Add the notion of a node selector for routing

client/node_selector.go

@@ -8,10 +8,18 @@ import (
 	"github.com/micro/go-micro/registry"
 )
 
+// NodeSelector is used to retrieve a node to which a request
+// should be routed. It takes a list of services and selects
+// a single node. If a node cannot be selected it should return
+// an error. A list of services is provided as a service may
+// have 1 or more versions.
+type NodeSelector func(service []*registry.Service) (*registry.Node, error)
+
 func init() {
 	rand.Seed(time.Now().UnixNano())
 }
 
+// Built in random hashed node selector
 func nodeSelector(service []*registry.Service) (*registry.Node, error) {
 	if len(service) == 0 {
 		return nil, errors.NotFound("go.micro.client", "Service not found")

client/options.go

@@ -14,6 +14,7 @@ type options struct {
 	registry    registry.Registry
 	transport   transport.Transport
 	wrappers    []Wrapper
+	selector    NodeSelector
 }
 
 // Broker to be used for pub/sub
@@ -51,6 +52,13 @@ func Transport(t transport.Transport) Option {
 	}
 }
 
+// Selector is used to select a node to route a request to
+func Selector(s NodeSelector) Option {
+	return func(o *options) {
+		o.selector = s
+	}
+}
+
 // Adds a Wrapper to a list of options passed into the client
 func Wrap(w Wrapper) Option {
 	return func(o *options) {

client/rpc_client.go

@@ -43,6 +43,10 @@ func newRpcClient(opt ...Option) Client {
 		opts.broker = broker.DefaultBroker
 	}
 
+	if opts.selector == nil {
+		opts.selector = nodeSelector
+	}
+
 	rc := &rpcClient{
 		once: once,
 		opts: opts,
@@ -146,7 +150,7 @@ func (r *rpcClient) Call(ctx context.Context, request Request, response interfac
 		return errors.InternalServerError("go.micro.client", err.Error())
 	}
 
-	node, err := nodeSelector(service)
+	node, err := r.opts.selector(service)
 	if err != nil {
 		return err
 	}
@@ -169,7 +173,7 @@ func (r *rpcClient) Stream(ctx context.Context, request Request, responseChan in
 		return nil, errors.InternalServerError("go.micro.client", err.Error())
 	}
 
-	node, err := nodeSelector(service)
+	node, err := r.opts.selector(service)
 	if err != nil {
 		return nil, err
 	}

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)
+	}
+}