Rewrite fan-in/out messaging patterns

messaging/fan_in.md

@@ -0,0 +1,40 @@
+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.
+
+```go
+// 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.