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// This is a simple AMQP broker implemented using the event-driven proton package.
// It maintains a set of named in-memory queues of messages. Clients can send
// messages to queues or subscribe to receive messages from them.
// TODO: show how to handle acknowledgements from receivers and put rejected or
// un-acknowledged messages back on their queues.
package main
import (
// Usage and command-line flags
func usage() {
fmt.Fprintf(os.Stderr, `
Usage: %s
A simple broker-like demo. Queues are created automatically for sender or receiver addresses.
`, os.Args[0])
var addr = flag.String("addr", ":amqp", "Listening address")
var credit = flag.Int("credit", 100, "Receiver credit window")
var qsize = flag.Int("qsize", 1000, "Max queue size")
var debug = flag.Bool("debug", false, "Print detailed debug output")
var debugf = func(format string, data ...interface{}) {} // Default no debugging output
func main() {
flag.Usage = usage
if *debug {
debugf = func(format string, data ...interface{}) { log.Printf(format, data...) }
b := &broker{makeQueues(*qsize)}
if err :=; err != nil {
// State for the broker
type broker struct {
queues queues
// Listens for connections and starts a proton.Engine for each one.
func (b *broker) run() error {
listener, err := net.Listen("tcp", *addr)
if err != nil {
return err
defer listener.Close()
fmt.Printf("Listening on %s\n", listener.Addr())
for {
conn, err := listener.Accept()
if err != nil {
debugf("Accept error: %v", err)
adapter := proton.NewMessagingAdapter(newHandler(&b.queues))
// We want to accept messages when they are enqueued, not just when they
// are received, so we turn off auto-accept and prefetch by the adapter.
adapter.Prefetch = 0
adapter.AutoAccept = false
engine, err := proton.NewEngine(conn, adapter)
if err != nil {
debugf("Connection error: %v", err)
engine.Server() // Enable server-side protocol negotiation.
debugf("Accepted connection %s", engine)
go func() { // Start goroutine to run the engine event loop
debugf("Closed %s", engine)
// handler handles AMQP events. There is one handler per connection. The
// handler does not need to be concurrent-safe as proton.Engine will serialize
// all calls to the handler. We use channels to communicate between the handler
// goroutine and other goroutines sending and receiving messages.
type handler struct {
queues *queues
receivers map[proton.Link]*receiver
senders map[proton.Link]*sender
injecter proton.Injecter
func newHandler(queues *queues) *handler {
return &handler{
queues: queues,
receivers: make(map[proton.Link]*receiver),
senders: make(map[proton.Link]*sender),
// HandleMessagingEvent handles an event, called in the handler goroutine.
func (h *handler) HandleMessagingEvent(t proton.MessagingEvent, e proton.Event) {
switch t {
case proton.MStart:
h.injecter = e.Injecter()
case proton.MLinkOpening:
if e.Link().IsReceiver() {
} else {
case proton.MLinkClosed:
h.linkClosed(e.Link(), e.Link().RemoteCondition().Error())
case proton.MSendable:
if s, ok := h.senders[e.Link()]; ok {
s.sendable() // Signal the send goroutine that we have credit.
} else {
proton.CloseError(e.Link(), amqp.Errorf(amqp.NotFound, "link %s sender not found", e.Link()))
case proton.MMessage:
m, err := e.Delivery().Message() // Message() must be called while handling the MMessage event.
if err != nil {
proton.CloseError(e.Link(), err)
r, ok := h.receivers[e.Link()]
if !ok {
proton.CloseError(e.Link(), amqp.Errorf(amqp.NotFound, "link %s receiver not found", e.Link()))
// This will not block as AMQP credit is set to the buffer capacity.
r.buffer <- receivedMessage{e.Delivery(), m}
debugf("link %s received %#v", e.Link(), m)
case proton.MConnectionClosed, proton.MDisconnected:
for l, _ := range h.receivers {
h.linkClosed(l, nil)
for l, _ := range h.senders {
h.linkClosed(l, nil)
// linkClosed is called when a link has been closed by both ends.
// It removes the link from the handlers maps and stops its goroutine.
func (h *handler) linkClosed(l proton.Link, err error) {
if s, ok := h.senders[l]; ok {
delete(h.senders, l)
} else if r, ok := h.receivers[l]; ok {
delete(h.receivers, l)
// link has some common data and methods that are used by the sender and receiver types.
// An active link is represented by a sender or receiver value and a goroutine
// running its run() method. The run() method communicates with the handler via
// channels.
type link struct {
l proton.Link
q queue
h *handler
func makeLink(l proton.Link, q queue, h *handler) link {
lnk := link{l: l, q: q, h: h}
return lnk
// receiver has a channel to buffer messages that have been received by the
// handler and are waiting to go on the queue. AMQP credit ensures that the
// handler does not overflow the buffer and block.
type receiver struct {
buffer chan receivedMessage
// receivedMessage holds a message and a Delivery so that the message can be
// acknowledged when it is put on the queue.
type receivedMessage struct {
delivery proton.Delivery
message amqp.Message
// startReceiver creates a receiver and a goroutine for its run() method.
func (h *handler) startReceiver(e proton.Event) {
q := h.queues.Get(e.Link().RemoteTarget().Address())
r := &receiver{
link: makeLink(e.Link(), q, h),
buffer: make(chan receivedMessage, *credit),
h.receivers[r.l] = r
r.l.Flow(cap(r.buffer)) // Give credit to fill the buffer to capacity.
// run runs in a separate goroutine. It moves messages from the buffer to the
// queue for a receiver link, and injects a handler function to acknowledge the
// message and send a credit.
func (r *receiver) run() {
for rm := range r.buffer {
r.q <- rm.message
d :=
// We are not in the handler goroutine so we Inject the accept function as a closure.
r.h.injecter.Inject(func() {
// Check that the receiver is still open, it may have been closed by the remote end.
if r == r.h.receivers[r.l] {
d.Accept() // Accept the delivery
r.l.Flow(1) // Add one credit
// stop closes the buffer channel and waits for the run() goroutine to stop.
func (r *receiver) stop() {
// sender has a channel that is used to signal when there is credit to send messages.
type sender struct {
credit chan struct{} // Channel to signal availability of credit.
// startSender creates a sender and starts a goroutine for
func (h *handler) startSender(e proton.Event) {
q := h.queues.Get(e.Link().RemoteSource().Address())
s := &sender{
link: makeLink(e.Link(), q, h),
credit: make(chan struct{}, 1), // Capacity of 1 for signalling.
h.senders[e.Link()] = s
// stop closes the credit channel and waits for the run() goroutine to stop.
func (s *sender) stop() {
// sendable signals that the sender has credit, it does not block.
// has capacity 1, if it is already full we carry on.
func (s *sender) sendable() {
select { // Non-blocking
case <- struct{}{}:
// run runs in a separate goroutine. It monitors the queue for messages and injects
// a function to send them when there is credit
func (s *sender) run() {
var q queue // q is nil initially as we have no credit.
for {
select {
case _, ok := <
if !ok { // sender closed
q = s.q // We have credit, enable selecting on the queue.
case m, ok := <-q: // q is only enabled when we have credit.
if !ok { // queue closed
q = nil // Assume all credit will be used used, will be signaled otherwise.
s.h.injecter.Inject(func() { // Inject handler function to actually send
if s.h.senders[s.l] != s { // The sender has been closed by the remote end.
q.PutBack(m) // Put the message back on the queue but don't block
if s.sendOne(m) != nil {
// Send as many more messages as we can without blocking
for s.l.Credit() > 0 {
select { // Non blocking receive from q
case m, ok := <-s.q:
if ok {
default: // Queue is empty but we have credit, signal the run() goroutine.
// sendOne runs in the handler goroutine. It sends a single message.
func (s *sender) sendOne(m amqp.Message) error {
delivery, err := s.l.Send(m)
if err == nil {
delivery.Settle() // Pre-settled, unreliable.
debugf("link %s sent %#v", s.l, m)
} else {
s.q.PutBack(m) // Put the message back on the queue, don't block
return err
// Use a buffered channel as a very simple queue.
type queue chan amqp.Message
// Put a message back on the queue, does not block.
func (q queue) PutBack(m amqp.Message) {
select {
case q <- m:
// Not an efficient implementation but ensures we don't block the caller.
go func() { q <- m }()
// Concurrent-safe map of queues.
type queues struct {
queueSize int
m map[string]queue
lock sync.Mutex
func makeQueues(queueSize int) queues {
return queues{queueSize: queueSize, m: make(map[string]queue)}
// Create a queue if not found.
func (qs *queues) Get(name string) queue {
defer qs.lock.Unlock()
q := qs.m[name]
if q == nil {
q = make(queue, qs.queueSize)
qs.m[name] = q
return q