Implement the ServerDiscovery.WatchServers gRPC endpoint (#12819)

* Implement the ServerDiscovery.WatchServers gRPC endpoint
* Fix the ConnectCA.Sign gRPC endpoints metadata forwarding.
* Unify public gRPC endpoints around the public.TraceID function for request_id logging

.changelog/12819.txt

@@ -0,0 +1,3 @@
+```release-note:improvement
+grpc: Add a new ServerDiscovery.WatchServers gRPC endpoint for being notified when the set of ready servers has changed.
+```

agent/consul/grpc_integration_test.go

@@ -2,16 +2,15 @@ package consul
 
 import (
 	"context"
-	"net"
-	"os"
 	"testing"
+	"time"
 
 	"github.com/stretchr/testify/require"
-	"google.golang.org/grpc"
 
 	"github.com/hashicorp/consul/agent/connect"
+	"github.com/hashicorp/consul/agent/grpc/public"
 	"github.com/hashicorp/consul/proto-public/pbconnectca"
-	"github.com/hashicorp/consul/testrpc"
+	"github.com/hashicorp/consul/proto-public/pbserverdiscovery"
 )
 
 func TestGRPCIntegration_ConnectCA_Sign(t *testing.T) {
@@ -19,8 +18,6 @@ func TestGRPCIntegration_ConnectCA_Sign(t *testing.T) {
 		t.Skip("too slow for testing.Short")
 	}
 
-	t.Parallel()
-
 	// The gRPC endpoint itself well-tested with mocks. This test checks we're
 	// correctly wiring everything up in the server by:
 	//
@@ -28,42 +25,24 @@ func TestGRPCIntegration_ConnectCA_Sign(t *testing.T) {
 	//	* Making a request to a follower's public gRPC port.
 	//	* Ensuring that the request is correctly forwarded to the leader.
 	//	* Ensuring we get a valid certificate back (so it went through the CAManager).
-	dir1, server1 := testServerWithConfig(t, func(c *Config) {
+	server1, conn1 := testGRPCIntegrationServer(t, func(c *Config) {
 		c.Bootstrap = false
 		c.BootstrapExpect = 2
 	})
-	defer os.RemoveAll(dir1)
-	defer server1.Shutdown()
 
-	dir2, server2 := testServerWithConfig(t, func(c *Config) {
+	server2, conn2 := testGRPCIntegrationServer(t, func(c *Config) {
 		c.Bootstrap = false
 	})
-	defer os.RemoveAll(dir2)
-	defer server2.Shutdown()
 
 	joinLAN(t, server2, server1)
 
-	testrpc.WaitForLeader(t, server1.RPC, "dc1")
+	waitForLeaderEstablishment(t, server1, server2)
 
-	var follower *Server
+	conn := conn2
-	if server1.IsLeader() {
+	if server2.IsLeader() {
-		follower = server2
+		conn = conn1
-	} else {
-		follower = server1
 	}
 
-	// publicGRPCServer is bound to a listener by the wrapping agent code, so we
-	// need to do it ourselves here.
-	lis, err := net.Listen("tcp", "127.0.0.1:0")
-	require.NoError(t, err)
-	go func() {
-		require.NoError(t, follower.publicGRPCServer.Serve(lis))
-	}()
-	t.Cleanup(follower.publicGRPCServer.Stop)
-
-	conn, err := grpc.Dial(lis.Addr().String(), grpc.WithInsecure())
-	require.NoError(t, err)
-
 	client := pbconnectca.NewConnectCAServiceClient(conn)
 
 	csr, _ := connect.TestCSR(t, &connect.SpiffeIDService{
@@ -73,8 +52,13 @@ func TestGRPCIntegration_ConnectCA_Sign(t *testing.T) {
 		Service:    "foo",
 	})
 
+	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
+	t.Cleanup(cancel)
+
+	ctx = public.ContextWithToken(ctx, TestDefaultInitialManagementToken)
+
 	// This would fail if it wasn't forwarded to the leader.
-	rsp, err := client.Sign(context.Background(), &pbconnectca.SignRequest{
+	rsp, err := client.Sign(ctx, &pbconnectca.SignRequest{
 		Csr: csr,
 	})
 	require.NoError(t, err)
@@ -82,3 +66,52 @@ func TestGRPCIntegration_ConnectCA_Sign(t *testing.T) {
 	_, err = connect.ParseCert(rsp.CertPem)
 	require.NoError(t, err)
 }
+
+func TestGRPCIntegration_ServerDiscovery_WatchServers(t *testing.T) {
+	if testing.Short() {
+		t.Skip("too slow for testing.Short")
+	}
+
+	// The gRPC endpoint itself well-tested with mocks. This test checks we're
+	// correctly wiring everything up in the server by:
+	//
+	//	* Starting a server
+	// * Initiating the gRPC stream
+	// * Validating the snapshot
+	// * Adding another server
+	// * Validating another message is sent.
+
+	server1, conn := testGRPCIntegrationServer(t, func(c *Config) {
+		c.Bootstrap = true
+		c.BootstrapExpect = 1
+	})
+	waitForLeaderEstablishment(t, server1)
+
+	client := pbserverdiscovery.NewServerDiscoveryServiceClient(conn)
+
+	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
+	t.Cleanup(cancel)
+
+	ctx = public.ContextWithToken(ctx, TestDefaultInitialManagementToken)
+
+	serverStream, err := client.WatchServers(ctx, &pbserverdiscovery.WatchServersRequest{Wan: false})
+	require.NoError(t, err)
+
+	rsp, err := serverStream.Recv()
+	require.NoError(t, err)
+	require.NotNil(t, rsp)
+	require.Len(t, rsp.Servers, 1)
+
+	_, server2, _ := testACLServerWithConfig(t, func(c *Config) {
+		c.Bootstrap = false
+	}, false)
+
+	// join the new server to the leader
+	joinLAN(t, server2, server1)
+
+	// now receive the event containing 2 servers
+	rsp, err = serverStream.Recv()
+	require.NoError(t, err)
+	require.NotNil(t, rsp)
+	require.Len(t, rsp.Servers, 2)
+}

agent/consul/server.go

@@ -46,6 +46,7 @@ import (
 	"github.com/hashicorp/consul/agent/grpc/private/services/subscribe"
 	"github.com/hashicorp/consul/agent/grpc/public/services/connectca"
 	"github.com/hashicorp/consul/agent/grpc/public/services/dataplane"
+	"github.com/hashicorp/consul/agent/grpc/public/services/serverdiscovery"
 	"github.com/hashicorp/consul/agent/metadata"
 	"github.com/hashicorp/consul/agent/pool"
 	"github.com/hashicorp/consul/agent/router"
@@ -683,6 +684,12 @@ func NewServer(config *Config, flat Deps, publicGRPCServer *grpc.Server) (*Serve
 		Datacenter:  s.config.Datacenter,
 	}).Register(s.publicGRPCServer)
 
+	serverdiscovery.NewServer(serverdiscovery.Config{
+		Publisher:   s.publisher,
+		ACLResolver: plainACLResolver{s.ACLResolver},
+		Logger:      logger.Named("grpc-api.server-discovery"),
+	}).Register(s.publicGRPCServer)
+
 	// Initialize private gRPC server.
 	//
 	// Note: some "public" gRPC services are also exposed on the private gRPC server

agent/consul/server_test.go

@@ -257,6 +257,26 @@ func testACLServerWithConfig(t *testing.T, cb func(*Config), initReplicationToke
 	return dir, srv, codec
 }
 
+func testGRPCIntegrationServer(t *testing.T, cb func(*Config)) (*Server, *grpc.ClientConn) {
+	_, srv, _ := testACLServerWithConfig(t, cb, false)
+
+	// Normally the gRPC server listener is created at the agent level and passed down into
+	// the Server creation. For our tests, we need to ensure
+	ln, err := net.Listen("tcp", "127.0.0.1:0")
+	require.NoError(t, err)
+	go func() {
+		_ = srv.publicGRPCServer.Serve(ln)
+	}()
+	t.Cleanup(srv.publicGRPCServer.Stop)
+
+	conn, err := grpc.Dial(ln.Addr().String(), grpc.WithInsecure())
+	require.NoError(t, err)
+
+	t.Cleanup(func() { _ = conn.Close() })
+
+	return srv, conn
+}
+
 func newServer(t *testing.T, c *Config) (*Server, error) {
 	return newServerWithDeps(t, c, newDefaultDeps(t, c))
 }

agent/grpc/public/forward.go

@@ -0,0 +1,16 @@
+package public
+
+import (
+	"context"
+
+	"google.golang.org/grpc/metadata"
+)
+
+func ForwardMetadataContext(ctx context.Context) context.Context {
+	md, ok := metadata.FromIncomingContext(ctx)
+	if !ok {
+		return ctx
+	}
+
+	return metadata.NewOutgoingContext(ctx, md)
+}

agent/grpc/public/services/connectca/server_test.go

@@ -2,17 +2,15 @@ package connectca
 
 import (
 	"context"
-	"net"
-	"sync"
 	"testing"
-	"time"
 
 	"github.com/stretchr/testify/require"
 	"google.golang.org/grpc"
 
 	"github.com/hashicorp/consul/agent/consul/state"
 	"github.com/hashicorp/consul/agent/consul/stream"
-	"github.com/hashicorp/consul/agent/structs"
+	"github.com/hashicorp/consul/agent/grpc/public/testutils"
+	structs "github.com/hashicorp/consul/agent/structs"
 	"github.com/hashicorp/consul/proto-public/pbconnectca"
 )
 
@@ -20,68 +18,26 @@ func noopForwardRPC(structs.RPCInfo, func(*grpc.ClientConn) error) (bool, error)
 	return false, nil
 }
 
-func testStateStore(t *testing.T, publisher state.EventPublisher) *state.Store {
+func setupFSMAndPublisher(t *testing.T) (*testutils.FakeFSM, state.EventPublisher) {
 	t.Helper()
 
-	gc, err := state.NewTombstoneGC(time.Second, time.Millisecond)
+	config := testutils.FakeFSMConfig{
-	require.NoError(t, err)
+		Register: func(fsm *testutils.FakeFSM, publisher *stream.EventPublisher) {
-
-	return state.NewStateStoreWithEventPublisher(gc, publisher)
-}
-
-type FakeFSM struct {
-	lock      sync.Mutex
-	store     *state.Store
-	publisher *stream.EventPublisher
-}
-
-func newFakeFSM(t *testing.T, publisher *stream.EventPublisher) *FakeFSM {
-	t.Helper()
-
-	store := testStateStore(t, publisher)
-
-	fsm := FakeFSM{store: store, publisher: publisher}
-
 			// register handlers
 			publisher.RegisterHandler(state.EventTopicCARoots, func(req stream.SubscribeRequest, buf stream.SnapshotAppender) (uint64, error) {
 				return fsm.GetStore().CARootsSnapshot(req, buf)
 			})
+		},
+		Refresh: []stream.Topic{state.EventTopicCARoots},
+	}
 
-	return &fsm
+	return testutils.SetupFSMAndPublisher(t, config)
-}
-
-func (f *FakeFSM) GetStore() *state.Store {
-	f.lock.Lock()
-	defer f.lock.Unlock()
-	return f.store
-}
-
-func (f *FakeFSM) ReplaceStore(store *state.Store) {
-	f.lock.Lock()
-	defer f.lock.Unlock()
-	oldStore := f.store
-	f.store = store
-	oldStore.Abandon()
-	f.publisher.RefreshTopic(state.EventTopicCARoots)
-}
-
-func setupFSMAndPublisher(t *testing.T) (*FakeFSM, state.EventPublisher) {
-	t.Helper()
-	publisher := stream.NewEventPublisher(10 * time.Second)
-
-	fsm := newFakeFSM(t, publisher)
-
-	ctx, cancel := context.WithCancel(context.Background())
-	t.Cleanup(cancel)
-	go publisher.Run(ctx)
-
-	return fsm, publisher
 }
 
 func testClient(t *testing.T, server *Server) pbconnectca.ConnectCAServiceClient {
 	t.Helper()
 
-	addr := runTestServer(t, server)
+	addr := testutils.RunTestServer(t, server)
 
 	conn, err := grpc.DialContext(context.Background(), addr.String(), grpc.WithInsecure())
 	require.NoError(t, err)
@@ -91,18 +47,3 @@ func testClient(t *testing.T, server *Server) pbconnectca.ConnectCAServiceClient
 
 	return pbconnectca.NewConnectCAServiceClient(conn)
 }
-
-func runTestServer(t *testing.T, server *Server) net.Addr {
-	t.Helper()
-
-	lis, err := net.Listen("tcp", "127.0.0.1:0")
-	require.NoError(t, err)
-
-	grpcServer := grpc.NewServer()
-	server.Register(grpcServer)
-
-	go grpcServer.Serve(lis)
-	t.Cleanup(grpcServer.Stop)
-
-	return lis.Addr()
-}

agent/grpc/public/services/connectca/sign.go

@@ -22,7 +22,7 @@ func (s *Server) Sign(ctx context.Context, req *pbconnectca.SignRequest) (*pbcon
 		return nil, err
 	}
 
-	logger := s.Logger.Named("sign").With("request_id", traceID())
+	logger := s.Logger.Named("sign").With("request_id", public.TraceID())
 	logger.Trace("request received")
 
 	token := public.TokenFromContext(ctx)
@@ -48,6 +48,7 @@ func (s *Server) Sign(ctx context.Context, req *pbconnectca.SignRequest) (*pbcon
 	var rsp *pbconnectca.SignResponse
 	handled, err := s.ForwardRPC(&rpcInfo, func(conn *grpc.ClientConn) error {
 		logger.Trace("forwarding RPC")
+		ctx := public.ForwardMetadataContext(ctx)
 		var err error
 		rsp, err = pbconnectca.NewConnectCAServiceClient(conn).Sign(ctx, req)
 		return err

agent/grpc/public/services/connectca/sign_test.go

@@ -15,6 +15,7 @@ import (
 
 	acl "github.com/hashicorp/consul/acl"
 	"github.com/hashicorp/consul/agent/connect"
+	"github.com/hashicorp/consul/agent/grpc/public/testutils"
 	"github.com/hashicorp/consul/agent/structs"
 	"github.com/hashicorp/consul/proto-public/pbconnectca"
 )
@@ -231,7 +232,7 @@ func TestSign_RPCForwarding(t *testing.T) {
 		ForwardRPC:     noopForwardRPC,
 		ConnectEnabled: true,
 	})
-	leaderConn, err := grpc.Dial(runTestServer(t, leader).String(), grpc.WithInsecure())
+	leaderConn, err := grpc.Dial(testutils.RunTestServer(t, leader).String(), grpc.WithInsecure())
 	require.NoError(t, err)
 
 	follower := NewServer(Config{

agent/grpc/public/services/connectca/watch_roots.go

@@ -11,7 +11,6 @@ import (
 	"google.golang.org/protobuf/types/known/timestamppb"
 
 	"github.com/hashicorp/go-hclog"
-	"github.com/hashicorp/go-uuid"
 
 	"github.com/hashicorp/consul/acl"
 	"github.com/hashicorp/consul/agent/connect"
@@ -30,7 +29,7 @@ func (s *Server) WatchRoots(_ *emptypb.Empty, serverStream pbconnectca.ConnectCA
 		return err
 	}
 
-	logger := s.Logger.Named("watch-roots").With("stream_id", traceID())
+	logger := s.Logger.Named("watch-roots").With("request_id", public.TraceID())
 	logger.Trace("starting stream")
 	defer logger.Trace("stream closed")
 
@@ -181,16 +180,6 @@ func (s *Server) authorize(token string) error {
 	return nil
 }
 
-// We tag logs with a unique identifier to ease debugging. In the future this
-// should probably be a real Open Telemetry trace ID.
-func traceID() string {
-	id, err := uuid.GenerateUUID()
-	if err != nil {
-		return ""
-	}
-	return id
-}
-
 func getTrustDomain(store StateStore, logger hclog.Logger) (string, error) {
 	_, cfg, err := store.CAConfig(nil)
 	switch {

agent/grpc/public/services/connectca/watch_roots_test.go

@@ -230,7 +230,7 @@ func TestWatchRoots_StateStoreAbandoned(t *testing.T) {
 	mustGetRoots(t, rspCh)
 
 	// Simulate a snapshot restore.
-	storeB := testStateStore(t, publisher)
+	storeB := testutils.TestStateStore(t, publisher)
 
 	rootB := connect.TestCA(t, nil)
 	_, err = storeB.CARootSetCAS(1, 0, structs.CARoots{rootB})

agent/grpc/public/services/dataplane/get_envoy_boostrap_params_test.go

@@ -69,7 +69,7 @@ func TestGetEnvoyBootstrapParams_Success(t *testing.T) {
 	}
 
 	run := func(t *testing.T, tc testCase) {
-		store := testStateStore(t)
+		store := testutils.TestStateStore(t, nil)
 		err := store.EnsureRegistration(1, tc.registerReq)
 		require.NoError(t, err)
 
@@ -148,7 +148,7 @@ func TestGetEnvoyBootstrapParams_Error(t *testing.T) {
 			Return(testutils.TestAuthorizerServiceRead(t, proxyServiceID), nil)
 		ctx := public.ContextWithToken(context.Background(), testToken)
 
-		store := testStateStore(t)
+		store := testutils.TestStateStore(t, nil)
 		registerReq := testRegisterRequestProxy(t)
 		err := store.EnsureRegistration(1, registerReq)
 		require.NoError(t, err)
@@ -217,7 +217,7 @@ func TestGetEnvoyBootstrapParams_Unauthenticated(t *testing.T) {
 	aclResolver.On("ResolveTokenAndDefaultMeta", mock.Anything, mock.Anything, mock.Anything).
 		Return(nil, acl.ErrNotFound)
 	ctx := public.ContextWithToken(context.Background(), testToken)
-	store := testStateStore(t)
+	store := testutils.TestStateStore(t, nil)
 	server := NewServer(Config{
 		GetStore:    func() StateStore { return store },
 		Logger:      hclog.NewNullLogger(),
@@ -236,7 +236,7 @@ func TestGetEnvoyBootstrapParams_PermissionDenied(t *testing.T) {
 	aclResolver.On("ResolveTokenAndDefaultMeta", testToken, mock.Anything, mock.Anything).
 		Return(acl.DenyAll(), nil)
 	ctx := public.ContextWithToken(context.Background(), testToken)
-	store := testStateStore(t)
+	store := testutils.TestStateStore(t, nil)
 	registerReq := structs.TestRegisterRequestProxy(t)
 	proxyServiceID := "web-sidecar-proxy"
 	registerReq.Service.ID = proxyServiceID

agent/grpc/public/services/dataplane/server_test.go

@@ -2,29 +2,18 @@ package dataplane
 
 import (
 	"context"
-	"net"
 	"testing"
-	"time"
 
-	"github.com/hashicorp/consul/agent/consul/state"
+	"github.com/hashicorp/consul/agent/grpc/public/testutils"
 	"github.com/hashicorp/consul/proto-public/pbdataplane"
 	"github.com/stretchr/testify/require"
 	"google.golang.org/grpc"
 )
 
-func testStateStore(t *testing.T) *state.Store {
-	t.Helper()
-
-	gc, err := state.NewTombstoneGC(time.Second, time.Millisecond)
-	require.NoError(t, err)
-
-	return state.NewStateStore(gc)
-}
-
 func testClient(t *testing.T, server *Server) pbdataplane.DataplaneServiceClient {
 	t.Helper()
 
-	addr := RunTestServer(t, server)
+	addr := testutils.RunTestServer(t, server)
 
 	conn, err := grpc.DialContext(context.Background(), addr.String(), grpc.WithInsecure())
 	require.NoError(t, err)
@@ -34,18 +23,3 @@ func testClient(t *testing.T, server *Server) pbdataplane.DataplaneServiceClient
 
 	return pbdataplane.NewDataplaneServiceClient(conn)
 }
-
-func RunTestServer(t *testing.T, server *Server) net.Addr {
-	t.Helper()
-
-	lis, err := net.Listen("tcp", "127.0.0.1:0")
-	require.NoError(t, err)
-
-	grpcServer := grpc.NewServer()
-	server.Register(grpcServer)
-
-	go grpcServer.Serve(lis)
-	t.Cleanup(grpcServer.Stop)
-
-	return lis.Addr()
-}

agent/grpc/public/services/serverdiscovery/mock_ACLResolver.go

@@ -0,0 +1,36 @@
+// Code generated by mockery v1.0.0. DO NOT EDIT.
+
+package serverdiscovery
+
+import (
+	acl "github.com/hashicorp/consul/acl"
+	mock "github.com/stretchr/testify/mock"
+)
+
+// MockACLResolver is an autogenerated mock type for the ACLResolver type
+type MockACLResolver struct {
+	mock.Mock
+}
+
+// ResolveTokenAndDefaultMeta provides a mock function with given fields: _a0, _a1, _a2
+func (_m *MockACLResolver) ResolveTokenAndDefaultMeta(_a0 string, _a1 *acl.EnterpriseMeta, _a2 *acl.AuthorizerContext) (acl.Authorizer, error) {
+	ret := _m.Called(_a0, _a1, _a2)
+
+	var r0 acl.Authorizer
+	if rf, ok := ret.Get(0).(func(string, *acl.EnterpriseMeta, *acl.AuthorizerContext) acl.Authorizer); ok {
+		r0 = rf(_a0, _a1, _a2)
+	} else {
+		if ret.Get(0) != nil {
+			r0 = ret.Get(0).(acl.Authorizer)
+		}
+	}
+
+	var r1 error
+	if rf, ok := ret.Get(1).(func(string, *acl.EnterpriseMeta, *acl.AuthorizerContext) error); ok {
+		r1 = rf(_a0, _a1, _a2)
+	} else {
+		r1 = ret.Error(1)
+	}
+
+	return r0, r1
+}

agent/grpc/public/services/serverdiscovery/server.go

@@ -0,0 +1,38 @@
+package serverdiscovery
+
+import (
+	"google.golang.org/grpc"
+
+	"github.com/hashicorp/go-hclog"
+
+	"github.com/hashicorp/consul/acl"
+	"github.com/hashicorp/consul/agent/consul/stream"
+	"github.com/hashicorp/consul/proto-public/pbserverdiscovery"
+)
+
+type Server struct {
+	Config
+}
+
+type Config struct {
+	Publisher   EventPublisher
+	Logger      hclog.Logger
+	ACLResolver ACLResolver
+}
+
+type EventPublisher interface {
+	Subscribe(*stream.SubscribeRequest) (*stream.Subscription, error)
+}
+
+//go:generate mockery -name ACLResolver -inpkg
+type ACLResolver interface {
+	ResolveTokenAndDefaultMeta(string, *acl.EnterpriseMeta, *acl.AuthorizerContext) (acl.Authorizer, error)
+}
+
+func NewServer(cfg Config) *Server {
+	return &Server{cfg}
+}
+
+func (s *Server) Register(grpcServer *grpc.Server) {
+	pbserverdiscovery.RegisterServerDiscoveryServiceServer(grpcServer, s)
+}

agent/grpc/public/services/serverdiscovery/server_test.go

@@ -0,0 +1,89 @@
+package serverdiscovery
+
+import (
+	"context"
+	"testing"
+	"time"
+
+	mock "github.com/stretchr/testify/mock"
+	"github.com/stretchr/testify/require"
+	"google.golang.org/grpc"
+
+	"github.com/hashicorp/consul/agent/consul/autopilotevents"
+	"github.com/hashicorp/consul/agent/consul/state"
+	"github.com/hashicorp/consul/agent/consul/stream"
+	"github.com/hashicorp/consul/agent/grpc/public/testutils"
+	"github.com/hashicorp/consul/proto-public/pbserverdiscovery"
+)
+
+type mockSnapshotHandler struct {
+	mock.Mock
+}
+
+func newMockSnapshotHandler(t *testing.T) *mockSnapshotHandler {
+	handler := &mockSnapshotHandler{}
+	t.Cleanup(func() {
+		handler.AssertExpectations(t)
+	})
+	return handler
+}
+
+func (m *mockSnapshotHandler) handle(req stream.SubscribeRequest, buf stream.SnapshotAppender) (uint64, error) {
+	ret := m.Called(req, buf)
+	return ret.Get(0).(uint64), ret.Error(1)
+}
+
+func (m *mockSnapshotHandler) expect(token string, requestIndex uint64, eventIndex uint64, payload autopilotevents.EventPayloadReadyServers) {
+	m.On("handle", stream.SubscribeRequest{
+		Topic:   autopilotevents.EventTopicReadyServers,
+		Subject: stream.SubjectNone,
+		Token:   token,
+		Index:   requestIndex,
+	}, mock.Anything).Once().Run(func(args mock.Arguments) {
+		buf := args.Get(1).(stream.SnapshotAppender)
+		buf.Append([]stream.Event{
+			{
+				Topic:   autopilotevents.EventTopicReadyServers,
+				Index:   eventIndex,
+				Payload: payload,
+			},
+		})
+	}).Return(eventIndex, nil)
+}
+
+func newMockACLResolver(t *testing.T) *MockACLResolver {
+	t.Helper()
+	m := &MockACLResolver{}
+	t.Cleanup(func() { m.AssertExpectations(t) })
+	return m
+}
+
+func setupPublisher(t *testing.T) (*mockSnapshotHandler, state.EventPublisher) {
+	t.Helper()
+
+	handler := newMockSnapshotHandler(t)
+
+	publisher := stream.NewEventPublisher(10 * time.Second)
+	publisher.RegisterHandler(autopilotevents.EventTopicReadyServers, handler.handle)
+
+	ctx, cancel := context.WithCancel(context.Background())
+	t.Cleanup(cancel)
+	go publisher.Run(ctx)
+
+	return handler, publisher
+
+}
+
+func testClient(t *testing.T, server *Server) pbserverdiscovery.ServerDiscoveryServiceClient {
+	t.Helper()
+
+	addr := testutils.RunTestServer(t, server)
+
+	conn, err := grpc.DialContext(context.Background(), addr.String(), grpc.WithInsecure())
+	require.NoError(t, err)
+	t.Cleanup(func() {
+		require.NoError(t, conn.Close())
+	})
+
+	return pbserverdiscovery.NewServerDiscoveryServiceClient(conn)
+}

agent/grpc/public/services/serverdiscovery/watch_servers.go

@@ -0,0 +1,146 @@
+package serverdiscovery
+
+import (
+	"context"
+	"errors"
+
+	"github.com/hashicorp/consul/acl"
+	"github.com/hashicorp/consul/agent/consul/autopilotevents"
+	"github.com/hashicorp/consul/agent/consul/stream"
+	"github.com/hashicorp/consul/agent/grpc/public"
+	"github.com/hashicorp/consul/agent/structs"
+	"github.com/hashicorp/consul/proto-public/pbserverdiscovery"
+	"github.com/hashicorp/go-hclog"
+	"google.golang.org/grpc/codes"
+	"google.golang.org/grpc/status"
+)
+
+// WatchServers provides a stream on which you can receive the list of servers
+// that are ready to receive incoming requests including stale queries. The
+// current set of ready servers are sent immediately at the start of the
+// stream and new updates will be sent whenver the set of ready servers changes.
+func (s *Server) WatchServers(req *pbserverdiscovery.WatchServersRequest, serverStream pbserverdiscovery.ServerDiscoveryService_WatchServersServer) error {
+	logger := s.Logger.Named("watch-servers").With("request_id", public.TraceID())
+
+	logger.Debug("starting stream")
+	defer logger.Trace("stream closed")
+
+	token := public.TokenFromContext(serverStream.Context())
+
+	// Serve the ready servers from an EventPublisher subscription. If the subscription is
+	// closed due to an ACL change, we'll attempt to re-authorize and resume it to
+	// prevent unnecessarily terminating the stream.
+	var idx uint64
+	for {
+		var err error
+		idx, err = s.serveReadyServers(token, idx, req, serverStream, logger)
+		if errors.Is(err, stream.ErrSubForceClosed) {
+			logger.Trace("subscription force-closed due to an ACL change or snapshot restore, will attempt to re-auth and resume")
+		} else {
+			return err
+		}
+	}
+}
+
+func (s *Server) serveReadyServers(token string, index uint64, req *pbserverdiscovery.WatchServersRequest, serverStream pbserverdiscovery.ServerDiscoveryService_WatchServersServer, logger hclog.Logger) (uint64, error) {
+	if err := s.authorize(token); err != nil {
+		return 0, err
+	}
+
+	// Start the subscription.
+	sub, err := s.Publisher.Subscribe(&stream.SubscribeRequest{
+		Topic:   autopilotevents.EventTopicReadyServers,
+		Subject: stream.SubjectNone,
+		Token:   token,
+		Index:   index,
+	})
+	if err != nil {
+		logger.Error("failed to subscribe to server discovery events", "error", err)
+		return 0, status.Error(codes.Internal, "failed to subscribe to server discovery events")
+	}
+	defer sub.Unsubscribe()
+
+	for {
+		event, err := sub.Next(serverStream.Context())
+		switch {
+		case errors.Is(err, stream.ErrSubForceClosed):
+			return index, err
+		case errors.Is(err, context.Canceled):
+			return 0, nil
+		case err != nil:
+			logger.Error("failed to read next event", "error", err)
+			return index, status.Error(codes.Internal, err.Error())
+		}
+
+		// We do not send framing events (e.g. EndOfSnapshot, NewSnapshotToFollow)
+		// because we send a full list of ready servers on every event, rather than expecting
+		// clients to maintain a state-machine in the way they do for service health.
+		if event.IsFramingEvent() {
+			continue
+		}
+
+		// Note: this check isn't strictly necessary because the event publishing
+		// machinery will ensure the index increases monotonically, but it can be
+		// tricky to faithfully reproduce this in tests (e.g. the EventPublisher
+		// garbage collects topic buffers and snapshots aggressively when streams
+		// disconnect) so this avoids a bunch of confusing setup code.
+		if event.Index <= index {
+			continue
+		}
+
+		index = event.Index
+
+		rsp, err := eventToResponse(req, event)
+		if err != nil {
+			logger.Error("failed to convert event to response", "error", err)
+			return index, status.Error(codes.Internal, err.Error())
+		}
+		if err := serverStream.Send(rsp); err != nil {
+			logger.Error("failed to send response", "error", err)
+			return index, err
+		}
+	}
+}
+
+func (s *Server) authorize(token string) error {
+	// Require the given ACL token to have `service:write` on any service (in any
+	// partition and namespace).
+	var authzContext acl.AuthorizerContext
+	entMeta := structs.WildcardEnterpriseMetaInPartition(structs.WildcardSpecifier)
+	authz, err := s.ACLResolver.ResolveTokenAndDefaultMeta(token, entMeta, &authzContext)
+	if err != nil {
+		return status.Error(codes.Unauthenticated, err.Error())
+	}
+	if err := authz.ToAllowAuthorizer().ServiceWriteAnyAllowed(&authzContext); err != nil {
+		return status.Error(codes.PermissionDenied, err.Error())
+	}
+	return nil
+}
+
+func eventToResponse(req *pbserverdiscovery.WatchServersRequest, event stream.Event) (*pbserverdiscovery.WatchServersResponse, error) {
+	readyServers, err := autopilotevents.ExtractEventPayload(event)
+	if err != nil {
+		return nil, err
+	}
+
+	var servers []*pbserverdiscovery.Server
+
+	for _, srv := range readyServers {
+		addr := srv.Address
+
+		wanAddr, ok := srv.TaggedAddresses[structs.TaggedAddressWAN]
+		if req.Wan && ok {
+			addr = wanAddr
+		}
+
+		servers = append(servers, &pbserverdiscovery.Server{
+			Id:      srv.ID,
+			Version: srv.Version,
+			Address: addr,
+		})
+	}
+
+	return &pbserverdiscovery.WatchServersResponse{
+		Servers: servers,
+	}, nil
+}

agent/grpc/public/services/serverdiscovery/watch_servers_test.go

@@ -0,0 +1,302 @@
+package serverdiscovery
+
+import (
+	"context"
+	"errors"
+	"io"
+	"testing"
+	"time"
+
+	acl "github.com/hashicorp/consul/acl"
+	"github.com/hashicorp/consul/agent/consul/autopilotevents"
+	"github.com/hashicorp/consul/agent/consul/stream"
+	"github.com/hashicorp/consul/agent/grpc/public"
+	"github.com/hashicorp/consul/agent/grpc/public/testutils"
+	"github.com/hashicorp/consul/proto-public/pbserverdiscovery"
+	"github.com/hashicorp/consul/proto/prototest"
+	"github.com/hashicorp/consul/sdk/testutil"
+	mock "github.com/stretchr/testify/mock"
+	"github.com/stretchr/testify/require"
+	"google.golang.org/grpc/codes"
+	"google.golang.org/grpc/status"
+)
+
+const testACLToken = "eb61f1ed-65a4-4da6-8d3d-0564bd16c965"
+
+func TestWatchServers_StreamLifeCycle(t *testing.T) {
+	// The flow for this test is roughly:
+	//
+	// 1. Open a WatchServers stream
+	// 2. Observe the snapshot message is sent back through
+	//    the stream.
+	// 3. Publish an event that changes to 2 servers.
+	// 4. See the corresponding message sent back through the stream.
+	// 5. Send a NewCloseSubscriptionEvent for the token secret.
+	// 6. See that a new snapshot is taken and the corresponding message
+	//    gets sent back. If there were multiple subscribers for the topic
+	//    then this should not happen. However with the current EventPublisher
+	//    implementation, whenever the last subscriber for a topic has its
+	//    subscription closed then the publisher will delete the whole topic
+	//    buffer. When that happens, resubscribing will see no snapshot
+	//    cache, or latest event in the buffer and force creating a new snapshot.
+	// 7. Publish another event to move to 3 servers.
+	// 8. Ensure that the message gets sent through the stream. Also
+	//    this will validate that no other 1 or 2 server event is
+	//    seen after stream reinitialization.
+
+	srv1 := autopilotevents.ReadyServerInfo{
+		ID:      "9aeb73f6-e83e-43c1-bdc9-ca5e43efe3e4",
+		Address: "198.18.0.1",
+		Version: "1.12.0",
+	}
+	srv2 := autopilotevents.ReadyServerInfo{
+		ID:      "eec8721f-c42b-48da-a5a5-07565158015e",
+		Address: "198.18.0.2",
+		Version: "1.12.3",
+	}
+	srv3 := autopilotevents.ReadyServerInfo{
+		ID:      "256796f2-3a38-4f80-8cef-375c3cb3aa1f",
+		Address: "198.18.0.3",
+		Version: "1.12.3",
+	}
+
+	oneServerEventPayload := autopilotevents.EventPayloadReadyServers{srv1}
+	twoServerEventPayload := autopilotevents.EventPayloadReadyServers{srv1, srv2}
+	threeServerEventPayload := autopilotevents.EventPayloadReadyServers{srv1, srv2, srv3}
+
+	oneServerResponse := &pbserverdiscovery.WatchServersResponse{
+		Servers: []*pbserverdiscovery.Server{
+			{
+				Id:      srv1.ID,
+				Address: srv1.Address,
+				Version: srv1.Version,
+			},
+		},
+	}
+
+	twoServerResponse := &pbserverdiscovery.WatchServersResponse{
+		Servers: []*pbserverdiscovery.Server{
+			{
+				Id:      srv1.ID,
+				Address: srv1.Address,
+				Version: srv1.Version,
+			},
+			{
+				Id:      srv2.ID,
+				Address: srv2.Address,
+				Version: srv2.Version,
+			},
+		},
+	}
+
+	threeServerResponse := &pbserverdiscovery.WatchServersResponse{
+		Servers: []*pbserverdiscovery.Server{
+			{
+				Id:      srv1.ID,
+				Address: srv1.Address,
+				Version: srv1.Version,
+			},
+			{
+				Id:      srv2.ID,
+				Address: srv2.Address,
+				Version: srv2.Version,
+			},
+			{
+				Id:      srv3.ID,
+				Address: srv3.Address,
+				Version: srv3.Version,
+			},
+		},
+	}
+
+	// setup the event publisher and snapshot handler
+	handler, publisher := setupPublisher(t)
+	// we only expect this to be called once. For the rest of the
+	// test we ought to be able to resume the stream.
+	handler.expect(testACLToken, 0, 1, oneServerEventPayload)
+	handler.expect(testACLToken, 2, 3, twoServerEventPayload)
+
+	// setup the mock ACLResolver and its expectations
+	// 2 times authorization should succeed and the third should fail.
+	resolver := newMockACLResolver(t)
+	resolver.On("ResolveTokenAndDefaultMeta", testACLToken, mock.Anything, mock.Anything).
+		Return(testutils.TestAuthorizerServiceWriteAny(t), nil).Twice()
+
+	// add the token to the requests context
+	ctx := public.ContextWithToken(context.Background(), testACLToken)
+
+	// setup the server
+	server := NewServer(Config{
+		Publisher:   publisher,
+		Logger:      testutil.Logger(t),
+		ACLResolver: resolver,
+	})
+
+	// Run the server and get a test client for it
+	client := testClient(t, server)
+
+	// 1. Open the WatchServers stream
+	serverStream, err := client.WatchServers(ctx, &pbserverdiscovery.WatchServersRequest{Wan: false})
+	require.NoError(t, err)
+
+	rspCh := handleReadyServersStream(t, serverStream)
+
+	// 2. Observe the snapshot message is sent back through the stream.
+	rsp := mustGetServers(t, rspCh)
+	require.NotNil(t, rsp)
+	prototest.AssertDeepEqual(t, oneServerResponse, rsp)
+
+	// 3. Publish an event that changes to 2 servers.
+	publisher.Publish([]stream.Event{
+		{
+			Topic:   autopilotevents.EventTopicReadyServers,
+			Index:   2,
+			Payload: twoServerEventPayload,
+		},
+	})
+
+	// 4. See the corresponding message sent back through the stream.
+	rsp = mustGetServers(t, rspCh)
+	require.NotNil(t, rsp)
+	prototest.AssertDeepEqual(t, twoServerResponse, rsp)
+
+	// 5. Send a NewCloseSubscriptionEvent for the token secret.
+	publisher.Publish([]stream.Event{
+		stream.NewCloseSubscriptionEvent([]string{testACLToken}),
+	})
+
+	// 6. Observe another snapshot message
+	rsp = mustGetServers(t, rspCh)
+	require.NotNil(t, rsp)
+	prototest.AssertDeepEqual(t, twoServerResponse, rsp)
+
+	// 7. Publish another event to move to 3 servers.
+	publisher.Publish([]stream.Event{
+		{
+			Topic:   autopilotevents.EventTopicReadyServers,
+			Index:   4,
+			Payload: threeServerEventPayload,
+		},
+	})
+
+	// 8. Ensure that the message gets sent through the stream. Also
+	//    this will validate that no other 1 or 2 server event is
+	//    seen after stream reinitialization.
+	rsp = mustGetServers(t, rspCh)
+	require.NotNil(t, rsp)
+	prototest.AssertDeepEqual(t, threeServerResponse, rsp)
+}
+
+func TestWatchServers_ACLToken_PermissionDenied(t *testing.T) {
+	// setup the event publisher and snapshot handler
+	_, publisher := setupPublisher(t)
+
+	resolver := newMockACLResolver(t)
+	resolver.On("ResolveTokenAndDefaultMeta", testACLToken, mock.Anything, mock.Anything).
+		Return(acl.DenyAll(), nil).Once()
+
+	// add the token to the requests context
+	ctx := public.ContextWithToken(context.Background(), testACLToken)
+
+	// setup the server
+	server := NewServer(Config{
+		Publisher:   publisher,
+		Logger:      testutil.Logger(t),
+		ACLResolver: resolver,
+	})
+
+	// Run the server and get a test client for it
+	client := testClient(t, server)
+
+	// 1. Open the WatchServers stream
+	serverStream, err := client.WatchServers(ctx, &pbserverdiscovery.WatchServersRequest{Wan: false})
+	require.NoError(t, err)
+	rspCh := handleReadyServersStream(t, serverStream)
+
+	// Expect to get an Unauthenticated error immediately.
+	err = mustGetError(t, rspCh)
+	require.Equal(t, codes.PermissionDenied.String(), status.Code(err).String())
+}
+
+func TestWatchServers_ACLToken_Unauthenticated(t *testing.T) {
+	// setup the event publisher and snapshot handler
+	_, publisher := setupPublisher(t)
+
+	resolver := newMockACLResolver(t)
+	resolver.On("ResolveTokenAndDefaultMeta", testACLToken, mock.Anything, mock.Anything).
+		Return(nil, acl.ErrNotFound).Once()
+
+	// add the token to the requests context
+	ctx := public.ContextWithToken(context.Background(), testACLToken)
+
+	// setup the server
+	server := NewServer(Config{
+		Publisher:   publisher,
+		Logger:      testutil.Logger(t),
+		ACLResolver: resolver,
+	})
+
+	// Run the server and get a test client for it
+	client := testClient(t, server)
+
+	// 1. Open the WatchServers stream
+	serverStream, err := client.WatchServers(ctx, &pbserverdiscovery.WatchServersRequest{Wan: false})
+	require.NoError(t, err)
+	rspCh := handleReadyServersStream(t, serverStream)
+
+	// Expect to get an Unauthenticated error immediately.
+	err = mustGetError(t, rspCh)
+	require.Equal(t, codes.Unauthenticated.String(), status.Code(err).String())
+}
+
+func handleReadyServersStream(t *testing.T, stream pbserverdiscovery.ServerDiscoveryService_WatchServersClient) <-chan serversOrError {
+	t.Helper()
+
+	rspCh := make(chan serversOrError)
+	go func() {
+		for {
+			rsp, err := stream.Recv()
+			if errors.Is(err, io.EOF) ||
+				errors.Is(err, context.Canceled) ||
+				errors.Is(err, context.DeadlineExceeded) {
+				return
+			}
+			rspCh <- serversOrError{
+				rsp: rsp,
+				err: err,
+			}
+		}
+	}()
+	return rspCh
+}
+
+func mustGetServers(t *testing.T, ch <-chan serversOrError) *pbserverdiscovery.WatchServersResponse {
+	t.Helper()
+
+	select {
+	case rsp := <-ch:
+		require.NoError(t, rsp.err)
+		return rsp.rsp
+	case <-time.After(1 * time.Second):
+		t.Fatal("timeout waiting for WatchServersResponse")
+		return nil
+	}
+}
+
+func mustGetError(t *testing.T, ch <-chan serversOrError) error {
+	t.Helper()
+
+	select {
+	case rsp := <-ch:
+		require.Error(t, rsp.err)
+		return rsp.err
+	case <-time.After(1 * time.Second):
+		t.Fatal("timeout waiting for WatchServersResponse")
+		return nil
+	}
+}
+
+type serversOrError struct {
+	rsp *pbserverdiscovery.WatchServersResponse
+	err error
+}

agent/grpc/public/testutils/fsm.go

@@ -0,0 +1,81 @@
+package testutils
+
+import (
+	"context"
+	"sync"
+	"testing"
+	"time"
+
+	"github.com/hashicorp/consul/agent/consul/state"
+	"github.com/hashicorp/consul/agent/consul/stream"
+	"github.com/stretchr/testify/require"
+)
+
+func TestStateStore(t *testing.T, publisher state.EventPublisher) *state.Store {
+	t.Helper()
+
+	gc, err := state.NewTombstoneGC(time.Second, time.Millisecond)
+	require.NoError(t, err)
+
+	if publisher == nil {
+		publisher = stream.NoOpEventPublisher{}
+	}
+
+	return state.NewStateStoreWithEventPublisher(gc, publisher)
+}
+
+type Registrar func(*FakeFSM, *stream.EventPublisher)
+
+type FakeFSMConfig struct {
+	Register  Registrar
+	Refresh   []stream.Topic
+	publisher *stream.EventPublisher
+}
+
+type FakeFSM struct {
+	config FakeFSMConfig
+	lock   sync.Mutex
+	store  *state.Store
+}
+
+func newFakeFSM(t *testing.T, config FakeFSMConfig) *FakeFSM {
+	t.Helper()
+
+	store := TestStateStore(t, config.publisher)
+
+	fsm := &FakeFSM{store: store, config: config}
+
+	config.Register(fsm, fsm.config.publisher)
+
+	return fsm
+}
+
+func (f *FakeFSM) GetStore() *state.Store {
+	f.lock.Lock()
+	defer f.lock.Unlock()
+	return f.store
+}
+
+func (f *FakeFSM) ReplaceStore(store *state.Store) {
+	f.lock.Lock()
+	defer f.lock.Unlock()
+	oldStore := f.store
+	f.store = store
+	oldStore.Abandon()
+	for _, topic := range f.config.Refresh {
+		f.config.publisher.RefreshTopic(topic)
+	}
+}
+
+func SetupFSMAndPublisher(t *testing.T, config FakeFSMConfig) (*FakeFSM, state.EventPublisher) {
+	t.Helper()
+	config.publisher = stream.NewEventPublisher(10 * time.Second)
+
+	fsm := newFakeFSM(t, config)
+
+	ctx, cancel := context.WithCancel(context.Background())
+	t.Cleanup(cancel)
+	go config.publisher.Run(ctx)
+
+	return fsm, config.publisher
+}

agent/grpc/public/testutils/server.go

@@ -0,0 +1,30 @@
+package testutils
+
+import (
+	"net"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+	"google.golang.org/grpc"
+)
+
+type GRPCService interface {
+	Register(*grpc.Server)
+}
+
+func RunTestServer(t *testing.T, services ...GRPCService) net.Addr {
+	t.Helper()
+
+	lis, err := net.Listen("tcp", "127.0.0.1:0")
+	require.NoError(t, err)
+
+	grpcServer := grpc.NewServer()
+	for _, svc := range services {
+		svc.Register(grpcServer)
+	}
+
+	go grpcServer.Serve(lis)
+	t.Cleanup(grpcServer.Stop)
+
+	return lis.Addr()
+}

proto-public/pbserverdiscovery/serverdiscovery.pb.binary.go

@@ -0,0 +1,38 @@
+// Code generated by protoc-gen-go-binary. DO NOT EDIT.
+// source: proto-public/pbserverdiscovery/serverdiscovery.proto
+
+package pbserverdiscovery
+
+import (
+	"github.com/golang/protobuf/proto"
+)
+
+// MarshalBinary implements encoding.BinaryMarshaler
+func (msg *WatchServersRequest) MarshalBinary() ([]byte, error) {
+	return proto.Marshal(msg)
+}
+
+// UnmarshalBinary implements encoding.BinaryUnmarshaler
+func (msg *WatchServersRequest) UnmarshalBinary(b []byte) error {
+	return proto.Unmarshal(b, msg)
+}
+
+// MarshalBinary implements encoding.BinaryMarshaler
+func (msg *WatchServersResponse) MarshalBinary() ([]byte, error) {
+	return proto.Marshal(msg)
+}
+
+// UnmarshalBinary implements encoding.BinaryUnmarshaler
+func (msg *WatchServersResponse) UnmarshalBinary(b []byte) error {
+	return proto.Unmarshal(b, msg)
+}
+
+// MarshalBinary implements encoding.BinaryMarshaler
+func (msg *Server) MarshalBinary() ([]byte, error) {
+	return proto.Marshal(msg)
+}
+
+// UnmarshalBinary implements encoding.BinaryUnmarshaler
+func (msg *Server) UnmarshalBinary(b []byte) error {
+	return proto.Unmarshal(b, msg)
+}

proto-public/pbserverdiscovery/serverdiscovery.pb.go

@@ -0,0 +1,437 @@
+// Package serverdiscovery provides a service on Consul servers to discover the set of servers
+// currently able to handle incoming requests.
+
+// Code generated by protoc-gen-go. DO NOT EDIT.
+// versions:
+// 	protoc-gen-go v1.23.0
+// 	protoc        v3.15.8
+// source: proto-public/pbserverdiscovery/serverdiscovery.proto
+
+package pbserverdiscovery
+
+import (
+	context "context"
+	proto "github.com/golang/protobuf/proto"
+	grpc "google.golang.org/grpc"
+	codes "google.golang.org/grpc/codes"
+	status "google.golang.org/grpc/status"
+	protoreflect "google.golang.org/protobuf/reflect/protoreflect"
+	protoimpl "google.golang.org/protobuf/runtime/protoimpl"
+	reflect "reflect"
+	sync "sync"
+)
+
+const (
+	// Verify that this generated code is sufficiently up-to-date.
+	_ = protoimpl.EnforceVersion(20 - protoimpl.MinVersion)
+	// Verify that runtime/protoimpl is sufficiently up-to-date.
+	_ = protoimpl.EnforceVersion(protoimpl.MaxVersion - 20)
+)
+
+// This is a compile-time assertion that a sufficiently up-to-date version
+// of the legacy proto package is being used.
+const _ = proto.ProtoPackageIsVersion4
+
+type WatchServersRequest struct {
+	state         protoimpl.MessageState
+	sizeCache     protoimpl.SizeCache
+	unknownFields protoimpl.UnknownFields
+
+	// Wan being set to true will cause WAN addresses to be sent in the response
+	// instead of the LAN addresses which are the default
+	Wan bool `protobuf:"varint,1,opt,name=wan,proto3" json:"wan,omitempty"`
+}
+
+func (x *WatchServersRequest) Reset() {
+	*x = WatchServersRequest{}
+	if protoimpl.UnsafeEnabled {
+		mi := &file_proto_public_pbserverdiscovery_serverdiscovery_proto_msgTypes[0]
+		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
+		ms.StoreMessageInfo(mi)
+	}
+}
+
+func (x *WatchServersRequest) String() string {
+	return protoimpl.X.MessageStringOf(x)
+}
+
+func (*WatchServersRequest) ProtoMessage() {}
+
+func (x *WatchServersRequest) ProtoReflect() protoreflect.Message {
+	mi := &file_proto_public_pbserverdiscovery_serverdiscovery_proto_msgTypes[0]
+	if protoimpl.UnsafeEnabled && x != nil {
+		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
+		if ms.LoadMessageInfo() == nil {
+			ms.StoreMessageInfo(mi)
+		}
+		return ms
+	}
+	return mi.MessageOf(x)
+}
+
+// Deprecated: Use WatchServersRequest.ProtoReflect.Descriptor instead.
+func (*WatchServersRequest) Descriptor() ([]byte, []int) {
+	return file_proto_public_pbserverdiscovery_serverdiscovery_proto_rawDescGZIP(), []int{0}
+}
+
+func (x *WatchServersRequest) GetWan() bool {
+	if x != nil {
+		return x.Wan
+	}
+	return false
+}
+
+type WatchServersResponse struct {
+	state         protoimpl.MessageState
+	sizeCache     protoimpl.SizeCache
+	unknownFields protoimpl.UnknownFields
+
+	// Servers is the list of server address information.
+	Servers []*Server `protobuf:"bytes,1,rep,name=servers,proto3" json:"servers,omitempty"`
+}
+
+func (x *WatchServersResponse) Reset() {
+	*x = WatchServersResponse{}
+	if protoimpl.UnsafeEnabled {
+		mi := &file_proto_public_pbserverdiscovery_serverdiscovery_proto_msgTypes[1]
+		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
+		ms.StoreMessageInfo(mi)
+	}
+}
+
+func (x *WatchServersResponse) String() string {
+	return protoimpl.X.MessageStringOf(x)
+}
+
+func (*WatchServersResponse) ProtoMessage() {}
+
+func (x *WatchServersResponse) ProtoReflect() protoreflect.Message {
+	mi := &file_proto_public_pbserverdiscovery_serverdiscovery_proto_msgTypes[1]
+	if protoimpl.UnsafeEnabled && x != nil {
+		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
+		if ms.LoadMessageInfo() == nil {
+			ms.StoreMessageInfo(mi)
+		}
+		return ms
+	}
+	return mi.MessageOf(x)
+}
+
+// Deprecated: Use WatchServersResponse.ProtoReflect.Descriptor instead.
+func (*WatchServersResponse) Descriptor() ([]byte, []int) {
+	return file_proto_public_pbserverdiscovery_serverdiscovery_proto_rawDescGZIP(), []int{1}
+}
+
+func (x *WatchServersResponse) GetServers() []*Server {
+	if x != nil {
+		return x.Servers
+	}
+	return nil
+}
+
+type Server struct {
+	state         protoimpl.MessageState
+	sizeCache     protoimpl.SizeCache
+	unknownFields protoimpl.UnknownFields
+
+	// id is the unique string identifying this server for all time.
+	Id string `protobuf:"bytes,1,opt,name=id,proto3" json:"id,omitempty"`
+	// address on the network of the server
+	Address string `protobuf:"bytes,2,opt,name=address,proto3" json:"address,omitempty"`
+	// the consul version of the server
+	Version string `protobuf:"bytes,3,opt,name=version,proto3" json:"version,omitempty"`
+}
+
+func (x *Server) Reset() {
+	*x = Server{}
+	if protoimpl.UnsafeEnabled {
+		mi := &file_proto_public_pbserverdiscovery_serverdiscovery_proto_msgTypes[2]
+		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
+		ms.StoreMessageInfo(mi)
+	}
+}
+
+func (x *Server) String() string {
+	return protoimpl.X.MessageStringOf(x)
+}
+
+func (*Server) ProtoMessage() {}
+
+func (x *Server) ProtoReflect() protoreflect.Message {
+	mi := &file_proto_public_pbserverdiscovery_serverdiscovery_proto_msgTypes[2]
+	if protoimpl.UnsafeEnabled && x != nil {
+		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
+		if ms.LoadMessageInfo() == nil {
+			ms.StoreMessageInfo(mi)
+		}
+		return ms
+	}
+	return mi.MessageOf(x)
+}
+
+// Deprecated: Use Server.ProtoReflect.Descriptor instead.
+func (*Server) Descriptor() ([]byte, []int) {
+	return file_proto_public_pbserverdiscovery_serverdiscovery_proto_rawDescGZIP(), []int{2}
+}
+
+func (x *Server) GetId() string {
+	if x != nil {
+		return x.Id
+	}
+	return ""
+}
+
+func (x *Server) GetAddress() string {
+	if x != nil {
+		return x.Address
+	}
+	return ""
+}
+
+func (x *Server) GetVersion() string {
+	if x != nil {
+		return x.Version
+	}
+	return ""
+}
+
+var File_proto_public_pbserverdiscovery_serverdiscovery_proto protoreflect.FileDescriptor
+
+var file_proto_public_pbserverdiscovery_serverdiscovery_proto_rawDesc = []byte{
+	0x0a, 0x34, 0x70, 0x72, 0x6f, 0x74, 0x6f, 0x2d, 0x70, 0x75, 0x62, 0x6c, 0x69, 0x63, 0x2f, 0x70,
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+	0x22, 0x00, 0x30, 0x01, 0x42, 0x3c, 0x5a, 0x3a, 0x67, 0x69, 0x74, 0x68, 0x75, 0x62, 0x2e, 0x63,
+	0x6f, 0x6d, 0x2f, 0x68, 0x61, 0x73, 0x68, 0x69, 0x63, 0x6f, 0x72, 0x70, 0x2f, 0x63, 0x6f, 0x6e,
+	0x73, 0x75, 0x6c, 0x2f, 0x70, 0x72, 0x6f, 0x74, 0x6f, 0x2d, 0x70, 0x75, 0x62, 0x6c, 0x69, 0x63,
+	0x2f, 0x70, 0x62, 0x73, 0x65, 0x72, 0x76, 0x65, 0x72, 0x64, 0x69, 0x73, 0x63, 0x6f, 0x76, 0x65,
+	0x72, 0x79, 0x62, 0x06, 0x70, 0x72, 0x6f, 0x74, 0x6f, 0x33,
+}
+
+var (
+	file_proto_public_pbserverdiscovery_serverdiscovery_proto_rawDescOnce sync.Once
+	file_proto_public_pbserverdiscovery_serverdiscovery_proto_rawDescData = file_proto_public_pbserverdiscovery_serverdiscovery_proto_rawDesc
+)
+
+func file_proto_public_pbserverdiscovery_serverdiscovery_proto_rawDescGZIP() []byte {
+	file_proto_public_pbserverdiscovery_serverdiscovery_proto_rawDescOnce.Do(func() {
+		file_proto_public_pbserverdiscovery_serverdiscovery_proto_rawDescData = protoimpl.X.CompressGZIP(file_proto_public_pbserverdiscovery_serverdiscovery_proto_rawDescData)
+	})
+	return file_proto_public_pbserverdiscovery_serverdiscovery_proto_rawDescData
+}
+
+var file_proto_public_pbserverdiscovery_serverdiscovery_proto_msgTypes = make([]protoimpl.MessageInfo, 3)
+var file_proto_public_pbserverdiscovery_serverdiscovery_proto_goTypes = []interface{}{
+	(*WatchServersRequest)(nil),  // 0: serverdiscovery.WatchServersRequest
+	(*WatchServersResponse)(nil), // 1: serverdiscovery.WatchServersResponse
+	(*Server)(nil),               // 2: serverdiscovery.Server
+}
+var file_proto_public_pbserverdiscovery_serverdiscovery_proto_depIdxs = []int32{
+	2, // 0: serverdiscovery.WatchServersResponse.servers:type_name -> serverdiscovery.Server
+	0, // 1: serverdiscovery.ServerDiscoveryService.WatchServers:input_type -> serverdiscovery.WatchServersRequest
+	1, // 2: serverdiscovery.ServerDiscoveryService.WatchServers:output_type -> serverdiscovery.WatchServersResponse
+	2, // [2:3] is the sub-list for method output_type
+	1, // [1:2] is the sub-list for method input_type
+	1, // [1:1] is the sub-list for extension type_name
+	1, // [1:1] is the sub-list for extension extendee
+	0, // [0:1] is the sub-list for field type_name
+}
+
+func init() { file_proto_public_pbserverdiscovery_serverdiscovery_proto_init() }
+func file_proto_public_pbserverdiscovery_serverdiscovery_proto_init() {
+	if File_proto_public_pbserverdiscovery_serverdiscovery_proto != nil {
+		return
+	}
+	if !protoimpl.UnsafeEnabled {
+		file_proto_public_pbserverdiscovery_serverdiscovery_proto_msgTypes[0].Exporter = func(v interface{}, i int) interface{} {
+			switch v := v.(*WatchServersRequest); i {
+			case 0:
+				return &v.state
+			case 1:
+				return &v.sizeCache
+			case 2:
+				return &v.unknownFields
+			default:
+				return nil
+			}
+		}
+		file_proto_public_pbserverdiscovery_serverdiscovery_proto_msgTypes[1].Exporter = func(v interface{}, i int) interface{} {
+			switch v := v.(*WatchServersResponse); i {
+			case 0:
+				return &v.state
+			case 1:
+				return &v.sizeCache
+			case 2:
+				return &v.unknownFields
+			default:
+				return nil
+			}
+		}
+		file_proto_public_pbserverdiscovery_serverdiscovery_proto_msgTypes[2].Exporter = func(v interface{}, i int) interface{} {
+			switch v := v.(*Server); i {
+			case 0:
+				return &v.state
+			case 1:
+				return &v.sizeCache
+			case 2:
+				return &v.unknownFields
+			default:
+				return nil
+			}
+		}
+	}
+	type x struct{}
+	out := protoimpl.TypeBuilder{
+		File: protoimpl.DescBuilder{
+			GoPackagePath: reflect.TypeOf(x{}).PkgPath(),
+			RawDescriptor: file_proto_public_pbserverdiscovery_serverdiscovery_proto_rawDesc,
+			NumEnums:      0,
+			NumMessages:   3,
+			NumExtensions: 0,
+			NumServices:   1,
+		},
+		GoTypes:           file_proto_public_pbserverdiscovery_serverdiscovery_proto_goTypes,
+		DependencyIndexes: file_proto_public_pbserverdiscovery_serverdiscovery_proto_depIdxs,
+		MessageInfos:      file_proto_public_pbserverdiscovery_serverdiscovery_proto_msgTypes,
+	}.Build()
+	File_proto_public_pbserverdiscovery_serverdiscovery_proto = out.File
+	file_proto_public_pbserverdiscovery_serverdiscovery_proto_rawDesc = nil
+	file_proto_public_pbserverdiscovery_serverdiscovery_proto_goTypes = nil
+	file_proto_public_pbserverdiscovery_serverdiscovery_proto_depIdxs = nil
+}
+
+// Reference imports to suppress errors if they are not otherwise used.
+var _ context.Context
+var _ grpc.ClientConnInterface
+
+// This is a compile-time assertion to ensure that this generated file
+// is compatible with the grpc package it is being compiled against.
+const _ = grpc.SupportPackageIsVersion6
+
+// ServerDiscoveryServiceClient is the client API for ServerDiscoveryService service.
+//
+// For semantics around ctx use and closing/ending streaming RPCs, please refer to https://godoc.org/google.golang.org/grpc#ClientConn.NewStream.
+type ServerDiscoveryServiceClient interface {
+	// WatchServers will stream back sets of ready servers as they change such as
+	// when new servers are added or older ones removed. A ready server is one that
+	// should be considered ready for sending general RPC requests towards that would
+	// catalog queries, xDS proxy configurations and similar services.
+	WatchServers(ctx context.Context, in *WatchServersRequest, opts ...grpc.CallOption) (ServerDiscoveryService_WatchServersClient, error)
+}
+
+type serverDiscoveryServiceClient struct {
+	cc grpc.ClientConnInterface
+}
+
+func NewServerDiscoveryServiceClient(cc grpc.ClientConnInterface) ServerDiscoveryServiceClient {
+	return &serverDiscoveryServiceClient{cc}
+}
+
+func (c *serverDiscoveryServiceClient) WatchServers(ctx context.Context, in *WatchServersRequest, opts ...grpc.CallOption) (ServerDiscoveryService_WatchServersClient, error) {
+	stream, err := c.cc.NewStream(ctx, &_ServerDiscoveryService_serviceDesc.Streams[0], "/serverdiscovery.ServerDiscoveryService/WatchServers", opts...)
+	if err != nil {
+		return nil, err
+	}
+	x := &serverDiscoveryServiceWatchServersClient{stream}
+	if err := x.ClientStream.SendMsg(in); err != nil {
+		return nil, err
+	}
+	if err := x.ClientStream.CloseSend(); err != nil {
+		return nil, err
+	}
+	return x, nil
+}
+
+type ServerDiscoveryService_WatchServersClient interface {
+	Recv() (*WatchServersResponse, error)
+	grpc.ClientStream
+}
+
+type serverDiscoveryServiceWatchServersClient struct {
+	grpc.ClientStream
+}
+
+func (x *serverDiscoveryServiceWatchServersClient) Recv() (*WatchServersResponse, error) {
+	m := new(WatchServersResponse)
+	if err := x.ClientStream.RecvMsg(m); err != nil {
+		return nil, err
+	}
+	return m, nil
+}
+
+// ServerDiscoveryServiceServer is the server API for ServerDiscoveryService service.
+type ServerDiscoveryServiceServer interface {
+	// WatchServers will stream back sets of ready servers as they change such as
+	// when new servers are added or older ones removed. A ready server is one that
+	// should be considered ready for sending general RPC requests towards that would
+	// catalog queries, xDS proxy configurations and similar services.
+	WatchServers(*WatchServersRequest, ServerDiscoveryService_WatchServersServer) error
+}
+
+// UnimplementedServerDiscoveryServiceServer can be embedded to have forward compatible implementations.
+type UnimplementedServerDiscoveryServiceServer struct {
+}
+
+func (*UnimplementedServerDiscoveryServiceServer) WatchServers(*WatchServersRequest, ServerDiscoveryService_WatchServersServer) error {
+	return status.Errorf(codes.Unimplemented, "method WatchServers not implemented")
+}
+
+func RegisterServerDiscoveryServiceServer(s *grpc.Server, srv ServerDiscoveryServiceServer) {
+	s.RegisterService(&_ServerDiscoveryService_serviceDesc, srv)
+}
+
+func _ServerDiscoveryService_WatchServers_Handler(srv interface{}, stream grpc.ServerStream) error {
+	m := new(WatchServersRequest)
+	if err := stream.RecvMsg(m); err != nil {
+		return err
+	}
+	return srv.(ServerDiscoveryServiceServer).WatchServers(m, &serverDiscoveryServiceWatchServersServer{stream})
+}
+
+type ServerDiscoveryService_WatchServersServer interface {
+	Send(*WatchServersResponse) error
+	grpc.ServerStream
+}
+
+type serverDiscoveryServiceWatchServersServer struct {
+	grpc.ServerStream
+}
+
+func (x *serverDiscoveryServiceWatchServersServer) Send(m *WatchServersResponse) error {
+	return x.ServerStream.SendMsg(m)
+}
+
+var _ServerDiscoveryService_serviceDesc = grpc.ServiceDesc{
+	ServiceName: "serverdiscovery.ServerDiscoveryService",
+	HandlerType: (*ServerDiscoveryServiceServer)(nil),
+	Methods:     []grpc.MethodDesc{},
+	Streams: []grpc.StreamDesc{
+		{
+			StreamName:    "WatchServers",
+			Handler:       _ServerDiscoveryService_WatchServers_Handler,
+			ServerStreams: true,
+		},
+	},
+	Metadata: "proto-public/pbserverdiscovery/serverdiscovery.proto",
+}

proto-public/pbserverdiscovery/serverdiscovery.proto

@@ -0,0 +1,37 @@
+// Package serverdiscovery provides a service on Consul servers to discover the set of servers 
+// currently able to handle incoming requests.
+
+syntax = "proto3";
+
+package serverdiscovery;
+
+option go_package = "github.com/hashicorp/consul/proto-public/pbserverdiscovery";
+
+service ServerDiscoveryService {
+   // WatchServers will stream back sets of ready servers as they change such as
+   // when new servers are added or older ones removed. A ready server is one that
+   // should be considered ready for sending general RPC requests towards that would
+   // catalog queries, xDS proxy configurations and similar services.
+   rpc WatchServers(WatchServersRequest) returns (stream WatchServersResponse) {};
+}
+
+message WatchServersRequest {
+   // Wan being set to true will cause WAN addresses to be sent in the response
+   // instead of the LAN addresses which are the default
+   bool wan = 1;
+}
+
+message WatchServersResponse{
+   // Servers is the list of server address information.
+   repeated Server servers = 1;
+}
+
+message Server {
+   // id is the unique string identifying this server for all time.
+   string id = 1;
+   // address on the network of the server
+   string address = 2;
+   // the consul version of the server
+   string version = 3;
+}
+