Move to a selector package

client/options.go

@@ -4,6 +4,7 @@ import (
 	"github.com/micro/go-micro/broker"
 	"github.com/micro/go-micro/codec"
 	"github.com/micro/go-micro/registry"
+	"github.com/micro/go-micro/selector"
 	"github.com/micro/go-micro/transport"
 )
 
@@ -12,13 +13,13 @@ type options struct {
 	broker      broker.Broker
 	codecs      map[string]codec.NewCodec
 	registry    registry.Registry
-	selector    registry.Selector
+	selector    selector.Selector
 	transport   transport.Transport
 	wrappers    []Wrapper
 }
 
 type callOptions struct {
-	selectOptions []registry.SelectOption
+	selectOptions []selector.SelectOption
 }
 
 type publishOptions struct{}
@@ -59,7 +60,7 @@ func Transport(t transport.Transport) Option {
 }
 
 // Select is used to select a node to route a request to
-func Selector(s registry.Selector) Option {
+func Selector(s selector.Selector) Option {
 	return func(o *options) {
 		o.selector = s
 	}
@@ -74,7 +75,7 @@ func Wrap(w Wrapper) Option {
 
 // Call Options
 
-func WithSelectOption(so registry.SelectOption) CallOption {
+func WithSelectOption(so selector.SelectOption) CallOption {
 	return func(o *callOptions) {
 		o.selectOptions = append(o.selectOptions, so)
 	}

client/rpc_client.go

@@ -10,6 +10,7 @@ import (
 	c "github.com/micro/go-micro/context"
 	"github.com/micro/go-micro/errors"
 	"github.com/micro/go-micro/registry"
+	"github.com/micro/go-micro/selector"
 	"github.com/micro/go-micro/transport"
 
 	"golang.org/x/net/context"
@@ -44,8 +45,8 @@ func newRpcClient(opt ...Option) Client {
 	}
 
 	if opts.selector == nil {
-		opts.selector = registry.NewRandomSelector(
+		opts.selector = selector.NewSelector(
-			registry.SelectorRegistry(opts.registry),
+			selector.Registry(opts.registry),
 		)
 	}
 
@@ -156,14 +157,14 @@ func (r *rpcClient) Call(ctx context.Context, request Request, response interfac
 	}
 
 	next, err := r.opts.selector.Select(request.Service(), copts.selectOptions...)
-	if err != nil && err == registry.ErrNotFound {
+	if err != nil && err == selector.ErrNotFound {
 		return errors.NotFound("go.micro.client", err.Error())
 	} else if err != nil {
 		return errors.InternalServerError("go.micro.client", err.Error())
 	}
 
 	node, err := next()
-	if err != nil && err == registry.ErrNotFound {
+	if err != nil && err == selector.ErrNotFound {
 		return errors.NotFound("go.micro.client", err.Error())
 	} else if err != nil {
 		return errors.InternalServerError("go.micro.client", err.Error())
@@ -190,14 +191,14 @@ func (r *rpcClient) Stream(ctx context.Context, request Request, responseChan in
 	}
 
 	next, err := r.opts.selector.Select(request.Service(), copts.selectOptions...)
-	if err != nil && err == registry.ErrNotFound {
+	if err != nil && err == selector.ErrNotFound {
 		return nil, errors.NotFound("go.micro.client", err.Error())
 	} else if err != nil {
 		return nil, errors.InternalServerError("go.micro.client", err.Error())
 	}
 
 	node, err := next()
-	if err != nil && err == registry.ErrNotFound {
+	if err != nil && err == selector.ErrNotFound {
 		return nil, errors.NotFound("go.micro.client", err.Error())
 	} else if err != nil {
 		return nil, errors.InternalServerError("go.micro.client", err.Error())

examples/client/dc_filter/dc_filter.go

@@ -8,9 +8,11 @@ import (
 	"github.com/micro/go-micro/client"
 	"github.com/micro/go-micro/cmd"
 	c "github.com/micro/go-micro/context"
-	example "github.com/micro/go-micro/examples/server/proto/example"
 	"github.com/micro/go-micro/registry"
+	"github.com/micro/go-micro/selector"
 	"golang.org/x/net/context"
+
+	example "github.com/micro/go-micro/examples/server/proto/example"
 )
 
 func init() {
@@ -38,7 +40,9 @@ func (dc *dcWrapper) Call(ctx context.Context, req client.Request, rsp interface
 		return services
 	}
 
-	callOptions := append(opts, client.WithSelectOption(registry.SelectFilter(filter)))
+	callOptions := append(opts, client.WithSelectOption(
+		selector.Filter(filter),
+	))
 
 	fmt.Printf("[DC Wrapper] filtering for datacenter %s\n", md["datacenter"])
 	return dc.Client.Call(ctx, req, rsp, callOptions...)

examples/client/dc_selector/dc_selector.go

@@ -8,14 +8,16 @@ import (
 
 	"github.com/micro/go-micro/client"
 	"github.com/micro/go-micro/cmd"
-	example "github.com/micro/go-micro/examples/server/proto/example"
 	"github.com/micro/go-micro/registry"
+	"github.com/micro/go-micro/selector"
 	"golang.org/x/net/context"
+
+	example "github.com/micro/go-micro/examples/server/proto/example"
 )
 
 // Built in random hashed node selector
 type dcSelector struct {
-	opts registry.SelectorOptions
+	opts selector.Options
 }
 
 var (
@@ -26,14 +28,14 @@ func init() {
 	rand.Seed(time.Now().Unix())
 }
 
-func (n *dcSelector) Select(service string, opts ...registry.SelectOption) (registry.SelectNext, error) {
+func (n *dcSelector) Select(service string, opts ...selector.SelectOption) (selector.Next, error) {
 	services, err := n.opts.Registry.GetService(service)
 	if err != nil {
 		return nil, err
 	}
 
 	if len(services) == 0 {
-		return nil, registry.ErrNotFound
+		return nil, selector.ErrNotFound
 	}
 
 	var nodes []*registry.Node
@@ -48,7 +50,7 @@ func (n *dcSelector) Select(service string, opts ...registry.SelectOption) (regi
 	}
 
 	if len(nodes) == 0 {
-		return nil, registry.ErrNotFound
+		return nil, selector.ErrNotFound
 	}
 
 	var i int
@@ -75,8 +77,8 @@ func (n *dcSelector) Close() error {
 }
 
 // Return a new first node selector
-func DCSelector(opts ...registry.SelectorOption) registry.Selector {
+func DCSelector(opts ...selector.Option) selector.Selector {
-	var sopts registry.SelectorOptions
+	var sopts selector.Options
 	for _, opt := range opts {
 		opt(&sopts)
 	}

examples/client/selector/selector.go

@@ -9,6 +9,7 @@ import (
 	"github.com/micro/go-micro/cmd"
 	example "github.com/micro/go-micro/examples/server/proto/example"
 	"github.com/micro/go-micro/registry"
+	"github.com/micro/go-micro/selector"
 	"golang.org/x/net/context"
 )
 
@@ -18,20 +19,20 @@ func init() {
 
 // Built in random hashed node selector
 type firstNodeSelector struct {
-	opts registry.SelectorOptions
+	opts selector.Options
 }
 
-func (n *firstNodeSelector) Select(service string, opts ...registry.SelectOption) (registry.SelectNext, error) {
+func (n *firstNodeSelector) Select(service string, opts ...selector.SelectOption) (selector.Next, error) {
 	services, err := n.opts.Registry.GetService(service)
 	if err != nil {
 		return nil, err
 	}
 
 	if len(services) == 0 {
-		return nil, registry.ErrNotFound
+		return nil, selector.ErrNotFound
 	}
 
-	var sopts registry.SelectOptions
+	var sopts selector.SelectOptions
 	for _, opt := range opts {
 		opt(&sopts)
 	}
@@ -41,11 +42,11 @@ func (n *firstNodeSelector) Select(service string, opts ...registry.SelectOption
 	}
 
 	if len(services) == 0 {
-		return nil, registry.ErrNotFound
+		return nil, selector.ErrNotFound
 	}
 
 	if len(services[0].Nodes) == 0 {
-		return nil, registry.ErrNotFound
+		return nil, selector.ErrNotFound
 	}
 
 	return func() (*registry.Node, error) {
@@ -66,8 +67,8 @@ func (n *firstNodeSelector) Close() error {
 }
 
 // Return a new first node selector
-func FirstNodeSelector(opts ...registry.SelectorOption) registry.Selector {
+func FirstNodeSelector(opts ...selector.Option) selector.Selector {
-	var sopts registry.SelectorOptions
+	var sopts selector.Options
 	for _, opt := range opts {
 		opt(&sopts)
 	}

registry/mock/mock.go

@@ -0,0 +1,70 @@
+package mock
+
+import (
+	"github.com/micro/go-micro/registry"
+)
+
+type MockRegistry struct{}
+
+func (m *MockRegistry) GetService(service string) ([]*registry.Service, error) {
+	return []*registry.Service{
+		{
+			Name:    "foo",
+			Version: "1.0.0",
+			Nodes: []*registry.Node{
+				{
+					Id:      "foo-1.0.0-123",
+					Address: "localhost",
+					Port:    9999,
+				},
+				{
+					Id:      "foo-1.0.0-321",
+					Address: "localhost",
+					Port:    9999,
+				},
+			},
+		},
+		{
+			Name:    "foo",
+			Version: "1.0.1",
+			Nodes: []*registry.Node{
+				{
+					Id:      "foo-1.0.1-321",
+					Address: "localhost",
+					Port:    6666,
+				},
+			},
+		},
+		{
+			Name:    "foo",
+			Version: "1.0.3",
+			Nodes: []*registry.Node{
+				{
+					Id:      "foo-1.0.3-345",
+					Address: "localhost",
+					Port:    8888,
+				},
+			},
+		},
+	}, nil
+}
+
+func (m *MockRegistry) ListServices() ([]*registry.Service, error) {
+	return []*registry.Service{}, nil
+}
+
+func (m *MockRegistry) Register(s *registry.Service) error {
+	return nil
+}
+
+func (m *MockRegistry) Deregister(s *registry.Service) error {
+	return nil
+}
+
+func (m *MockRegistry) Watch() (registry.Watcher, error) {
+	return nil, nil
+}
+
+func NewRegistry() *MockRegistry {
+	return &MockRegistry{}
+}

registry/mock_registry.go

@@ -1,62 +0,0 @@
-package registry
-
-type mockRegistry struct{}
-
-func (m *mockRegistry) GetService(service string) ([]*Service, error) {
-	return []*Service{
-		{
-			Name:    "foo",
-			Version: "1.0.0",
-			Nodes: []*Node{
-				{
-					Id:      "foo-1.0.0-123",
-					Address: "localhost",
-					Port:    9999,
-				},
-				{
-					Id:      "foo-1.0.0-321",
-					Address: "localhost",
-					Port:    9999,
-				},
-			},
-		},
-		{
-			Name:    "foo",
-			Version: "1.0.1",
-			Nodes: []*Node{
-				{
-					Id:      "foo-1.0.1-321",
-					Address: "localhost",
-					Port:    6666,
-				},
-			},
-		},
-		{
-			Name:    "foo",
-			Version: "1.0.3",
-			Nodes: []*Node{
-				{
-					Id:      "foo-1.0.3-345",
-					Address: "localhost",
-					Port:    8888,
-				},
-			},
-		},
-	}, nil
-}
-
-func (m *mockRegistry) ListServices() ([]*Service, error) {
-	return []*Service{}, nil
-}
-
-func (m *mockRegistry) Register(s *Service) error {
-	return nil
-}
-
-func (m *mockRegistry) Deregister(s *Service) error {
-	return nil
-}
-
-func (m *mockRegistry) Watch() (Watcher, error) {
-	return nil, nil
-}

registry/registry.go

@@ -1,9 +1,5 @@
 package registry
 
-import (
-	"errors"
-)
-
 type Registry interface {
 	Register(*Service) error
 	Deregister(*Service) error
@@ -18,9 +14,6 @@ type Option func(*options)
 
 var (
 	DefaultRegistry = newConsulRegistry([]string{})
-
-	ErrNotFound      = errors.New("not found")
-	ErrNoneAvailable = errors.New("none available")
 )
 
 func NewRegistry(addrs []string, opt ...Option) Registry {

registry/selector.go

@@ -1,44 +0,0 @@
-package registry
-
-// Selector builds on the registry as a mechanism to pick nodes
-// and mark their status. This allows host pools and other things
-// to be built using various algorithms.
-type Selector interface {
-	Select(service string, opts ...SelectOption) (SelectNext, error)
-	Mark(service string, node *Node, err error)
-	Reset(service string)
-	Close() error
-}
-
-// SelectNext is a function that returns the next node
-// based on the selector's algorithm
-type SelectNext func() (*Node, error)
-
-type SelectorOptions struct {
-	Registry Registry
-}
-
-type SelectOptions struct {
-	Filters []func([]*Service) []*Service
-}
-
-// Option used to initialise the selector
-type SelectorOption func(*SelectorOptions)
-
-// Option used when making a select call
-type SelectOption func(*SelectOptions)
-
-// SelectorRegistry sets the registry used by the selector
-func SelectorRegistry(r Registry) SelectorOption {
-	return func(o *SelectorOptions) {
-		o.Registry = r
-	}
-}
-
-// SelectFilter adds a filter function to the list of filters
-// used during the Select call.
-func SelectFilter(fn func([]*Service) []*Service) SelectOption {
-	return func(o *SelectOptions) {
-		o.Filters = append(o.Filters, fn)
-	}
-}

selector/blacklist/black_list_selector.go

@@ -1,9 +1,12 @@
-package registry
+package blacklist
 
 import (
 	"math/rand"
 	"sync"
 	"time"
+
+	"github.com/micro/go-micro/registry"
+	"github.com/micro/go-micro/selector"
 )
 
 type blackListNode struct {
@@ -13,7 +16,7 @@ type blackListNode struct {
 }
 
 type blackListSelector struct {
-	so   SelectorOptions
+	so   selector.Options
 	ttl  int64
 	exit chan bool
 	once sync.Once
@@ -51,8 +54,8 @@ func (r *blackListSelector) run() {
 	}
 }
 
-func (r *blackListSelector) Select(service string, opts ...SelectOption) (SelectNext, error) {
+func (r *blackListSelector) Select(service string, opts ...selector.SelectOption) (selector.Next, error) {
-	var sopts SelectOptions
+	var sopts selector.SelectOptions
 	for _, opt := range opts {
 		opt(&sopts)
 	}
@@ -70,10 +73,10 @@ func (r *blackListSelector) Select(service string, opts ...SelectOption) (Select
 
 	// if there's nothing left, return
 	if len(services) == 0 {
-		return nil, ErrNotFound
+		return nil, selector.ErrNotFound
 	}
 
-	var nodes []*Node
+	var nodes []*registry.Node
 
 	for _, service := range services {
 		for _, node := range service.Nodes {
@@ -82,11 +85,11 @@ func (r *blackListSelector) Select(service string, opts ...SelectOption) (Select
 	}
 
 	if len(nodes) == 0 {
-		return nil, ErrNotFound
+		return nil, selector.ErrNotFound
 	}
 
-	return func() (*Node, error) {
+	return func() (*registry.Node, error) {
-		var viable []*Node
+		var viable []*registry.Node
 
 		r.RLock()
 		for _, node := range nodes {
@@ -97,14 +100,14 @@ func (r *blackListSelector) Select(service string, opts ...SelectOption) (Select
 		r.RUnlock()
 
 		if len(viable) == 0 {
-			return nil, ErrNoneAvailable
+			return nil, selector.ErrNoneAvailable
 		}
 
 		return viable[rand.Int()%len(viable)], nil
 	}, nil
 }
 
-func (r *blackListSelector) Mark(service string, node *Node, err error) {
+func (r *blackListSelector) Mark(service string, node *registry.Node, err error) {
 	r.Lock()
 	defer r.Unlock()
 	if err == nil {
@@ -138,15 +141,15 @@ func (r *blackListSelector) Close() error {
 	return nil
 }
 
-func NewBlackListSelector(opts ...SelectorOption) Selector {
+func NewSelector(opts ...selector.Option) selector.Selector {
-	var sopts SelectorOptions
+	var sopts selector.Options
 
 	for _, opt := range opts {
 		opt(&sopts)
 	}
 
 	if sopts.Registry == nil {
-		sopts.Registry = DefaultRegistry
+		sopts.Registry = registry.DefaultRegistry
 	}
 
 	var once sync.Once

selector/blacklist/black_list_selector_test.go

@@ -1,17 +1,20 @@
-package registry
+package blacklist
 
 import (
 	"errors"
 	"testing"
 	"time"
+
+	"github.com/micro/go-micro/registry/mock"
+	"github.com/micro/go-micro/selector"
 )
 
 func TestBlackListSelector(t *testing.T) {
 	counts := map[string]int{}
 
 	bl := &blackListSelector{
-		so: SelectorOptions{
+		so: selector.Options{
-			Registry: &mockRegistry{},
+			Registry: mock.NewRegistry(),
 		},
 		ttl:  2,
 		bl:   make(map[string]blackListNode),
@@ -44,7 +47,7 @@ func TestBlackListSelector(t *testing.T) {
 		}
 		bl.Mark("foo", node, errors.New("blacklist"))
 	}
-	if node, err := next(); err != ErrNoneAvailable {
+	if node, err := next(); err != selector.ErrNoneAvailable {
 		t.Errorf("Expected none available err, got node %v err %v", node, err)
 	}
 	time.Sleep(time.Second * time.Duration(bl.ttl) * 2)
@@ -60,7 +63,7 @@ func TestBlackListSelector(t *testing.T) {
 		}
 		bl.Mark("foo", node, errors.New("blacklist"))
 	}
-	if node, err := next(); err != ErrNoneAvailable {
+	if node, err := next(); err != selector.ErrNoneAvailable {
 		t.Errorf("Expected none available err, got node %v err %v", node, err)
 	}
 	bl.Reset("foo")

selector/options.go

@@ -0,0 +1,34 @@
+package selector
+
+import (
+	"github.com/micro/go-micro/registry"
+)
+
+type Options struct {
+	Registry registry.Registry
+}
+
+type SelectOptions struct {
+	Filters []SelectFilter
+}
+
+// Option used to initialise the selector
+type Option func(*Options)
+
+// SelectOption used when making a select call
+type SelectOption func(*SelectOptions)
+
+// Registry sets the registry used by the selector
+func Registry(r registry.Registry) Option {
+	return func(o *Options) {
+		o.Registry = r
+	}
+}
+
+// Filter adds a filter function to the list of filters
+// used during the Select call.
+func Filter(fn SelectFilter) SelectOption {
+	return func(o *SelectOptions) {
+		o.Filters = append(o.Filters, fn)
+	}
+}

selector/random/random.go

@@ -0,0 +1,7 @@
+package random
+
+import "github.com/micro/go-micro/selector"
+
+func NewSelector(opts ...selector.Option) selector.Selector {
+	return selector.NewSelector(opts...)
+}

selector/random_selector.go

@@ -1,19 +1,21 @@
-package registry
+package selector
 
 import (
 	"math/rand"
 	"time"
+
+	"github.com/micro/go-micro/registry"
 )
 
 type randomSelector struct {
-	so SelectorOptions
+	so Options
 }
 
 func init() {
 	rand.Seed(time.Now().Unix())
 }
 
-func (r *randomSelector) Select(service string, opts ...SelectOption) (SelectNext, error) {
+func (r *randomSelector) Select(service string, opts ...SelectOption) (Next, error) {
 	var sopts SelectOptions
 	for _, opt := range opts {
 		opt(&sopts)
@@ -35,7 +37,7 @@ func (r *randomSelector) Select(service string, opts ...SelectOption) (SelectNex
 		return nil, ErrNotFound
 	}
 
-	var nodes []*Node
+	var nodes []*registry.Node
 
 	for _, service := range services {
 		for _, node := range service.Nodes {
@@ -47,7 +49,7 @@ func (r *randomSelector) Select(service string, opts ...SelectOption) (SelectNex
 		return nil, ErrNotFound
 	}
 
-	return func() (*Node, error) {
+	return func() (*registry.Node, error) {
 		i := rand.Int()
 		j := i % len(services)
 
@@ -60,7 +62,7 @@ func (r *randomSelector) Select(service string, opts ...SelectOption) (SelectNex
 	}, nil
 }
 
-func (r *randomSelector) Mark(service string, node *Node, err error) {
+func (r *randomSelector) Mark(service string, node *registry.Node, err error) {
 	return
 }
 
@@ -72,15 +74,15 @@ func (r *randomSelector) Close() error {
 	return nil
 }
 
-func NewRandomSelector(opts ...SelectorOption) Selector {
+func newRandomSelector(opts ...Option) Selector {
-	var sopts SelectorOptions
+	var sopts Options
 
 	for _, opt := range opts {
 		opt(&sopts)
 	}
 
 	if sopts.Registry == nil {
-		sopts.Registry = DefaultRegistry
+		sopts.Registry = registry.DefaultRegistry
 	}
 
 	return &randomSelector{sopts}

selector/random_selector_test.go

@@ -1,19 +1,21 @@
-package registry
+package selector
 
 import (
 	"testing"
+
+	"github.com/micro/go-micro/registry/mock"
 )
 
 func TestRandomSelector(t *testing.T) {
 	counts := map[string]int{}
 
-	rr := &randomSelector{
+	bl := &randomSelector{
-		so: SelectorOptions{
+		so: Options{
-			Registry: &mockRegistry{},
+			Registry: mock.NewRegistry(),
 		},
 	}
 
-	next, err := rr.Select("foo")
+	next, err := bl.Select("foo")
 	if err != nil {
 		t.Errorf("Unexpected error calling random select: %v", err)
 	}

selector/roundrobin/round_robin_selector.go

@@ -1,15 +1,18 @@
-package registry
+package roundrobin
 
 import (
 	"sync"
+
+	"github.com/micro/go-micro/registry"
+	"github.com/micro/go-micro/selector"
 )
 
 type roundRobinSelector struct {
-	so SelectorOptions
+	so selector.Options
 }
 
-func (r *roundRobinSelector) Select(service string, opts ...SelectOption) (SelectNext, error) {
+func (r *roundRobinSelector) Select(service string, opts ...selector.SelectOption) (selector.Next, error) {
-	var sopts SelectOptions
+	var sopts selector.SelectOptions
 	for _, opt := range opts {
 		opt(&sopts)
 	}
@@ -27,10 +30,10 @@ func (r *roundRobinSelector) Select(service string, opts ...SelectOption) (Selec
 
 	// if there's nothing left, return
 	if len(services) == 0 {
-		return nil, ErrNotFound
+		return nil, selector.ErrNotFound
 	}
 
-	var nodes []*Node
+	var nodes []*registry.Node
 
 	for _, service := range services {
 		for _, node := range service.Nodes {
@@ -39,13 +42,13 @@ func (r *roundRobinSelector) Select(service string, opts ...SelectOption) (Selec
 	}
 
 	if len(nodes) == 0 {
-		return nil, ErrNotFound
+		return nil, selector.ErrNotFound
 	}
 
 	var i int
 	var mtx sync.Mutex
 
-	return func() (*Node, error) {
+	return func() (*registry.Node, error) {
 		mtx.Lock()
 		defer mtx.Unlock()
 		i++
@@ -53,7 +56,7 @@ func (r *roundRobinSelector) Select(service string, opts ...SelectOption) (Selec
 	}, nil
 }
 
-func (r *roundRobinSelector) Mark(service string, node *Node, err error) {
+func (r *roundRobinSelector) Mark(service string, node *registry.Node, err error) {
 	return
 }
 
@@ -65,15 +68,15 @@ func (r *roundRobinSelector) Close() error {
 	return nil
 }
 
-func NewRoundRobinSelector(opts ...SelectorOption) Selector {
+func NewRoundRobinSelector(opts ...selector.Option) selector.Selector {
-	var sopts SelectorOptions
+	var sopts selector.Options
 
 	for _, opt := range opts {
 		opt(&sopts)
 	}
 
 	if sopts.Registry == nil {
-		sopts.Registry = DefaultRegistry
+		sopts.Registry = registry.DefaultRegistry
 	}
 
 	return &roundRobinSelector{sopts}

selector/roundrobin/round_robin_selector_test.go

@@ -1,15 +1,18 @@
-package registry
+package roundrobin
 
 import (
 	"testing"
+
+	"github.com/micro/go-micro/registry/mock"
+	"github.com/micro/go-micro/selector"
 )
 
 func TestRoundRobinSelector(t *testing.T) {
 	counts := map[string]int{}
 
 	rr := &roundRobinSelector{
-		so: SelectorOptions{
+		so: selector.Options{
-			Registry: &mockRegistry{},
+			Registry: mock.NewRegistry(),
 		},
 	}
 

selector/selector.go

@@ -0,0 +1,93 @@
+/*
+The Selector package provides a way to algorithmically filter and return
+nodes required by the client or any other system. Selector's implemented
+by Micro build on the registry but it's of optional use. One could
+provide a static Selector that has a fixed pool.
+
+	func (r *randomSelector) Select(service string, opts ...SelectOption) (Next, error) {
+		var sopts SelectOptions
+		for _, opt := range opts {
+			opt(&sopts)
+		}
+
+		// get the service
+		services, err := r.so.Registry.GetService(service)
+		if err != nil {
+			return nil, err
+		}
+
+		// apply the filters
+		for _, filter := range sopts.Filters {
+			services = filter(services)
+		}
+
+		// if there's nothing left, return
+		if len(services) == 0 {
+			return nil, ErrNotFound
+		}
+
+		var nodes []*registry.Node
+
+		for _, service := range services {
+			for _, node := range service.Nodes {
+				nodes = append(nodes, node)
+			}
+		}
+
+		if len(nodes) == 0 {
+			return nil, ErrNotFound
+		}
+
+		return func() (*registry.Node, error) {
+			i := rand.Int()
+			j := i % len(services)
+
+			if len(services[j].Nodes) == 0 {
+				return nil, ErrNotFound
+			}
+
+			k := i % len(services[j].Nodes)
+			return services[j].Nodes[k], nil
+		}, nil
+	}
+
+
+*/
+package selector
+
+import (
+	"errors"
+	"github.com/micro/go-micro/registry"
+)
+
+// Selector builds on the registry as a mechanism to pick nodes
+// and mark their status. This allows host pools and other things
+// to be built using various algorithms.
+type Selector interface {
+	// Select returns a function which should return the next node
+	Select(service string, opts ...SelectOption) (Next, error)
+	// Mark sets the success/error against a node
+	Mark(service string, node *registry.Node, err error)
+	// Reset returns state back to zero for a service
+	Reset(service string)
+	// Close renders the selector unusable
+	Close() error
+}
+
+// Next is a function that returns the next node
+// based on the selector's algorithm
+type Next func() (*registry.Node, error)
+
+// SelectFilter is used to filter a service during the selection process
+type SelectFilter func([]*registry.Service) []*registry.Service
+
+var (
+	DefaultSelector = newRandomSelector()
+
+	ErrNotFound      = errors.New("not found")
+	ErrNoneAvailable = errors.New("none available")
+)
+
+func NewSelector(opts ...Option) Selector {
+	return newRandomSelector(opts...)
+}