Broadcast a channel through multiple channel in Go - go

Question

I would like to broadcast data received from a channel to a list of channel. The list of channel is dynamic an can be modified during the run phase.
As a new developper in Go, I wrote this code. I found it quite heavy for what I want. Is-there a better way to do this?
package utils
import "sync"
// StringChannelBroadcaster broadcasts string data from a channel to multiple channels
type StringChannelBroadcaster struct {
Source chan string
Subscribers map[string]*StringChannelSubscriber
stopChannel chan bool
mutex sync.Mutex
capacity uint64
}
// NewStringChannelBroadcaster creates a StringChannelBroadcaster
func NewStringChannelBroadcaster(capacity uint64) (b *StringChannelBroadcaster) {
return &StringChannelBroadcaster{
Source: make(chan string, capacity),
Subscribers: make(map[string]*StringChannelSubscriber),
capacity: capacity,
}
}
// Dispatch starts dispatching message
func (b *StringChannelBroadcaster) Dispatch() {
b.stopChannel = make(chan bool)
for {
select {
case val, ok := <-b.Source:
if ok {
b.mutex.Lock()
for _, value := range b.Subscribers {
value.Channel <- val
}
b.mutex.Unlock()
}
case <-b.stopChannel:
return
}
}
}
// Stop stops the Broadcaster
func (b *StringChannelBroadcaster) Stop() {
close(b.stopChannel)
}
// StringChannelSubscriber defines a subscriber to a StringChannelBroadcaster
type StringChannelSubscriber struct {
Key string
Channel chan string
}
// NewSubscriber returns a new subsriber to the StringChannelBroadcaster
func (b *StringChannelBroadcaster) NewSubscriber() *StringChannelSubscriber {
key := RandString(20)
newSubscriber := StringChannelSubscriber{
Key: key,
Channel: make(chan string, b.capacity),
}
b.mutex.Lock()
b.Subscribers[key] = &newSubscriber
b.mutex.Unlock()
return &newSubscriber
}
// RemoveSubscriber removes a subscrber from the StringChannelBroadcaster
func (b *StringChannelBroadcaster) RemoveSubscriber(subscriber *StringChannelSubscriber) {
b.mutex.Lock()
delete(b.Subscribers, subscriber.Key)
b.mutex.Unlock()
}
Thank you,
Julien

Answer 1

I think you can simplify it a bit: get rid of stopChannel and the Stop method. You can just close Source instead of calling Stop, and detect that in Dispatch (ok will be false) to quit (you can just range over the source channel actually).
You can get rid of Dispatch, and just start a goroutine in NewStringChannelBroadcaster with the for cycle, so external code doesn't have to start the dispatch cycle separately.
You can use a channel type as the map key, so your map can become map[chan string]struct{} (empty struct because you don't need the map value). So your NewSubscriber can take a channel type parameter (or create a new channel and return it), and insert that into the map, you don't need the random string or the StringChannelSubscriber type.
I also made some improvements, like closing the subscriber channels:
package main
import "sync"
import (
"fmt"
"time"
)
// StringChannelBroadcaster broadcasts string data from a channel to multiple channels
type StringChannelBroadcaster struct {
Source chan string
Subscribers map[chan string]struct{}
mutex sync.Mutex
capacity uint64
}
// NewStringChannelBroadcaster creates a StringChannelBroadcaster
func NewStringChannelBroadcaster(capacity uint64) *StringChannelBroadcaster {
b := &StringChannelBroadcaster{
Source: make(chan string, capacity),
Subscribers: make(map[chan string]struct{}),
capacity: capacity,
}
go b.dispatch()
return b
}
// Dispatch starts dispatching message
func (b *StringChannelBroadcaster) dispatch() {
// for iterates until the channel is closed
for val := range b.Source {
b.mutex.Lock()
for ch := range b.Subscribers {
ch <- val
}
b.mutex.Unlock()
}
b.mutex.Lock()
for ch := range b.Subscribers {
close(ch)
// you shouldn't be calling RemoveSubscriber after closing b.Source
// but it's better to be safe than sorry
delete(b.Subscribers, ch)
}
b.Subscribers = nil
b.mutex.Unlock()
}
func (b *StringChannelBroadcaster) NewSubscriber() chan string {
ch := make(chan string, b.capacity)
b.mutex.Lock()
if b.Subscribers == nil {
panic(fmt.Errorf("NewSubscriber called on closed broadcaster"))
}
b.Subscribers[ch] = struct{}{}
b.mutex.Unlock()
return ch
}
// RemoveSubscriber removes a subscrber from the StringChannelBroadcaster
func (b *StringChannelBroadcaster) RemoveSubscriber(ch chan string) {
b.mutex.Lock()
if _, ok := b.Subscribers[ch]; ok {
close(ch) // this line does have to be inside the if to prevent close of closed channel, in case RemoveSubscriber is called twice on the same channel
delete(b.Subscribers, ch) // this line doesn't need to be inside the if
}
b.mutex.Unlock()
}
func main() {
b := NewStringChannelBroadcaster(0)
var toberemoved chan string
for i := 0; i < 3; i++ {
i := i
ch := b.NewSubscriber()
if i == 1 {
toberemoved = ch
}
go func() {
for v := range ch {
fmt.Printf("receive %v: %v\n", i, v)
}
fmt.Printf("Exit %v\n", i)
}()
}
b.Source <- "Test 1"
b.Source <- "Test 2"
// This is a race condition: the second reader may or may not receive the first two messages.
b.RemoveSubscriber(toberemoved)
b.Source <- "Test 3"
// let the reader goroutines receive the last message
time.Sleep(2 * time.Second)
close(b.Source)
// let the reader goroutines write close message
time.Sleep(1 * time.Second)
}
https://play.golang.org/p/X-NcikvbDM
Edit: I've added your edit to fix the panic when calling RemoveSubscriber after closing Source, but you shouldn't be doing that, you should let the struct and everything in it be garbage collected after the channel is closed.
I've also added a panic to NewSubscriber if it's called after closing Source. Previously you could do that and it'd leak the created channel and presumably the goroutine that will block forever on that channel.
If you can call NewSubscriber (or RemoveSubscriber) on an already closed broadcaster, that probably means there's an error in your code somewhere, since you're holding on to a broadcaster that you shouldn't be.