Building APIs in Go with OpenAPI: A Guide for Beginners

Introduction

Hi Codementor community! I’m Patricio, a seasoned backend engineer with over five years of experience specializing in Golang, RestAPI design, and clean architecture. My journey has been fueled by curiosity and a passion for building scalable, efficient solutions. Today, I’m excited to share insights into building APIs in Go using OpenAPI tools.

If you’re new to Golang or API development, this guide is for you. Together, we’ll design, generate, and implement a simple TODO list API using OpenAPI. Along the way, I’ll also highlight how I approach mentoring—providing practical advice, breaking down complex concepts, and fostering an environment where learning thrives.

Why OpenAPI and Go?

APIs are the backbone of modern applications, enabling seamless communication between services. OpenAPI simplifies API development by providing a clear, machine-readable contract for your endpoints.

Golang, with its simplicity and performance, is a great fit for building robust APIs. Pairing OpenAPI with Go allows us to focus on implementing logic without getting bogged down in boilerplate code.

A Simple TODO List API

Let’s dive into a practical example: building a TODO list API. This project demonstrates the full lifecycle of API development—from designing the contract to writing the implementation.

Step 1: Design the API Contract

The api.yaml file defines the structure of our API. Here’s an excerpt:

openapi: "3.0.0"
info:
  title: "TODO List API"
  version: "1.0.0"
paths:
  /todos:
    get:
      summary: "Retrieve all TODOs"
      responses:
        '200':
          description: "A list of TODO items."

This contract ensures everyone interacting with our API knows exactly how it works.

Step 2: Generate Boilerplate Code

Using oapi-codegen, we generate Go code for the API. This saves time and aligns implementation with the defined contract.

oapi-codegen -generate models,gin-server -package openapi -o ./openapi/api.gen.go api.yaml

Step 3: Implement the API

Here’s where the magic happens. Using Golang’s gin framework, we implement endpoints like retrieving, creating, and deleting TODOs. For example:

func (s *Server) GetTodos(c *gin.Context) {
    c.JSON(http.StatusOK, s.todos)
}

This method retrieves all TODO items stored in memory.

Step 4: Test and Iterate

With tools like restish or Postman, we test the API to ensure it behaves as expected:

restish get /todos

Mentorship Philosophy

My goal as a mentor is to empower learners by providing:

• Guidance on Real-World Projects: Practical, hands-on learning through projects like the TODO API.
• Tailored Learning Paths: Understanding your goals and crafting a roadmap that suits you.
• Supportive Environment: Encouraging questions and celebrating progress at every step.

Whether you’re a complete beginner or looking to refine your skills, I’ll work with you to unlock your potential.

Join Me on This Learning Journey

If you found this guide helpful, imagine what we can achieve together through one-on-one mentorship. Whether you’re diving into Golang, exploring Domain-Driven Design, or tackling real-world challenges, I’m here to guide you.

Check out my Codementor profile to schedule a session. Let’s build something amazing together!

Next Steps

Curious about the full implementation of the TODO list API? Check out the project repository here and try it yourself.

Got questions or suggestions? Drop a comment below—I’d love to hear from you!