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
 	}