go-pattern-examples/gomore/04_fan_in/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 goroutines have been started, Merge a goroutine is started to close the main channel.