Add the notion of a node selector for routing

examples/client/node_selector/node_selector.go

@@ -0,0 +1,84 @@
+package main
+
+import (
+	"fmt"
+	"math/rand"
+	"time"
+
+	"github.com/micro/go-micro/client"
+	"github.com/micro/go-micro/cmd"
+	c "github.com/micro/go-micro/context"
+	"github.com/micro/go-micro/errors"
+	example "github.com/micro/go-micro/examples/server/proto/example"
+	"github.com/micro/go-micro/registry"
+	"golang.org/x/net/context"
+)
+
+func init() {
+	rand.Seed(time.Now().Unix())
+}
+
+// A random node selector
+func randomSelector(s []*registry.Service) (*registry.Node, error) {
+	if len(s) == 0 {
+		return nil, errors.NotFound("go.micro.client", "Service not found")
+	}
+
+	i := rand.Int()
+	j := i % len(s)
+
+	if len(s[j].Nodes) == 0 {
+		return nil, errors.NotFound("go.micro.client", "Service not found")
+	}
+
+	n := i % len(s[j].Nodes)
+	return s[j].Nodes[n], nil
+}
+
+// Wraps the node selector so that it will log what node was selected
+func wrapSelector(fn client.NodeSelector) client.NodeSelector {
+	return func(s []*registry.Service) (*registry.Node, error) {
+		n, err := fn(s)
+		if err != nil {
+			return nil, err
+		}
+		fmt.Printf("Selected node %v\n", n)
+		return n, nil
+	}
+}
+
+func call(i int) {
+	// Create new request to service go.micro.srv.example, method Example.Call
+	req := client.NewRequest("go.micro.srv.example", "Example.Call", &example.Request{
+		Name: "John",
+	})
+
+	// create context with metadata
+	ctx := c.WithMetadata(context.Background(), map[string]string{
+		"X-User-Id": "john",
+		"X-From-Id": "script",
+	})
+
+	rsp := &example.Response{}
+
+	// Call service
+	if err := client.Call(ctx, req, rsp); err != nil {
+		fmt.Println("call err: ", err, rsp)
+		return
+	}
+
+	fmt.Println("Call:", i, "rsp:", rsp.Msg)
+}
+
+func main() {
+	cmd.Init()
+
+	client.DefaultClient = client.NewClient(
+		client.Selector(wrapSelector(randomSelector)),
+	)
+
+	fmt.Println("\n--- Call example ---\n")
+	for i := 0; i < 10; i++ {
+		call(i)
+	}
+}