broadcast

A broadcasting library for Go.

What?

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

Why?

Why not Channels?

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 broadcasted 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

Why not sync.Cond?

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

How?

Step by Step

First, we need to create a Relay:

relay := broadcast.NewRelay() // Create a new relay

Once a Relay is created, we can create a new listener using the Listen 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 two different manners:

• Blocking: using the Notify method
• Non-blocking: using the Broadcast method

The main difference is that Notify guarantees that a listener will receive the notification. This method blocks until the notification have been sent to all the Listener. Conversely, Broadcast isn't blocking:

relay.Notify() // Send a blocking notification (delivery is guaranteed)
relay.Broadcast() // Send a non-blocking notification (delivery is not guaranteed)

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

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

We can close a Listener using Close:

list.Close() // Close a listener

Please note that this operation is purposely not thread-safe. It can be called multiple times by the same goroutine but shouldn't be called in parallel by multiple goroutines as it can lead to a panic (trying to close the same channel multiple times).

Last but not least, we can close a Relay using Close:

relay.Close() // Close a relay

Example

relay := broadcast.NewRelay() // Create a relay
defer relay.Close()

list1 := relay.Listener(1) // Create a listener with one capacity
defer list1.Close()
list2 := relay.Listener(1) // Create a listener with one capacity
defer list2.Close()

// Listener goroutines
f := func(i int, list *broadcast.Listener) {
	for range list.Ch() { // Waits for receiving notifications
		fmt.Printf("listener %d has received a notification\n", i)
	}
}
go f(1, list1)
go f(2, list2)

// Notifier goroutine
for i := 0; i < 5; i++ {
	time.Sleep(time.Second)
	relay.Notify() // Send notifications with guaranteed delivery
}
for i := 0; i < 5; i++ {
	time.Sleep(time.Second)
	relay.Broadcast() // Send notifications without guaranteed delivery
}