fan_out.md

Fan-Out Messaging Pattern

========================= Fan-Out is a messaging pattern used for distributing work amongst workers (producer: source, consumers: destination).

We can model fan-out using the Go channels.

// Split a channel into n channels that receive messages in a round-robin fashion.
func Split(ch <-chan int, n int) []<-chan int {
    cs := make([]chan int)
    for i := 0; i < n; i++ {
        cs = append(cs, make(chan int))
    }

    // Distributes the work in a round robin fashion among the stated number
    // of channels until the main channel has been closed. In that case, close
    // all channels and return.
    distributeToChannels := func(ch <-chan int, cs []chan<- int) {
        // Close every channel when the execution ends.
        defer func(cs []chan<- int) {
            for _, c := range cs {
                close(c)
            }
        }(cs)

        for {
            for _, c := range cs {
                select {
                case val, ok := <-ch:
                    if !ok {
                        return
                    }

                    c <- val
                }
            }
        }
    }

    go distributeToChannels(ch, cs)

    return cs
}

The Split function converts a single channel into a list of channels by using a goroutine to copy received values to channels in the list in a round-robin fashion.