Source Code This is a step by step guide on how to receive webhooks from QStash in your Golang application running on fly.io.

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0. Prerequisites

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1. Create a new project

Let’s create a new folder called flyio-go and initialize a new project. 
mkdir flyio-go
cd flyio-go
go mod init flyio-go

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2. Creating the main function

In this example we will show how to receive a webhook from QStash and verify the signature using the popular golang-jwt/jwt library. First, let’s import everything we need: 
package main

import (
	"crypto/sha256"
	"encoding/base64"
	"fmt"
	"github.com/golang-jwt/jwt/v4"
	"io"
	"net/http"
	"os"
	"time"
)
Next we create main.go. Ignore the verify function for now. We will add that next. In the handler we will prepare all necessary variables that we need for verification. This includes the signature and the signing keys. Then we try to verify the request using the current signing key and if that fails we will try the next one. If the signature could be verified, we can start processing the request. 
func main() {
	port := os.Getenv("PORT")
	if port == "" {
		port = "8080"
	}

	http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
		defer r.Body.Close()

		currentSigningKey := os.Getenv("QSTASH_CURRENT_SIGNING_KEY")
		nextSigningKey := os.Getenv("QSTASH_NEXT_SIGNING_KEY")
		tokenString := r.Header.Get("Upstash-Signature")

		body, err := io.ReadAll(r.Body)
		if err != nil {
			http.Error(w, err.Error(), http.StatusInternalServerError)
			return
		}

		err = verify(body, tokenString, currentSigningKey)
		if err != nil {
			fmt.Printf("Unable to verify signature with current signing key: %v", err)
			err = verify(body, tokenString, nextSigningKey)
		}

		if err != nil {
			http.Error(w, err.Error(), http.StatusUnauthorized)
			return
		}

		// handle your business logic here

		w.WriteHeader(http.StatusOK)

	})

	fmt.Println("listening on", port)
	err := http.ListenAndServe(":"+port, nil)
	if err != nil {
		panic(err)
	}
}
The verify function will handle verification of the JWT, that includes claims about the request. See here. 
func verify(body []byte, tokenString, signingKey string) error {
	token, err := jwt.Parse(
		tokenString,
		func(token *jwt.Token) (interface{}, error) {
			if _, ok := token.Method.(*jwt.SigningMethodHMAC); !ok {
				return nil, fmt.Errorf("Unexpected signing method: %v", token.Header["alg"])
			}
			return []byte(signingKey), nil
		})

	if err != nil {
		return err
	}

	claims, ok := token.Claims.(jwt.MapClaims)
	if !ok || !token.Valid {
		return fmt.Errorf("Invalid token")
	}

	if !claims.VerifyIssuer("Upstash", true) {
		return fmt.Errorf("invalid issuer")
	}
	if !claims.VerifyExpiresAt(time.Now().Unix(), true) {
		return fmt.Errorf("token has expired")
	}
	if !claims.VerifyNotBefore(time.Now().Unix(), true) {
		return fmt.Errorf("token is not valid yet")
	}

	bodyHash := sha256.Sum256(body)
	if claims["body"] != base64.URLEncoding.EncodeToString(bodyHash[:]) {
		return fmt.Errorf("body hash does not match")
	}

	return nil
}
You can find the complete file here. That’s it, now we can deploy our API and test it.

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3. Create app on fly.io

Login with flyctl and then flyctl launch the new app. This will create the necessary fly.toml for us. It will ask you a bunch of questions. I chose all defaults here except for the last question. We do not want to deploy just yet. 
$ flyctl launch
Creating app in /Users/andreasthomas/github/upstash/qstash-examples/fly.io/go
Scanning source code
Detected a Go app
Using the following build configuration:
        Builder: paketobuildpacks/builder:base
        Buildpacks: gcr.io/paketo-buildpacks/go
? App Name (leave blank to use an auto-generated name):
Automatically selected personal organization: Andreas Thomas
? Select region: fra (Frankfurt, Germany)
Created app winer-cherry-9545 in organization personal
Wrote config file fly.toml
? Would you like to setup a Postgresql database now? No
? Would you like to deploy now? No
Your app is ready. Deploy with `flyctl deploy`

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4. Set Environment Variables

Get your current and next signing key from the Upstash Console Then set them using flyctl secrets set ... 
flyctl secrets set QSTASH_CURRENT_SIGNING_KEY=...
flyctl secrets set QSTASH_NEXT_SIGNING_KEY=...

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5. Deploy the app

Fly.io made this step really simple. Just flyctl deploy and enjoy. 
flyctl deploy

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6. Publish a message

Now you can publish a message to QStash. Note the destination url is basically your app name, if you are not sure what it is, you can go to fly.io/dashboard and find out. In my case the app is named “winter-cherry-9545” and the public url is “https://winter-cherry-9545.fly.dev”. 
curl --request POST "https://qstash.upstash.io/v2/publish/https://winter-cherry-9545.fly.dev" \
     -H "Authorization: Bearer <QSTASH_TOKEN>" \
     -H "Content-Type: application/json" \
     -d "{ \"hello\": \"world\"}"

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Next Steps

That’s it, you have successfully created a Go API hosted on fly.io, that receives and verifies incoming webhooks from qstash. Learn more about publishing a message to qstash here