path: root/main.go

package main

import (
	"flag"
	"fmt"
	"io/ioutil"
	"log"
	"net"
	"net/http"
	"os"
	"strings"
	"time"

	"github.com/nats-io/nats.go"
)

// printHelp outputs the usage information.
func printHelp() {
	fmt.Println("Usage: HTTP-to-NATS proxy server")
	fmt.Println("\nThe following environment variables are supported:")
	fmt.Println("    NATS_URL - NATS connection URL (default: nats://127.0.0.1:4222)")
	fmt.Println("    NATS_USER - NATS username for authentication (optional)")
	fmt.Println("    NATS_PASSWORD - NATS password for authentication (optional)")
	fmt.Println("    NATS_TOKEN - NATS token for authentication (optional)")
	fmt.Println("    NATS_NKEY - NATS NKEY for authentication (optional)")
	fmt.Println("    NATS_NKEY_SEED - NATS NKEY seed for authentication (optional)")
	fmt.Println("    NATS_CREDS_FILE - Path to NATS credentials file (optional)")
	fmt.Println("    NATS_INBOX_PREFIX - Subject prefix for NATS messages (default: _INBOX)")
	fmt.Println("    HTTP_PORT - HTTP port to listen on (default: 8080)")
}

func main() {
	helpFlag := flag.Bool("help", false, "Display help information about available environment variables")
	flag.Parse()
	if *helpFlag {
		printHelp()
		os.Exit(0)
	}

	// Read NATS connection info from environment variables
	natsURL := os.Getenv("NATS_URL")
	if natsURL == "" {
		natsURL = nats.DefaultURL // defaults to "nats://127.0.0.1:4222"
	}
	natsUser := os.Getenv("NATS_USER")
	natsPassword := os.Getenv("NATS_PASSWORD")
	natsToken := os.Getenv("NATS_TOKEN")
	natsNkey := os.Getenv("NATS_NKEY")
	natsNkeySeed := os.Getenv("NATS_NKEY_SEED")
	natsCredsFile := os.Getenv("NATS_CREDS_FILE")
	natsInboxPrefix := os.Getenv("NATS_INBOX_PREFIX")

	// Read HTTP port from environment variables
	httpPort := os.Getenv("HTTP_PORT")
	if httpPort == "" {
		httpPort = "8080"
	}

	// Set up NATS connection options
	opts := []nats.Option{nats.Name("Web proxy")}

	if natsUser != "" && natsPassword != "" {
		opts = append(opts, nats.UserInfo(natsUser, natsPassword))
	} else if natsToken != "" {
		opts = append(opts, nats.Token(natsToken))
	} else if natsNkey != "" && natsNkeySeed != "" {
		log.Fatalln("NKEY connection not supported")
	} else if natsCredsFile != "" {
		opts = append(opts, nats.UserCredentials(natsCredsFile))
	}

	if natsInboxPrefix != "" {
		opts = append(opts, nats.CustomInboxPrefix(natsInboxPrefix))
	}

	nc, err := nats.Connect(natsURL, opts...)
	if err != nil {
		log.Fatal("Error connecting to NATS:", err)
	}
	defer nc.Close()

	// HTTP handler to proxy all requests to NATS
	http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
		// Read the entire request body (if any)
		body, err := ioutil.ReadAll(r.Body)
		if err != nil {
			http.Error(w, "Error reading request body", http.StatusInternalServerError)
			return
		}
		defer r.Body.Close()

		// Create the NATS subject.
		// Remove the leading slash from the path and replace remaining slashes with dots.
		// The subject is prefixed with "http.<host>.<method>.", where <method> is lower-case.

		// Extract host and reverse domain components
		host := r.Host
		// Remove port if present
		if h, _, err := net.SplitHostPort(host); err == nil {
			host = h
		}
		// Use "_" instead of "." in the domain to make it a single token.
		domainParts := strings.ReplaceAll(host, ".", "_")

		// Process path component
		path := strings.TrimPrefix(r.URL.Path, "/")
		// Replace all "." with "_" and then all "/" with ".".
		subjectPath := strings.ReplaceAll(strings.ReplaceAll(path, ".", "_"), "/", ".")

		// Build final subject
		subjectBase := "http"
		subjectParts := []string{subjectBase, domainParts, strings.ToLower(r.Method)}
		if subjectPath != "" {
			subjectParts = append(subjectParts, subjectPath)
		}
		subject := strings.Join(subjectParts, ".")

		log.Println("Forwarding HTTP", r.Method, "request on", r.URL.Path, "to NATS subject:", subject)

		// Create a new NATS message with the HTTP request body
		msg := nats.Msg{
			Subject: subject,
			Data:    body,
			Header:  nats.Header{},
		}

		// Copy over all the HTTP request headers to the NATS message headers
		for key, values := range r.Header {
			for _, value := range values {
				msg.Header.Add(key, value)
			}
		}

		// Add additional HTTP meta-data as headers
		msg.Header.Set("X-HTTP-Method", r.Method)
		msg.Header.Set("X-HTTP-Path", r.URL.Path)
		msg.Header.Set("X-HTTP-Query", r.URL.RawQuery)
		msg.Header.Set("X-HTTP-Host", r.Host)
		msg.Header.Set("X-Remote-Addr", r.RemoteAddr)

		// Send the NATS request and wait synchronously for a reply (timeout: 30 seconds)
		reply, err := nc.RequestMsg(&msg, 30*time.Second)
		if err != nil {
			log.Println("NATS request error:", err)

			// Handle specific NATS error cases
			if err == nats.ErrNoResponders || strings.Contains(err.Error(), "no responders") {
				http.Error(w, "No service available to handle request", http.StatusNotFound)
			} else {
				http.Error(w, "Error processing request", http.StatusInternalServerError)
			}
			return
		}

		// Set any response headers from the NATS reply on the HTTP response
		for key, values := range reply.Header {
			for _, value := range values {
				w.Header().Add(key, value)
			}
		}

		// Write the reply body (from NATS) back to the HTTP client
		w.Write(reply.Data)
	})

	// Start the HTTP server
	fmt.Println("Server is running on http://localhost:8080")
	log.Fatal(http.ListenAndServe(":8080", nil))
}