Subscriber who only wants N responses

Wants N

// Copyright 2012-2018 The NATS Authors
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package main

import (
	"flag"
	"log"
	"runtime"
	"time"

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

// NOTE: Can test with demo servers.
// nats-wantn -s demo.nats.io -n N
// nats-wantn -s demo.nats.io:4443 -n N  (TLS version)

func usage() {
	log.Fatalf("Usage: nats-wantn [-s server] [-t] [-n N]")
}

func printMsg(m *nats.Msg, i int) {
	log.Printf("[#%d] Received on [%s]: '%s'", i, m.Subject, string(m.Data))
}

func main() {
	var urls = flag.String("s", nats.DefaultURL, "The nats server URLs (separated by comma)")
	var nkeyFile = flag.String("nkey", "", "Use the nkey seed file for authentication")
	var showTime = flag.Bool("t", false, "Display timestamps")
	var wantsN = flag.Int("n", 1, "Number of messages to receive")
	log.SetFlags(0)
	flag.Usage = usage
	flag.Parse()

	// Connect Options.
	opts := []nats.Option{nats.Name("NATS Sample N Responses Subscriber")}
	opts = setupConnOptions(opts)

	// Use Nkey authentication.
	if *nkeyFile != "" {
		opt, err := nats.NkeyOptionFromSeed(*nkeyFile)
		if err != nil {
			log.Fatal(err)
		}
		opts = append(opts, opt)
	}

	// Connect to NATS
	nc, err := nats.Connect(*urls, opts...)
	if err != nil {
		log.Fatal(err)
	}

	i := 0
	inbox := nc.NewRespInbox()

	sub, _ := nc.Subscribe(inbox, func(msg *nats.Msg) {
		i += 1
		printMsg(msg, i)
		if i == *wantsN {
			log.Fatalf("Reached max responses of %d, exiting", *wantsN)
		}
	})
	sub.AutoUnsubscribe(*wantsN)
	nc.Flush()
	if err := nc.LastError(); err != nil {
		log.Fatal(err)
	}

	log.Printf("Listening on [%s]", inbox)
	if *showTime {
		log.SetFlags(log.LstdFlags)
	}

	runtime.Goexit()
}

func setupConnOptions(opts []nats.Option) []nats.Option {
	totalWait := 10 * time.Minute
	reconnectDelay := time.Second

	opts = append(opts, nats.ReconnectWait(reconnectDelay))
	opts = append(opts, nats.MaxReconnects(int(totalWait/reconnectDelay)))
	opts = append(opts, nats.DisconnectHandler(func(nc *nats.Conn) {
		log.Printf("Disconnected")
		log.Printf("Reconnecting for next %.0fm", totalWait.Minutes())
	}))
	opts = append(opts, nats.ReconnectHandler(func(nc *nats.Conn) {
		log.Printf("Reconnected [%s]", nc.ConnectedUrl())
	}))
	opts = append(opts, nats.ClosedHandler(func(nc *nats.Conn) {
		log.Fatal("Exiting, no servers available")
	}))
	return opts
}