oapi-codegen

README

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 microservices, it's important to have an API contract which servers and clients both imprement 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 Echo as our HTTP routing engine, due to its speed and simplicity for the generated stubs.

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.

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 get github.com/deepmap/oapi-codegen/cmd/oapi-codegen
oapi-codegen petstore-expanded.yaml  > petstore.gen.go

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,omitempty:"tag"`
}

// 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 server.

type ServerInterface interface {
    //  (GET /pets)
    FindPets(ctx echo.Context, params FindPetsParams) error
    //  (POST /pets)
    AddPet(ctx echo.Context) error
    //  (DELETE /pets/{id})
    DeletePet(ctx echo.Context, id int64) error
    //  (GET /pets/{id})
    FindPetById(ctx echo.Context, id int64) error
}

These are the functions which you will implement yourself in order to create a server conforming to the API specification. Normally, all the arguments and parameters are stored on the echo.Context in handlers, so we do the tedious work of of unmarshaling the JSON automatically, simply passing values into your handlers.

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"`
}

The usage of Echo is out of scope of this doc, but once you have an echo instance, we generate a utility function to help you associate your handlers with this autogenerated code. For the pet store, it looks like this:

func RegisterHandlers(router codegen.EchoRouter, si ServerInterface) {
	wrapper := ServerInterfaceWrapper{
		Handler: si,
	}
	router.GET("/pets", wrapper.FindPets)
	router.POST("/pets", wrapper.AddPet)
	router.DELETE("/pets/:id", wrapper.DeletePet)
	router.GET("/pets/:id", wrapper.FindPetById)
}

The wrapper functions referenced above contain generated code which pulls parameters off the Echo request context, and unmarshals them into Go objects.

You would register the generated handlers as follows:

func SetupHandler() {
    var myApi PetStoreImpl  // This implements the pet store interface
    e := echo.New()
    petstore.RegisterHandlers(e, &myApi)
    ...
}

What's missing

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.

• additionalProperties isn't supported, and will exit with an error. This should be possible to support in the future via a map[string]interface{} or map[string]string.

• patternProperties isn't yet supported and will exit with an error. This too should be possible to implement.

• Cookie parameters are not yet supported.

Making changes to code generation

The code generator uses a tool to inline all the template definitions into code, so that we don't have to deal with the location of the template files. When you update any of the files under the templates/ directory, you will need to regenerate the template inlines:

templates -s pkg/codegen/templates/ > pkg/codegen/templates/templates.gen.go

All this command does is inline the files ending in .tmpl into the specified Go file. This command is found here:

go get github.com/cyberdelia/templates

You can also run go generate, since we've set up those hooks.