Simple HTTP server that enqueues subscribers and sends published messages to the oldest subscriber (and enqueues messages in case there are no subscribers)

pubsub.go

package main

import (
	"io"
	"log"
	"net/http"
)

const (
	buffSize = 100
)

var (
	subs = make(chan string, buffSize)
	msgs = make(chan io.Reader, buffSize)
)

// send sends a message to a subscriber
func send(s string, m io.Reader) {
	log.Println("Sending data to", s)
	http.Post(s, "application/json", m)
}

// pub sends a message to the oldest registered subscriber or enqueues the message to be sent later
func pub(m io.Reader) {
	select {
	case s := <-subs:
		send(s, m)
	default:
		msgs <- m
	}
}

// sub registers a new subscriber or sends the subscriber the oldest enqueued message
func sub(s string) {
	select {
	case m := <-msgs:
		send(s, m)
	default:
		subs <- s
	}
}

func main() {
	http.HandleFunc("/pub", func(w http.ResponseWriter, r *http.Request) {
		if r.Method != "POST" {
			http.Error(w, r.Method+" not supported", http.StatusNotFound)
		}
		pub(r.Body)
	})
	http.HandleFunc("/sub", func(w http.ResponseWriter, r *http.Request) {
		if r.Method != "POST" {
			http.Error(w, r.Method+" not supported", http.StatusNotFound)
		}
		sub(r.RemoteAddr)
	})
	log.Fatal(http.ListenAndServe(":8080", nil))
}

Note that this isn't a conventional "pubsub" system since messages are not published to all subscribers, just the single oldest subscriber in the queue.