OpenAPI Server Code Generator

This package contains a set of utilities for generating Go boilerplate code for services based on OpenAPI 3.0 API definitions. When working with services, it's important to have an API contract which servers and clients both implement to minimize the chances of incompatibilities. It's tedious to generate Go models which precisely correspond to OpenAPI specifications, so let our code generator do that work for you, so that you can focus on implementing the business logic for your service.

We have chosen to use Chi as our HTTP routing engine, due to its speed, simplicity, and compatibility with net/http.

This package tries to be too simple rather than too generic, so we've made some design decisions in favor of simplicity, knowing that we can't generate strongly typed Go code for all possible OpenAPI Schemas.

This repository is a hard fork of deepmap/oapi-codegen. This new version plans to diverge from the original repository with different design goals and more emphasis on go-chi.

Overview

We're going to use the OpenAPI example of the Expanded Petstore in the descriptions below, please have a look at it.

In order to create a Go server to serve this exact schema, you would have to write a lot of boilerplate code to perform all the marshalling and unmarshalling into objects which match the OpenAPI 3.0 definition. The code generator in this directory does a lot of that for you. You would run it like so:

go install github.com/discord-gophers/goapi-gen@latest
goapi-gen --out petstore.gen.go petstore-expanded.yaml

Let's go through that petstore.gen.go file to show you everything which was generated.

Generated Server Boilerplate

The /components/schemas section in OpenAPI defines reusable objects, so Go types are generated for these. The Pet Store example defines Error, Pet, Pets and NewPet, so we do the same in Go:

// Type definition for component schema "Error"
type Error struct {
    Code    int32  `json:"code"`
    Message string `json:"message"`
}

// Type definition for component schema "NewPet"
type NewPet struct {
    Name string  `json:"name"`
    Tag  *string `json:"tag,omitempty"`
}

// Type definition for component schema "Pet"
type Pet struct {
    // Embedded struct due to allOf(#/components/schemas/NewPet)
    NewPet
    // Embedded fields due to inline allOf schema
    Id int64 `json:"id"`
}

// Type definition for component schema "Pets"
type Pets []Pet

It's best to define objects under /components field in the schema, since those will be turned into named Go types. If you use inline types in your handler definitions, we will generate inline, anonymous Go types, but those are more tedious to deal with since you will have to redeclare them at every point of use.

For each element in the paths map in OpenAPI, we will generate a Go handler function in an interface object. Here is the generated Go interface for our Chi server.

type ServerInterface interface {
    //  (GET /pets)
    FindPets(w http.ResponseWriter, r *http.Request, params FindPetsParams)
    //  (POST /pets)
    AddPet(w http.ResponseWriter, r *http.Request)
    //  (DELETE /pets/{id})
    DeletePet(w http.ResponseWriter, r *http.Request, id int64)
    //  (GET /pets/{id})
    FindPetById(w http.ResponseWriter, r *http.Request, id int64)
}

These are the functions which you will implement yourself in order to create a server conforming to the API specification.

Notice that FindPetById takes a parameter id int64. All path arguments will be passed as arguments to your function, since they are mandatory.

Remaining arguments can be passed in headers, query arguments or cookies. Those will be written to a params object. Look at the FindPets function above, it takes as input FindPetsParams, which is defined as follows:

// Parameters object for FindPets
type FindPetsParams struct {
   Tags  *[]string `json:"tags,omitempty"`
   Limit *int32   `json:"limit,omitempty"`
}

The HTTP query parameter limit turns into a Go field named Limit. It is passed by pointer, since it is an optional parameter. If the parameter is specified, the pointer will be non-nil, and you can read its value.

If you changed the OpenAPI specification to make the parameter required, the FindPetsParams structure will contain the type by value:

type FindPetsParams struct {
    Tags  *[]string `json:"tags,omitempty"`
    Limit int32     `json:"limit"`
}

Registering handlers

You can register handlers when generating a server with -generate server.

type PetStoreImpl struct {}
func (*PetStoreImpl) GetPets(w http.ResponseWriter, r *http.Request) {
    // Implement me
}

func SetupHandler() {
    var myApi PetStoreImpl

    r := chi.NewRouter()
    r.Mount("/", Handler(&myApi))
}

type PetStoreImpl struct {}
func (*PetStoreImpl) GetPets(w http.ResponseWriter, r *http.Request) {
    // Implement me
}

func SetupHandler() {
    var myApi PetStoreImpl

    http.Handle("/", Handler(&myApi))
}

Additional Properties in type definitions

OpenAPI Schemas implicitly accept additionalProperties, meaning that any fields provided, but not explicitly defined via properties on the schema are accepted as input, and propagated. When unspecified, the additionalProperties field is assumed to be true.

Additional properties are tricky to support in Go with typing, and require lots of boilerplate code, so in this library, we assume that additionalProperties defaults to false and we don't generate this boilerplate. If you would like an object to accept additionalProperties, specify a schema for additionalProperties.

Say we declared NewPet above like so:

NewPet:
  required:
    - name
  properties:
    name:
      type: string
    tag:
      type: string
  additionalProperties:
    type: string

The Go code for NewPet would now look like this:

// NewPet defines model for NewPet.
type NewPet struct {
	Name                 string            `json:"name"`
	Tag                  *string           `json:"tag,omitempty"`
	AdditionalProperties map[string]string `json:"-"`
}

The additionalProperties, of type string become map[string]string, which maps field names to instances of the additionalProperties schema.

// Getter for additional properties for NewPet. Returns the specified
// element and whether it was found
func (a NewPet) Get(fieldName string) (value string, found bool) {...}

// Setter for additional properties for NewPet
func (a *NewPet) Set(fieldName string, value string) {...}

// Override default JSON handling for NewPet to handle additionalProperties
func (a *NewPet) UnmarshalJSON(b []byte) error {...}

// Override default JSON handling for NewPet to handle additionalProperties
func (a NewPet) MarshalJSON() ([]byte, error) {...}w

There are many special cases for additionalProperties, such as having to define types for inner fields which themselves support additionalProperties, and all of them are tested via the internal/test/components schemas and tests. Please look through those tests for more usage examples.

Extensions

goapi-gen supports the following extended properties:

• x-go-type: specifies Go type name. It allows you to specify the type name for a schema, and will override any default value. When using this with external types the fields type,import are required while alias is only needed if your import collides with any existing imports. If no alias is given, the generator assumes that the import name is the last word from the import path (github.com/example/time => time). This can produce issues as some projects (like chi with /v5) have different import names than paths. To be sure, always declare an alias. For those cases, please attach a :[import-name] to the path like github.com/go-chi/chi/v5:chi, this will then also be imported as chi.* without the use of an alias. This extended property isn't supported in all parts of OpenAPI, so please refer to the spec as to where it's allowed. Swagger validation tools will flag incorrect usage of this property.

  components:
    schemas:
      Object:
        properties:
          name:
            type: string
            x-go-type: MyCustomString
          time:
            type: integer
            x-go-type:
              type: MyCustomTime
              import: github.com/example/time
              alias: time2

• x-go-extra-tags: adds extra Go field tags to the generated struct field. This is useful for interfacing with tag based ORM or validation libraries. The extra tags that are added are in addition to the regular json tags that are generated. If you specify your own json tag, you will override the default one.

  components:
    schemas:
      Object:
        properties:
          name:
            type: string
            x-go-extra-tags:
              tag1: value1
              tag2: value2

  In the example above, field name will be declared as:

  Name string `json:"name" tag1:"value1" tag2:"value2"`

• x-go-middlewares: specifies a list of tagged middlewares. These can be specific middlewares that are operation-specific, as well as path-specific. This is very useful when you want to give a specific routes middleware, but not to all operations. The middleware are always called in the order of definition. If the tagged middleware is not defined, panic will be called while calling Handler.

  /pets:
    x-go-middlewares: [validateJSON]
      get:
        x-go-middlewares: [limit]

  In the example above, the following middleware calls will be added to your handler:

  // Operation specific middleware
  handler = siw.Middlewares["validateJSON"](handler).ServeHTTP
  handler = siw.Middlewares["limit"](handler).ServeHTTP

• x-go-optional-value: boolean, forces the generator to output value types (as opposed to pointer nil-able types) for all optional fields. This is particularly useful for when there is no practical difference between an empty omitted string and an omitted nil *string.

  This property can go in either the property value or the object attribute itself:

  components:
    schemas:
      Object:
        x-go-optional-value: true # valid
        properties:
          name:
          type: string
          x-go-optional-value: false # valid, overrides

• x-go-string: boolean, makes the generator add a ,string attribute into the JSON struct tag of an object's field. This is useful for sending large numbers over JSON to JavaScript (or other dynamically-typed languages) where they would be parsed to floats otherwise.

• x-go-omitempty: boolean, makes the generator add a ,omitempty attribute into the JSON struct tag. Note that if a field doesn't appear in required, then it will have an omitempty by default and its type will be a nil-able pointer.

Using goapi-gen

Usage details

The default options for goapi-gen will generate everything; server, type definitions and embedded swagger spec, but you can generate subsets of those via the -generate flag. It defaults to types,server,spec, but you can specify any combination of those.

• types: generate all type definitions for all types in the OpenAPI spec. This will be everything under #components, as well as request parameter, request body, and response type objects.
• server: generate the Chi server boilerplate. This code is dependent on that produced by the types target.
• spec: embed the OpenAPI spec into the generated code as a gzipped blob. This
• skip-fmt: skip running goimports on the generated code. This is useful for debugging the generated file in case the spec contains weird strings.
• skip-prune: skip pruning unused components from the spec prior to generating the code.
• import-mapping: specifies a map of references external OpenAPI specs to go Go include paths. Please see below.

So, for example, if you would like to produce only the server code, you could run goapi-gen --generate types,server. You could generate types and server into separate files, but both are required for the server code.

goapi-gen can filter paths base on their tags in the openapi definition. Use either --include-tags or --exclude-tags followed by a comma-separated list of tags. For instance, to generate a server that serves all paths except those tagged with auth or admin, use the argument, --exclude-tags="auth,admin". To generate a server that only handles admin paths, use the argument --include-tags="admin". When neither of these arguments is present, all paths are generated.

goapi-gen can filter schemas based on the option --exclude-schemas, which is a comma separated list of schema names. For instance, --exclude-schemas=Pet,NewPet will exclude from generation schemas Pet and NewPet. This allow to have a in the same package a manually defined structure or interface and refer to it in the openapi spec.

Since go generate commands must be a single line, all the options above can make them pretty unwieldy, so you can specify all of the options in a configuration file via the --config option. Please see the test under /internal/test/externalref/ for an example. The structure of the file is as follows:

output: externalref.gen.go
package: externalref
generate:
  - types
  - skip-prune
import-mapping:
  ./packageA/spec.yaml: github.com/discord-gophers/goapi-gen/internal/test/externalref/packageA
  ./packageB/spec.yaml: github.com/discord-gophers/goapi-gen/internal/test/externalref/packageB

Have a look at parse.go to see all the fields on the configuration structure.

Import Mappings

OpenAPI specifications may contain references to other OpenAPI specifications, and we need some additional information in order to be able to generate correct Go code.

An external reference looks like this:

$ref: ./some_spec.yaml#/components/schemas/Type

We assume that you have already generated the boilerplate code for ./some_spec.yaml using goapi-gen, and you have a package which contains the generated code, let's call it github.com/discord-gophers/some-package. You need to tell goapi-gen that some_spec.yaml corresponds to this package, and you would do it by specifying this command line argument:

-import-mapping=./some_spec.yaml:github.com/discord-gophers/some-package

This tells us that in order to resolve references generated from some_spec.yaml we need to import github.com/discord-gophers/some-package. You may specify multiple mappings by comma separating them in the form key1:value1,key2:value2.

What's missing or incomplete

This code is still young, and not complete, since we're filling it in as we need it. We've not yet implemented several things:

• oneOf, anyOf are not supported with strong Go typing. This schema:

    schema:
      oneOf:
        - $ref: '#/components/schemas/Cat'
        - $ref: '#/components/schemas/Dog'
  

  will result in a Go type of interface{}. It will be up to you to validate whether it conforms to Cat and/or Dog, depending on the keyword. It's not clear if we can do anything much better here given the limits of Go typing.

  allOf is supported, by taking the union of all the fields in all the component schemas. This is the most useful of these operations, and is commonly used to merge objects with an identifier, as in the petstore-expanded example.

• patternProperties isn't yet supported and will exit with an error. Pattern properties were defined in JSONSchema, and the kin-openapi Swagger object knows how to parse them, but they're not part of OpenAPI 3.0, so we've left them out, as support is very complicated.

Making changes to code generation

After updating any files under the pkg/codegen/templates directory, run go generate ./..., and the templates will be updated accordingly.

Alternatively, you can provide custom templates to override built-in ones using the -templates flag specifying a path to a directory containing templates files. These files must be named identically to built-in template files (see pkg/codegen/templates/*.tmpl in the source code), and will be interpreted on-the-fly at run time. Example:

$ ls -1 my-templates/
typedef.tmpl
$ goapi-gen \
    -templates my-templates/ \
    -generate types \
    petstore-expanded.yaml