Notification broadcaster in Go

broadcast is a library that allows sending repeated notifications to multiple goroutines with guaranteed delivery and user defined types.

The standard way to handle notifications is via a chan struct{}. However, sending a message to a channel is received by a single goroutine.

The only operation that is broadcast to multiple goroutines is a channel closure. Yet, if the channel is closed, there's no way to send a message again.

❌ Repeated notifications to multiple goroutines

✅ Guaranteed delivery

sync.Cond is the standard solution based on condition variables to set up containers of goroutines waiting for a specific condition.

There's one caveat to keep in mind, though: the Broadcast() method doesn't guarantee that a goroutine will receive the notification. Indeed, the notification will be lost if the listener goroutine isn't waiting on the Wait() method.

✅ Repeated notifications to multiple goroutines

❌ Guaranteed delivery

First, we need to create a Relay for a message type (empty struct in this case):

relay := broadcast.NewRelay[struct{}]()

Once a Relay is created, we can create a new listener using the Listener method. As the broadcast library relies internally on channels, it accepts a capacity:

list := relay.Listener(1) // Create a new listener based on a channel with a one capacity

A Relay can send a notification in three different manners:

• Notify: block until a notification is sent to all the listeners
• NotifyCtx: send a notification to all listeners unless the provided context times out or is canceled
• Broadcast: send a notification to all listeners in a non-blocking manner; delivery isn't guaranteed

On the Listener side, we can access the internal channel using Ch:

<-list.Ch() // Wait on a notification

We can close a Listener and a Relay using Close:

list.Close() 
relay.Close()

Closing a Relay and Listeners can be done concurrently in a safe manner.

type msg string
const (
    msgA msg = "A"
    msgB     = "B"
    msgC     = "C"
)

relay := broadcast.NewRelay[msg]() // Create a relay for msg values
defer relay.Close()

// Listener goroutines
for i := 0; i < 2; i++ {
    go func(i int) {
        l := relay.Listener(1)  // Create a listener with a buffer capacity of 1
        for n := range l.Ch() { // Ranges over notifications
            fmt.Printf("listener %d has received a notification: %v\n", i, n)
        }
    }(i)
}

// Notifiers
time.Sleep(time.Second)
relay.Notify(msgA)                                     // Send notification with guaranteed delivery
ctx, _ := context.WithTimeout(context.Background(), 0) // Context with immediate timeout
relay.NotifyCtx(ctx, msgB)                             // Send notification respecting context cancellation
time.Sleep(time.Second)                                // Allow time for previous messages to be processed
relay.Broadcast(msgC)                                  // Send notification without guaranteed delivery
time.Sleep(time.Second)                                // Allow time for previous messages to be processed