Our approach to Golang APIs

It’s hard to mantain docs and code in sync. Best case scenario, you use one as the source of truth and generate the other. This works well for example for python where you can use apispec to annotate the API in a structured way. In golang, this was always a struggle. One of the favorite tools for this task is swag that can generate openapi specification from the code (based on annotations in comment). The issue is, using comments for specification doesn’t work well. The comments get very long, it’s hard to maintain readable structure, and you can’t distinguish between documentation (what will be public) and actual comments (internal to repository).

At SumUp we have recently have the biggest success with oapi-codegen. Here the order is inverted, you write the specification first and generate the code from there. oapi-codegen requires you to use either echo or chi framework, if neither suits you, you can still use it to at least generate the models.

Our approach currently looks like this:

First you add specification of your API in a spec package. Such as spec/openapi.yaml:

openapi: 3.0.0
info:
  title: Sample API
servers:
  - url: http://api.example.com/v1
paths:
  /user/{id}:
    get:
      summary: Get user.
      oprationId: GetUser
      responses:
        '200':
          content:
            application/json:
              schema:
                $ref: "#/components/schemas/User"
components:
  schemas:
    User:
      type: object
      properties:
        required:
          - email
        email:
          type: string

Next, you add gen.go file that will be used with go generate command to mantain the generated code up to date:

package spec

//go:generate go run github.com/deepmap/oapi-codegen/cmd/oapi-codegen --package spec -o spec.gen.go openapi.yaml

Now any time you update your specs you can run go generate ./... to also update the generated code. oapi-codegen will by default generate for you the types, client implementation, and server interface. You still need to implement the server side though, but that is fairly easy with all the generated code. You implement the generated interface:

package api

import (
	"github.com/labstack/echo/v4"

	"github.com/you/your_package/api/spec"
)

type Handler struct {}

func (h *Handler) GetUsers(c echo.Context, id string) error {
	return c.JSON(http.StatusOK, spec.User{
    Email: "[email protected]",
  })
}

And finally you register it on an echo router

package main

import (
	"github.com/labstack/echo/v4"

	"github.com/you/your_package/api"
	"github.com/you/your_package/api/spec"
)

func main() {
	e := echo.New()
  spec.RegisterHandlers(g, &api.Handler{})
  e.Start(":8080")
}

That’s it, you are ready to roll. This approach saved us a few headaches already.