http-nats-proxy/main.go

package main

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

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

// 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)")
}

// URL to NATS subject conversion
func URLToNATS(urlPath string) (string, error) {
	segments := strings.Split(strings.Trim(urlPath, "/"), "/")
	for i, seg := range segments {
		// Decode existing encoding first to prevent double-encoding
		unescaped, err := url.PathUnescape(seg)
		if err != nil {
			return "", fmt.Errorf("failed to unescape segment: %w", err)
		}

		// Encode special NATS-sensitive characters
		encoded := url.PathEscape(unescaped)
		encoded = strings.ReplaceAll(encoded, ".", "%2E") // Critical for token separation
		encoded = strings.ReplaceAll(encoded, "*", "%2A") // Wildcard protection
		encoded = strings.ReplaceAll(encoded, ">", "%3E") // Wildcard protection

		segments[i] = encoded
	}
	return strings.Join(segments, "."), nil
}

// NATS subject to URL conversion
func NATSToURL(natsSubject string) (string, error) {
	tokens := strings.Split(natsSubject, ".")
	for i, token := range tokens {
		// Reverse the special character encoding
		decoded := strings.ReplaceAll(token, "%2E", ".")
		decoded = strings.ReplaceAll(decoded, "%2A", "*")
		decoded = strings.ReplaceAll(decoded, "%3E", ">")

		// Unescape remaining URL encoding
		unescaped, err := url.PathUnescape(decoded)
		if err != nil {
			return "", fmt.Errorf("failed to unescape token: %w", err)
		}
		tokens[i] = unescaped
	}
	return "/" + strings.Join(tokens, "/"), nil
}

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.TrimSuffix(strings.TrimPrefix(r.URL.Path, "/"), "/")
		subjectPath, err := URLToNATS(path)
		if err != nil {
			http.Error(w, "Error converting endpoint to NATS subject", http.StatusInternalServerError)
			log.Println("Could not convert endpoint to NATS subject", err)
			return

		}

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

		// 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)
	})

	_, err = micro.AddService(nc, micro.Config{
		Name:    "http-nats-proxy",
		Version: "0.1.0",
		Endpoint: &micro.EndpointConfig{
			Subject: "http.*.*.proxy.>",
			Handler: micro.HandlerFunc(func(natsReq micro.Request) {
				// http.host.method.proxy.host.endpoint.>
				httpPath := natsReq.Headers()["X-HTTP-Path"][0]
				httpReqURL := fmt.Sprintf("https:/%s", strings.TrimPrefix(httpPath, "/proxy"))
				httpBody := bytes.NewReader(natsReq.Data())
				httpMethod := natsReq.Headers()["X-HTTP-Method"][0]

				// Create a new request
				httpReq, err := http.NewRequest(httpMethod, httpReqURL, httpBody)
				if err != nil {
					log.Fatal(err)
				}
				client := &http.Client{}
				resp, err := client.Do(httpReq)
				if err != nil {
					log.Fatal(err)
				}
				defer resp.Body.Close()

				resBody, err := ioutil.ReadAll(resp.Body)
				if err != nil {
					log.Fatal(err)
				}

				natsReq.Respond(resBody)
			}),
		},
	})
	if err != nil {
		log.Fatal("Could not make NATS microservice:", err)
	}

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