Add traffic plugin

This allows for advanced loadbalancing and maybe geoIP loadbalancing.

Signed-off-by: Miek Gieben <miek@miek.nl>

plugin/traffic/assignment.go

@@ -0,0 +1,39 @@
+package traffic
+
+import (
+	"math/rand"
+	"net"
+)
+
+// assignment is an assignment for a single service. It contains multiple backends.
+type assignment struct {
+	service  string
+	backends []*backend
+}
+
+// backend is a backend specified by an address, port and a weight.
+type backend struct {
+	addr   net.IP
+	port   int
+	weight int
+}
+
+// Select selects a backend from a, using weighted random selection
+func (a assignment) Select() *backend {
+	total := 0
+	for _, b := range a.backends {
+		total += b.weight
+	}
+	if total == 0 {
+		return nil
+	}
+	r := rand.Intn(total) + 1
+
+	for _, b := range a.backends {
+		r -= b.weight
+		if r <= 0 {
+			return b
+		}
+	}
+	return nil
+}