awesome-patterns/messaging/fan_in.md

Fan-In Messaging Patterns

=================================== Fan-In is a messaging pattern used to create a funnel for work amongst workers (clients: source, server: destination).

We can model fan-in using the Go channels.

// Merge different channels in one channel
func Merge(cs ...<-chan int) <-chan int {
    var wg sync.WaitGroup

    out := make(chan int)

    // Start an send goroutine for each input channel in cs. send
    // copies values from c to out until c is closed, then calls wg.Done.
    send := func(c <-chan int) {
        for n := range c {
            out <- n
        }
        wg.Done()
    }

    wg.Add(len(cs))
    for _, c := range cs {
        go send(c)
    }

    // Start a goroutine to close out once all the send goroutines are
    // done.  This must start after the wg.Add call.
    go func() {
        wg.Wait()
        close(out)
    }()
    return out
}

The Merge function converts a list of channels to a single channel by starting a goroutine for each inbound channel that copies the values to the sole outbound channel.

Once all the output goroutine have been started, Merge a goroutine is started to close the main channel.