected]> Change-Id: I9f9968d381d516f0e189616871919ddb063f11d3 Reviewed-on: https://review.gerrithub.io/c/cue-lang/cuelang.org/+/1192024 Reviewed-by: Daniel Martí <[email protected]> TryBot-Result: CUEcueckoo <[email protected]> Reviewed-by: Paul Jolly <[email protected]>
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Diff for: ‎content/docs/concept/how-cue-works-with-openapi/en.md

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---
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title: How CUE works with OpenAPI
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tags:
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- encodings
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- cue command
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authors:
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- jpluscplusm
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- myitcv
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toc_hide: true
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---
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{{{with _script_ "en" "HIDDEN_ set up caches"}}}
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export GOMODCACHE=/caches/gomodcache
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export GOCACHE=/caches/gobuild
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{{{end}}}
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CUE works with the
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[OpenAPI 3.0.0 standard](https://github.com/OAI/OpenAPI-Specification/tree/3.0.0)
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for the description of REST APIs by supporting the use and import of OpenAPI
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`components.schemas` data schemas, and the export of CUE definitions into the
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same namespace.
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CUE is usually more clear and concise than the equivalent OpenAPI. However,
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given that they meet different needs for different types of users, CUE's
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ability to round-trip between CUE and OpenAPI's data schema subset acts as a
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useful bridge between their two worlds.
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## Reading and writing OpenAPI with the `cue` command
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The `cue` command can convert CUE schemas into OpenAPI's `components.schemas`.
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CUE files can be converted into OpenAPI so long as they only specify
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definitions and metadata (`info`, `$version`, etc) at their top-level.
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Let's start with a trivial CUE schema that we want to convert to OpenAPI:
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{{{with upload "en" "schema.cue"}}}
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-- schema.cue --
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// A schema for the pet API.
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package api
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$version: "v1.2.3"
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// A Pet is a pet that we handle.
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#Pet: {
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// A pet has a name.
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name!: string
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// We only handle certain kinds of pets.
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kind!: #Kind
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// Centenarian pets are not handled.
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age?: uint & <100 // TODO: increase limit if the tortoise proposal is accepted.
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...
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}
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// Kind encodes the different pets we handle.
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#Kind: "cat" | "dog" | "goldfish"
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{{{end}}}
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The [`cue def`]({{< relref "docs/reference/cli/cue-def" >}}) command normalizes
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the schema, and optionally converts it into another format:
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{{{with script "en" "cue def schema.cue"}}}
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cue def schema.cue -o api.pet.yaml --out openapi+yaml
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{{{end}}}
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{{{with _script_ "en" "HIDDEN_ move api.pet.yaml sideways"}}}
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mv api.pet.yaml .api.pet.yaml
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{{{end}}}
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The OpenAPI `info.title` field can be extracted from the top-level CUE comment,
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or can be specified directly.
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The same goes for OpenAPI's `info.version` field, which is extracted from CUE's
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top-level `$version` field if not specified directly.
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Be aware of just how *long* an equivalent OpenAPI definition can become - not
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all formats possess CUE's succinctness and economy of expression!
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The `cue def` command creates this file:
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{{{with upload "en" "api.pet.yaml"}}}
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-- api.pet.yaml --
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openapi: 3.0.0
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info:
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title: A schema for the pet API.
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version: v1.2.3
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paths: {}
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components:
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schemas:
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Kind:
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description: Kind encodes the different pets we handle.
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type: string
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enum:
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- cat
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- dog
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- goldfish
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Pet:
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description: A Pet is a pet that we handle.
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type: object
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required:
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- name
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- kind
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properties:
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name:
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description: A pet has a name.
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type: string
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kind:
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$ref: '#/components/schemas/Kind'
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age:
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description: Centenarian pets are not handled.
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type: integer
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minimum: 0
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maximum: 100
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exclusiveMaximum: true
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{{{end}}}
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{{{with _script_ "en" "HIDDEN_ diff api.pet.yaml"}}}
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diff -wu api.pet.yaml .api.pet.yaml
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rm .api.pet.yaml # tidy up
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{{{end}}}
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Because CUE is more expressive than OpenAPI, it isn't possible to generate a
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precise OpenAPI equivalent for *every* CUE schema. CUE does the best conversion
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it can, limited by what OpenAPI's data schemas can represent.
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[`cue import`]({{< relref "docs/reference/cli/cue-import" >}}) can perform the
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reverse operation, taking the OpenAPI definition emitted above and converting
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it back to CUE:
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{{{with script "en" "import openapi yaml"}}}
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cue import -p api api.pet.yaml
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{{{end}}}
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{{{with _script_ "en" "HIDDEN_ move api.pet.cue sideways"}}}
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mv api.pet.cue .api.pet.cue
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{{{end}}}
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This produces the following CUE, which is as close to the original `schema.cue`
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as OpenAPI's capabilities currently permit:
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{{{with upload "en" "api.pet.cue"}}}
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-- api.pet.cue --
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// A schema for the pet API.
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package api
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info: {
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title: *"A schema for the pet API." | string
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version: *"v1.2.3" | string
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}
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// Kind encodes the different pets we handle.
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#Kind: "cat" | "dog" | "goldfish"
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// A Pet is a pet that we handle.
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#Pet: {
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// A pet has a name.
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name: string
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kind: #Kind
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// Centenarian pets are not handled.
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age?: int & >=0 & <100
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...
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}
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{{{end}}}
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{{{with _script_ "en" "HIDDEN_ diff api.pet.cue"}}}
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diff -wu api.pet.cue .api.pet.cue
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rm .api.pet.cue # tidy up
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{{{end}}}
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## Using CUE's Go API
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CUE can also generate OpenAPI through its Go API.
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Generating an OpenAPI definition can be as simple as this:
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{{{with _script_ "en" "go mod init"}}}
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go mod init mod.example
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{{{end}}}
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{{{with upload "en" "go emit openapi main"}}}
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-- main.go --
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package main
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import (
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"bytes"
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"encoding/json"
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"fmt"
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"log"
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"cuelang.org/go/cue/cuecontext"
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"cuelang.org/go/cue/load"
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"cuelang.org/go/encoding/openapi"
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)
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func main() {
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ctx := cuecontext.New()
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insts := load.Instances([]string{"schema.cue"}, nil)
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v := ctx.BuildInstance(insts[0])
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// Generate the OpenAPI schema from the value loaded from schema.cue
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b, err := openapi.Gen(v, nil)
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if err != nil {
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log.Fatal(err)
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}
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// Render as indented JSON
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var out bytes.Buffer
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if err := json.Indent(&out, b, "", " "); err != nil {
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log.Fatal(err)
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}
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fmt.Printf("%s\n", out.Bytes())
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}
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{{{end}}}
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{{{with _script_ "en" "go mod tidy"}}}
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#ellipsis 0
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go get cuelang.org/go@${CUELANG_CUE_LATEST}
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#ellipsis 0
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go mod tidy
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{{{end}}}
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{{{with script "en" "go run"}}}
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#ellipsis 10
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go run .
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{{{end}}}
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The [`encoding/openapi`](https://pkg.go.dev/cuelang.org/go/encoding/openapi)
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package provides options to make a definition self-contained, to filter
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constraints, and so on. The *expanding references* option enables the
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"Structural OpenAPI" form required by CRDs targeting Kubernetes version 1.15
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and later.
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## Related content
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- CUE supports OpenAPI's `components.schemas` namespace, and metadata such as the `info` field --
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{{<issue 3133/>}} tracks the support of other namespaces defined by the OpenAPI standard
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- The [OpenAPI 3.0.0 specification](https://github.com/OAI/OpenAPI-Specification/tree/3.0.0)
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- {{< linkto/related/reference "cli/cue-def" >}}
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- {{< linkto/related/reference "cli/cue-import" >}}
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- The [`encoding/openapi`](https://pkg.go.dev/cuelang.org/go/encoding/openapi) package

Diff for: ‎content/docs/concept/how-cue-works-with-openapi/gen_cache.cue

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package site
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{
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content: {
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docs: {
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concept: {
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"how-cue-works-with-openapi": {
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page: {
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cache: {
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upload: {
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"schema.cue": "IhWYgY2d4XJ14dVZmRAuFEOvvNEGp0VQ4d4KmUiGPOI="
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"api.pet.yaml": "a59sa6kiZ8MIhIuFnjuABOwBKtJb6/0GvCczVlyVRrc="
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"api.pet.cue": "pfphGZPbu+Bc4/4G9UHYevOhe+M796iWteICO8ORgbg="
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"go emit openapi main": "iJ3M385hdYxCdbNbUSo2fUp8UxBgDb0Kra0vI/FTaFE="
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}
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multi_step: {
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hash: "3AL269A7I4O2UGG9L7R6CMJ7NQ924KQM0LG8COQNQG9S8BUJ8VEG===="
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scriptHash: "5JNL3Q47AL0NUVFJA6OLQ28R6PVKQCQTRNKUO3BRGGFHPLF77K4G===="
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steps: [{
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doc: ""
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cmd: "export GOMODCACHE=/caches/gomodcache"
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exitCode: 0
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output: ""
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}, {
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doc: ""
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cmd: "export GOCACHE=/caches/gobuild"
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exitCode: 0
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output: ""
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}, {
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doc: ""
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cmd: "cue def schema.cue -o api.pet.yaml --out openapi+yaml"
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exitCode: 0
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output: ""
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}, {
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doc: ""
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cmd: "mv api.pet.yaml .api.pet.yaml"
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exitCode: 0
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output: ""
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}, {
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doc: ""
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cmd: "diff -wu api.pet.yaml .api.pet.yaml"
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exitCode: 0
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output: ""
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}, {
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doc: ""
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cmd: "rm .api.pet.yaml # tidy up"
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exitCode: 0
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output: ""
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}, {
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doc: ""
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cmd: "cue import -p api api.pet.yaml"
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exitCode: 0
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output: ""
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}, {
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doc: ""
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cmd: "mv api.pet.cue .api.pet.cue"
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exitCode: 0
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output: ""
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}, {
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doc: ""
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cmd: "diff -wu api.pet.cue .api.pet.cue"
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exitCode: 0
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output: ""
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}, {
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doc: ""
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cmd: "rm .api.pet.cue # tidy up"
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exitCode: 0
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output: ""
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}, {
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doc: ""
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cmd: "go mod init mod.example"
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exitCode: 0
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output: """
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go: creating new go.mod: module mod.example
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"""
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}, {
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doc: "#ellipsis 0"
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cmd: "go get cuelang.org/[email protected]"
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exitCode: 0
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output: """
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...
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"""
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}, {
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doc: "#ellipsis 0"
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cmd: "go mod tidy"
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exitCode: 0
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output: """
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...
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"""
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}, {
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doc: "#ellipsis 10"
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cmd: "go run ."
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exitCode: 0
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output: """
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{
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"openapi": "3.0.0",
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"info": {
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"title": "A schema for the pet API.",
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"version": "v1.2.3"
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},
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"paths": {},
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"components": {
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"schemas": {
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"Kind": {
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...
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"""
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}]
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}
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}
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}
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}
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}
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}
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}
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}

Diff for: ‎content/docs/concept/how-cue-works-with-openapi/page.cue

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package site
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content: docs: concept: "how-cue-works-with-openapi": page: _

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