Swagger UI and Go


What is Swagger?

Swagger is a suite of tools to generate and represent RESTful APIs. It promotes the OpenAPI 2.0 spec which describes an API using JSON representation in a specification file called swagger.json. Since YAML is a superset of JSON, the specification file can conform to YAML.

In my projects, I want to be able to generate swagger.json from an annotated codebase to serve using Swagger UI.

Swagger Tools

  • go-swagger
    • enables generating a Go client or server from an existing Swagger spec
    • can scan an annotated codebase and generate a Swagger spec
  • Swagger UI
    • serves visual documentation of an API based on a given Swagger spec

Generating a Swagger spec

go-swagger is a CLI tool. The following is an example command for generating a Swagger spec

swagger generate spec -o ./swagger.json

Installing Swagger UI in a Go application

Based on the installation docs Swagger UI can be served as part of an application using plain HTML/CSS/JS.

How to serve plain HTML Swagger UI

The folder /dist includes all the HTML, CSS and JS files needed to run SwaggerUI on a static website or CMS, without requiring NPM.

  1. Download the latest release.
  2. Copy the contents of the /dist folder to your server.
  3. Open index.html in your HTML editor and replace “https://petstore.swagger.io/v2/swagger.json” with the URL for your OpenAPI 3.0 spec.

What is the purpose?

Reading and writing code is a big part of my daily life and the opportunity to put things together in Go is a fun break from the enterprise application development I do at C Spire. Swagger UI is a tool we use in our Spring/Spring Boot applications to easily share our API documentation with each other and the teams which integrate with us. This post is my personal documentation for how to integrate a tool I know and love in a new programming language.


Authored by Michael Lamb.
Published on 22 October 2021.
Category: Golang


Using OAuth2 in Go


Reference

Go Reference - Http "net/http"

Go Reference - OAuth2 for Go "golang.org/x/oauth2"

Go Reference - OAuth2 (Google APIs) "golang.org/x/oauth2/google"

Authorization Server Flow

Diagram of OAuth authorization flow

  1. A Go client will serve a redirect to a browser. This redirect will include a random state variable and a following redirect URL back to the Go client from the authorizing server.
  2. The user enters credentials into the browser (user agent).
  3. The user agent send credentials to the authorization server (u := config.AuthCodeURL(oauthState) prepares the URL, a call to http.Redirect() accepts u and the server redirects the browser)
  4. Authorization server returns an authorization code (r.FormValue("code"))
  5. The Go client will exchange the authorization code for an access token (the client will store the access token in a cookie).
  6. The Go client uses the access token to access scoped resource servers.

A Simple OAuth Implementation

Download the oauth2 library for import.

go get golang.org/x/oauth2

The app structure I followed is pretty simple:

1. In main.go use net/http to start a server

main.go

package main

import (
"fmt"
"log"
"net/http"

"foundry-automations/handlers"
)

func main() {
server := &http.Server{
    Addr:    fmt.Sprintf(":8000"),
    Handler: handlers.New(),
}

log.Printf("Starting HTTP Server. Listening at %q", server.Addr)
if err := server.ListenAndServe(); err != http.ErrServerClosed {
    log.Printf("%v", err)
  } else {
    log.Println("Server closed!")
  }
}

2. Create a module /handlers and add two .go files

  • base.go
  • <service>.go where <service> is a meaningful representation of the authorization server you are relying on (e.g., google apis)

3. Create a Handler

In base.go create func New() http.Handler {} and set up a mux with two handlers, "/auth/<service>/login" and "/auth/<service>/callback", then return mux. The helper functions passed as the second argument of mux.HandleFunc will live in your <service>.go implementation

base.go

package handlers

import (
{
    "net/http"
)

func New() http.Handler {
mux := http.NewServeMux()
// Root
mux.Handle("/", http.FileServer(http.Dir("templates/")))

// // OauthGoogle
// mux.HandleFunc("/auth/google/login", oauthGoogleLogin)
// mux.HandleFunc("/auth/google/callback", oauthGoogleCallback)
mux.HandleFunc("/auth/pco/login", pcoLogin)
mux.HandleFunc("/auth/pco/callback", pcoCallback)

return mux
  }
}

Authored by Michael Lamb.
Published on 07 October 2021.
Category: Golang


Error Handling


I am continuing to learn more about Go and its history.

The language follows idioms.

Idioms of Go

  • get the job done
  • one way of doing things
  • be explicit
  • build things by composing them

Fundamentals of Software Development

One of the most important aspects of my job is writing meaningful error messages. It’s something a lot of people don’t ever stop to consider: where do error messages come from?

Software developers create tools used by others and by nature of the broad interconnected tooling systems in computers and the internet at-large it cannot be difficult to imagine how collaboration is vitally important. Error messaging is how a software developer communicates with their users about expected and unexpected behavior. I believe that software which does not provide meaningful error messages (here’s what went wrong, here’s how to fix it) is not well-written software.

Error handling is a fundamental skill for software engineering.

Error Handling in Go

Review the following go packages for an example of returning an error. Source from golang.org

greetings.go

package greetings

import (
    "errors"
    "fmt"
)

// Hello returns a greeting for the named person.
func Hello(name string) (string, error) {
    // If no name was given, return an error with a message.
    if name == "" {
        return "", errors.New("empty name")
    }

    // If a name was received, return a value that embeds the name
    // in a greeting message.
    message := fmt.Sprintf("Hi, %v. Welcome!", name)
    return message, nil
}

main.go

package main

import (
    "fmt"
    "log"

    "example.com/greetings"
)

func main() {
    // Set properties of the predefined Logger, including
    // the log entry prefix and a flag to disable printing
    // the time, source file, and line number.
    log.SetPrefix("greetings: ")
    log.SetFlags(0)

    // Request a greeting message.
    message, err := greetings.Hello("")
    // If an error was returned, print it to the console and
    // exit the program.
    if err != nil {
        log.Fatal(err)
    }

    // If no error was returned, print the returned message
    // to the console.
    fmt.Println(message)
}

Notes

  • the log and errors packages are golang builtins
  • I would want to use constants for error messages, e.g., errors.New(UNEXPECTED_ERROR) or errors.New(EXTERNAL_SERVICE_ERROR) however Go does not support this pattern since it alternates between camelCase and PascalCase to denote variables which are global to a file vs. global to a package (respectively)

Willow

willow


Authored by Michael Lamb.
Published on 04 October 2021.
Category: Golang



Michael Lamb is a software engineer for privately-held telecommunications and technology company C Spire, headquartered in Ridgeland, Mississippi. He is a resident of Jackson, Mississippi, with his two cats and many plants. He graduated from Mississippi State University with a bachelor of science in Software Engineering. He is happily employed full-time creating microservices using Java and Spring technologies but is open to consulting.
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