rpc/server: add notification subscription

Note that the protocol differs a bit from #895 in its notifications format,
to avoid additional server-side processing we're omitting some metadata like:
 * block size and confirmations
 * transaction fees, confirmations, block hash and timestamp
 * application execution doesn't have ScriptHash populated

Some block fields may also differ in encoding compared to `getblock` results
(like nonce field).

I think these differences are unnoticieable for most use cases, so we can
leave them as is, but it can be changed in the future.

pkg/rpc/response/events.go

@@ -0,0 +1,79 @@
+package response
+
+import (
+	"encoding/json"
+
+	"github.com/pkg/errors"
+)
+
+type (
+	// EventID represents an event type happening on the chain.
+	EventID byte
+)
+
+const (
+	// InvalidEventID is an invalid event id that is the default value of
+	// EventID. It's only used as an initial value similar to nil.
+	InvalidEventID EventID = iota
+	// BlockEventID is a `block_added` event.
+	BlockEventID
+	// TransactionEventID corresponds to `transaction_added` event.
+	TransactionEventID
+	// NotificationEventID represents `notification_from_execution` events.
+	NotificationEventID
+	// ExecutionEventID is used for `transaction_executed` events.
+	ExecutionEventID
+)
+
+// String is a good old Stringer implementation.
+func (e EventID) String() string {
+	switch e {
+	case BlockEventID:
+		return "block_added"
+	case TransactionEventID:
+		return "transaction_added"
+	case NotificationEventID:
+		return "notification_from_execution"
+	case ExecutionEventID:
+		return "transaction_executed"
+	default:
+		return "unknown"
+	}
+}
+
+// GetEventIDFromString converts input string into an EventID if it's possible.
+func GetEventIDFromString(s string) (EventID, error) {
+	switch s {
+	case "block_added":
+		return BlockEventID, nil
+	case "transaction_added":
+		return TransactionEventID, nil
+	case "notification_from_execution":
+		return NotificationEventID, nil
+	case "transaction_executed":
+		return ExecutionEventID, nil
+	default:
+		return 255, errors.New("invalid stream name")
+	}
+}
+
+// MarshalJSON implements json.Marshaler interface.
+func (e EventID) MarshalJSON() ([]byte, error) {
+	return json.Marshal(e.String())
+}
+
+// UnmarshalJSON implements json.Unmarshaler interface.
+func (e *EventID) UnmarshalJSON(b []byte) error {
+	var s string
+
+	err := json.Unmarshal(b, &s)
+	if err != nil {
+		return err
+	}
+	id, err := GetEventIDFromString(s)
+	if err != nil {
+		return err
+	}
+	*e = id
+	return nil
+}

pkg/rpc/response/result/application_log.go

@@ -30,16 +30,22 @@ type NotificationEvent struct {
 	Item     smartcontract.Parameter `json:"state"`
 }
 
+// StateEventToResultNotification converts state.NotificationEvent to
+// result.NotificationEvent.
+func StateEventToResultNotification(event state.NotificationEvent) NotificationEvent {
+	seen := make(map[vm.StackItem]bool)
+	item := event.Item.ToContractParameter(seen)
+	return NotificationEvent{
+		Contract: event.ScriptHash,
+		Item:     item,
+	}
+}
+
 // NewApplicationLog creates a new ApplicationLog wrapper.
 func NewApplicationLog(appExecRes *state.AppExecResult, scriptHash util.Uint160) ApplicationLog {
 	events := make([]NotificationEvent, 0, len(appExecRes.Events))
 	for _, e := range appExecRes.Events {
-		seen := make(map[vm.StackItem]bool)
-		item := e.Item.ToContractParameter(seen)
-		events = append(events, NotificationEvent{
-			Contract: e.ScriptHash,
-			Item:     item,
-		})
+		events = append(events, StateEventToResultNotification(e))
 	}
 
 	triggerString := appExecRes.Trigger.String()

pkg/rpc/response/types.go

@@ -37,3 +37,12 @@ type GetRawTx struct {
 	HeaderAndError
 	Result *result.TransactionOutputRaw `json:"result"`
 }
+
+// Notification is a type used to represent wire format of events, they're
+// special in that they look like requests but they don't have IDs and their
+// "method" is actually an event name.
+type Notification struct {
+	JSONRPC string        `json:"jsonrpc"`
+	Event   EventID       `json:"method"`
+	Payload []interface{} `json:"params"`
+}

pkg/rpc/server/server.go

@@ -9,6 +9,7 @@ import (
 	"net"
 	"net/http"
 	"strconv"
+	"sync"
 	"time"
 
 	"github.com/gorilla/websocket"
@@ -42,6 +43,19 @@ type (
 		coreServer *network.Server
 		log        *zap.Logger
 		https      *http.Server
+		shutdown   chan struct{}
+
+		subsLock         sync.RWMutex
+		subscribers      map[*subscriber]bool
+		subsGroup        sync.WaitGroup
+		blockSubs        int
+		executionSubs    int
+		notificationSubs int
+		transactionSubs  int
+		blockCh          chan *block.Block
+		executionCh      chan *state.AppExecResult
+		notificationCh   chan *state.NotificationEvent
+		transactionCh    chan *transaction.Transaction
 	}
 )
 
@@ -57,6 +71,11 @@ const (
 
 	// Write deadline.
 	wsWriteLimit = wsPingPeriod / 2
+
+	// Maximum number of subscribers per Server. Each websocket client is
+	// treated like subscriber, so technically it's a limit on websocket
+	// connections.
+	maxSubscribers = 64
 )
 
 var rpcHandlers = map[string]func(*Server, request.Params) (interface{}, *response.Error){
@@ -92,6 +111,11 @@ var rpcHandlers = map[string]func(*Server, request.Params) (interface{}, *respon
 	"validateaddress":      (*Server).validateAddress,
 }
 
+var rpcWsHandlers = map[string]func(*Server, request.Params, *subscriber) (interface{}, *response.Error){
+	"subscribe":   (*Server).subscribe,
+	"unsubscribe": (*Server).unsubscribe,
+}
+
 var invalidBlockHeightError = func(index int, height int) *response.Error {
 	return response.NewRPCError(fmt.Sprintf("Param at index %d should be greater than or equal to 0 and less then or equal to current block height, got: %d", index, height), "", nil)
 }
@@ -120,6 +144,14 @@ func New(chain blockchainer.Blockchainer, conf rpc.Config, coreServer *network.S
 		coreServer: coreServer,
 		log:        log,
 		https:      tlsServer,
+		shutdown:   make(chan struct{}),
+
+		subscribers: make(map[*subscriber]bool),
+		// These are NOT buffered to preserve original order of events.
+		blockCh:        make(chan *block.Block),
+		executionCh:    make(chan *state.AppExecResult),
+		notificationCh: make(chan *state.NotificationEvent),
+		transactionCh:  make(chan *transaction.Transaction),
 	}
 }
 
@@ -134,6 +166,7 @@ func (s *Server) Start(errChan chan error) {
 	s.Handler = http.HandlerFunc(s.handleHTTPRequest)
 	s.log.Info("starting rpc-server", zap.String("endpoint", s.Addr))
 
+	go s.handleSubEvents()
 	if cfg := s.config.TLSConfig; cfg.Enabled {
 		s.https.Handler = http.HandlerFunc(s.handleHTTPRequest)
 		s.log.Info("starting rpc-server (https)", zap.String("endpoint", s.https.Addr))
@@ -156,6 +189,10 @@ func (s *Server) Start(errChan chan error) {
 // method.
 func (s *Server) Shutdown() error {
 	var httpsErr error
+
+	// Signal to websocket writer routines and handleSubEvents.
+	close(s.shutdown)
+
 	if s.config.TLSConfig.Enabled {
 		s.log.Info("shutting down rpc-server (https)", zap.String("endpoint", s.https.Addr))
 		httpsErr = s.https.Shutdown(context.Background())
@@ -163,6 +200,10 @@ func (s *Server) Shutdown() error {
 
 	s.log.Info("shutting down rpc-server", zap.String("endpoint", s.Addr))
 	err := s.Server.Shutdown(context.Background())
+
+	// Wait for handleSubEvents to finish.
+	<-s.executionCh
+
 	if err == nil {
 		return httpsErr
 	}
@@ -170,20 +211,40 @@ func (s *Server) Shutdown() error {
 }
 
 func (s *Server) handleHTTPRequest(w http.ResponseWriter, httpRequest *http.Request) {
+	req := request.NewIn()
+
 	if httpRequest.URL.Path == "/ws" && httpRequest.Method == "GET" {
+		// Technically there is a race between this check and
+		// s.subscribers modification 20 lines below, but it's tiny
+		// and not really critical to bother with it. Some additional
+		// clients may sneak in, no big deal.
+		s.subsLock.RLock()
+		numOfSubs := len(s.subscribers)
+		s.subsLock.RUnlock()
+		if numOfSubs >= maxSubscribers {
+			s.writeHTTPErrorResponse(
+				req,
+				w,
+				response.NewInternalServerError("websocket users limit reached", nil),
+			)
+			return
+		}
 		ws, err := upgrader.Upgrade(w, httpRequest, nil)
 		if err != nil {
 			s.log.Info("websocket connection upgrade failed", zap.Error(err))
 			return
 		}
 		resChan := make(chan response.Raw)
-		go s.handleWsWrites(ws, resChan)
-		s.handleWsReads(ws, resChan)
+		subChan := make(chan *websocket.PreparedMessage, notificationBufSize)
+		subscr := &subscriber{writer: subChan, ws: ws}
+		s.subsLock.Lock()
+		s.subscribers[subscr] = true
+		s.subsLock.Unlock()
+		go s.handleWsWrites(ws, resChan, subChan)
+		s.handleWsReads(ws, resChan, subscr)
 		return
 	}
 
-	req := request.NewIn()
-
 	if httpRequest.Method != "POST" {
 		s.writeHTTPErrorResponse(
 			req,
@@ -201,11 +262,14 @@ func (s *Server) handleHTTPRequest(w http.ResponseWriter, httpRequest *http.Requ
 		return
 	}
 
-	resp := s.handleRequest(req)
+	resp := s.handleRequest(req, nil)
 	s.writeHTTPServerResponse(req, w, resp)
 }
 
-func (s *Server) handleRequest(req *request.In) response.Raw {
+func (s *Server) handleRequest(req *request.In, sub *subscriber) response.Raw {
+	var res interface{}
+	var resErr *response.Error
+
 	reqParams, err := req.Params()
 	if err != nil {
 		return s.packResponseToRaw(req, nil, response.NewInvalidParamsError("Problem parsing request parameters", err))
@@ -217,20 +281,37 @@ func (s *Server) handleRequest(req *request.In) response.Raw {
 
 	incCounter(req.Method)
 
+	resErr = response.NewMethodNotFoundError(fmt.Sprintf("Method '%s' not supported", req.Method), nil)
 	handler, ok := rpcHandlers[req.Method]
-	if !ok {
-		return s.packResponseToRaw(req, nil, response.NewMethodNotFoundError(fmt.Sprintf("Method '%s' not supported", req.Method), nil))
+	if ok {
+		res, resErr = handler(s, *reqParams)
+	} else if sub != nil {
+		handler, ok := rpcWsHandlers[req.Method]
+		if ok {
+			res, resErr = handler(s, *reqParams, sub)
+		}
 	}
-	res, resErr := handler(s, *reqParams)
 	return s.packResponseToRaw(req, res, resErr)
 }
 
-func (s *Server) handleWsWrites(ws *websocket.Conn, resChan <-chan response.Raw) {
+func (s *Server) handleWsWrites(ws *websocket.Conn, resChan <-chan response.Raw, subChan <-chan *websocket.PreparedMessage) {
 	pingTicker := time.NewTicker(wsPingPeriod)
 	defer ws.Close()
 	defer pingTicker.Stop()
 	for {
 		select {
+		case <-s.shutdown:
+			// Signal to the reader routine.
+			ws.Close()
+			return
+		case event, ok := <-subChan:
+			if !ok {
+				return
+			}
+			ws.SetWriteDeadline(time.Now().Add(wsWriteLimit))
+			if err := ws.WritePreparedMessage(event); err != nil {
+				return
+			}
 		case res, ok := <-resChan:
 			if !ok {
 				return
@@ -248,22 +329,36 @@ func (s *Server) handleWsWrites(ws *websocket.Conn, resChan <-chan response.Raw)
 	}
 }
 
-func (s *Server) handleWsReads(ws *websocket.Conn, resChan chan<- response.Raw) {
+func (s *Server) handleWsReads(ws *websocket.Conn, resChan chan<- response.Raw, subscr *subscriber) {
 	ws.SetReadLimit(wsReadLimit)
 	ws.SetReadDeadline(time.Now().Add(wsPongLimit))
 	ws.SetPongHandler(func(string) error { ws.SetReadDeadline(time.Now().Add(wsPongLimit)); return nil })
+requestloop:
 	for {
 		req := new(request.In)
 		err := ws.ReadJSON(req)
 		if err != nil {
 			break
 		}
-		res := s.handleRequest(req)
+		res := s.handleRequest(req, subscr)
 		if res.Error != nil {
 			s.logRequestError(req, res.Error)
 		}
-		resChan <- res
+		select {
+		case <-s.shutdown:
+			break requestloop
+		case resChan <- res:
 		}
+
+	}
+	s.subsLock.Lock()
+	delete(s.subscribers, subscr)
+	for _, e := range subscr.feeds {
+		if e != response.InvalidEventID {
+			s.unsubscribeFromChannel(e)
+		}
+	}
+	s.subsLock.Unlock()
 	close(resChan)
 	ws.Close()
 }
@@ -1024,6 +1119,201 @@ func (s *Server) sendrawtransaction(reqParams request.Params) (interface{}, *res
 	return results, resultsErr
 }
 
+// subscribe handles subscription requests from websocket clients.
+func (s *Server) subscribe(reqParams request.Params, sub *subscriber) (interface{}, *response.Error) {
+	p, ok := reqParams.Value(0)
+	if !ok {
+		return nil, response.ErrInvalidParams
+	}
+	streamName, err := p.GetString()
+	if err != nil {
+		return nil, response.ErrInvalidParams
+	}
+	event, err := response.GetEventIDFromString(streamName)
+	if err != nil {
+		return nil, response.ErrInvalidParams
+	}
+	s.subsLock.Lock()
+	defer s.subsLock.Unlock()
+	select {
+	case <-s.shutdown:
+		return nil, response.NewInternalServerError("server is shutting down", nil)
+	default:
+	}
+	var id int
+	for ; id < len(sub.feeds); id++ {
+		if sub.feeds[id] == response.InvalidEventID {
+			break
+		}
+	}
+	if id == len(sub.feeds) {
+		return nil, response.NewInternalServerError("maximum number of subscriptions is reached", nil)
+	}
+	sub.feeds[id] = event
+	s.subscribeToChannel(event)
+	return strconv.FormatInt(int64(id), 10), nil
+}
+
+// subscribeToChannel subscribes RPC server to appropriate chain events if
+// it's not yet subscribed for them. It's supposed to be called with s.subsLock
+// taken by the caller.
+func (s *Server) subscribeToChannel(event response.EventID) {
+	switch event {
+	case response.BlockEventID:
+		if s.blockSubs == 0 {
+			s.chain.SubscribeForBlocks(s.blockCh)
+		}
+		s.blockSubs++
+	case response.TransactionEventID:
+		if s.transactionSubs == 0 {
+			s.chain.SubscribeForTransactions(s.transactionCh)
+		}
+		s.transactionSubs++
+	case response.NotificationEventID:
+		if s.notificationSubs == 0 {
+			s.chain.SubscribeForNotifications(s.notificationCh)
+		}
+		s.notificationSubs++
+	case response.ExecutionEventID:
+		if s.executionSubs == 0 {
+			s.chain.SubscribeForExecutions(s.executionCh)
+		}
+		s.executionSubs++
+	}
+}
+
+// unsubscribe handles unsubscription requests from websocket clients.
+func (s *Server) unsubscribe(reqParams request.Params, sub *subscriber) (interface{}, *response.Error) {
+	p, ok := reqParams.Value(0)
+	if !ok {
+		return nil, response.ErrInvalidParams
+	}
+	id, err := p.GetInt()
+	if err != nil || id < 0 {
+		return nil, response.ErrInvalidParams
+	}
+	s.subsLock.Lock()
+	defer s.subsLock.Unlock()
+	if len(sub.feeds) <= id || sub.feeds[id] == response.InvalidEventID {
+		return nil, response.ErrInvalidParams
+	}
+	event := sub.feeds[id]
+	sub.feeds[id] = response.InvalidEventID
+	s.unsubscribeFromChannel(event)
+	return true, nil
+}
+
+// unsubscribeFromChannel unsubscribes RPC server from appropriate chain events
+// if there are no other subscribers for it. It's supposed to be called with
+// s.subsLock taken by the caller.
+func (s *Server) unsubscribeFromChannel(event response.EventID) {
+	switch event {
+	case response.BlockEventID:
+		s.blockSubs--
+		if s.blockSubs == 0 {
+			s.chain.UnsubscribeFromBlocks(s.blockCh)
+		}
+	case response.TransactionEventID:
+		s.transactionSubs--
+		if s.transactionSubs == 0 {
+			s.chain.UnsubscribeFromTransactions(s.transactionCh)
+		}
+	case response.NotificationEventID:
+		s.notificationSubs--
+		if s.notificationSubs == 0 {
+			s.chain.UnsubscribeFromNotifications(s.notificationCh)
+		}
+	case response.ExecutionEventID:
+		s.executionSubs--
+		if s.executionSubs == 0 {
+			s.chain.UnsubscribeFromExecutions(s.executionCh)
+		}
+	}
+}
+
+func (s *Server) handleSubEvents() {
+chloop:
+	for {
+		var resp = response.Notification{
+			JSONRPC: request.JSONRPCVersion,
+			Payload: make([]interface{}, 1),
+		}
+		var msg *websocket.PreparedMessage
+		select {
+		case <-s.shutdown:
+			break chloop
+		case b := <-s.blockCh:
+			resp.Event = response.BlockEventID
+			resp.Payload[0] = b
+		case execution := <-s.executionCh:
+			resp.Event = response.ExecutionEventID
+			resp.Payload[0] = result.NewApplicationLog(execution, util.Uint160{})
+		case notification := <-s.notificationCh:
+			resp.Event = response.NotificationEventID
+			resp.Payload[0] = result.StateEventToResultNotification(*notification)
+		case tx := <-s.transactionCh:
+			resp.Event = response.TransactionEventID
+			resp.Payload[0] = tx
+		}
+		s.subsLock.RLock()
+	subloop:
+		for sub := range s.subscribers {
+			for _, subID := range sub.feeds {
+				if subID == resp.Event {
+					if msg == nil {
+						b, err := json.Marshal(resp)
+						if err != nil {
+							s.log.Error("failed to marshal notification",
+								zap.Error(err),
+								zap.String("type", resp.Event.String()))
+							break subloop
+						}
+						msg, err = websocket.NewPreparedMessage(websocket.TextMessage, b)
+						if err != nil {
+							s.log.Error("failed to prepare notification message",
+								zap.Error(err),
+								zap.String("type", resp.Event.String()))
+							break subloop
+						}
+					}
+					sub.writer <- msg
+					// The message is sent only once per subscriber.
+					break
+				}
+			}
+		}
+		s.subsLock.RUnlock()
+	}
+	// It's important to do it with lock held because no subscription routine
+	// should be running concurrently to this one. And even if one is to run
+	// after unlock, it'll see closed s.shutdown and won't subscribe.
+	s.subsLock.Lock()
+	// There might be no subscription in reality, but it's not a problem as
+	// core.Blockchain allows unsubscribing non-subscribed channels.
+	s.chain.UnsubscribeFromBlocks(s.blockCh)
+	s.chain.UnsubscribeFromTransactions(s.transactionCh)
+	s.chain.UnsubscribeFromNotifications(s.notificationCh)
+	s.chain.UnsubscribeFromExecutions(s.executionCh)
+	s.subsLock.Unlock()
+drainloop:
+	for {
+		select {
+		case <-s.blockCh:
+		case <-s.executionCh:
+		case <-s.notificationCh:
+		case <-s.transactionCh:
+		default:
+			break drainloop
+		}
+	}
+	// It's not required closing these, but since they're drained already
+	// this is safe and it also allows to give a signal to Shutdown routine.
+	close(s.blockCh)
+	close(s.transactionCh)
+	close(s.notificationCh)
+	close(s.executionCh)
+}
+
 func (s *Server) blockHeightFromParam(param *request.Param) (int, *response.Error) {
 	num, err := param.GetInt()
 	if err != nil {

pkg/rpc/server/server_helper_test.go

@@ -14,12 +14,11 @@ import (
 	"github.com/nspcc-dev/neo-go/pkg/network"
 	"github.com/nspcc-dev/neo-go/pkg/util"
 	"github.com/stretchr/testify/require"
+	"go.uber.org/zap"
 	"go.uber.org/zap/zaptest"
 )
 
-func initServerWithInMemoryChain(t *testing.T) (*core.Blockchain, *Server, *httptest.Server) {
-	var nBlocks uint32
-
+func getUnitTestChain(t *testing.T) (*core.Blockchain, config.Config, *zap.Logger) {
 	net := config.ModeUnitTestNet
 	configPath := "../../../config"
 	cfg, err := config.Load(configPath, net)
@@ -32,6 +31,10 @@ func initServerWithInMemoryChain(t *testing.T) (*core.Blockchain, *Server, *http
 
 	go chain.Run()
 
+	return chain, cfg, logger
+}
+
+func getTestBlocks(t *testing.T) []*block.Block {
 	// File "./testdata/testblocks.acc" was generated by function core._
 	// ("neo-go/pkg/core/helper_test.go").
 	// To generate new "./testdata/testblocks.acc", follow the steps:
@@ -41,15 +44,21 @@ func initServerWithInMemoryChain(t *testing.T) (*core.Blockchain, *Server, *http
 	f, err := os.Open("testdata/testblocks.acc")
 	require.Nil(t, err)
 	br := io.NewBinReaderFromIO(f)
-	nBlocks = br.ReadU32LE()
+	nBlocks := br.ReadU32LE()
 	require.Nil(t, br.Err)
+	blocks := make([]*block.Block, 0, int(nBlocks))
 	for i := 0; i < int(nBlocks); i++ {
 		_ = br.ReadU32LE()
 		b := &block.Block{}
 		b.DecodeBinary(br)
 		require.Nil(t, br.Err)
-		require.NoError(t, chain.AddBlock(b))
+		blocks = append(blocks, b)
 	}
+	return blocks
+}
+
+func initClearServerWithInMemoryChain(t *testing.T) (*core.Blockchain, *Server, *httptest.Server) {
+	chain, cfg, logger := getUnitTestChain(t)
 
 	serverConfig := network.NewServerConfig(cfg)
 	server, err := network.NewServer(serverConfig, chain, logger)
@@ -64,6 +73,15 @@ func initServerWithInMemoryChain(t *testing.T) (*core.Blockchain, *Server, *http
 	return chain, &rpcServer, srv
 }
 
+func initServerWithInMemoryChain(t *testing.T) (*core.Blockchain, *Server, *httptest.Server) {
+	chain, rpcServer, srv := initClearServerWithInMemoryChain(t)
+
+	for _, b := range getTestBlocks(t) {
+		require.NoError(t, chain.AddBlock(b))
+	}
+	return chain, rpcServer, srv
+}
+
 type FeerStub struct{}
 
 func (fs *FeerStub) IsLowPriority(util.Fixed8) bool {

pkg/rpc/server/subscription.go

@@ -0,0 +1,35 @@
+package server
+
+import (
+	"github.com/gorilla/websocket"
+	"github.com/nspcc-dev/neo-go/pkg/rpc/response"
+)
+
+type (
+	// subscriber is an event subscriber.
+	subscriber struct {
+		writer chan<- *websocket.PreparedMessage
+		ws     *websocket.Conn
+
+		// These work like slots as there is not a lot of them (it's
+		// cheaper doing it this way rather than creating a map),
+		// pointing to EventID is an obvious overkill at the moment, but
+		// that's not for long.
+		feeds [maxFeeds]response.EventID
+	}
+)
+
+const (
+	// Maximum number of subscriptions per one client.
+	maxFeeds = 16
+
+	// This sets notification messages buffer depth, it may seem to be quite
+	// big, but there is a big gap in speed between internal event processing
+	// and networking communication that is combined with spiky nature of our
+	// event generation process, which leads to lots of events generated in
+	// short time and they will put some pressure to this buffer (consider
+	// ~500 invocation txs in one block with some notifications). At the same
+	// time this channel is about sending pointers, so it's doesn't cost
+	// a lot in terms of memory used.
+	notificationBufSize = 1024
+)

pkg/rpc/server/subscription_test.go

@@ -0,0 +1,227 @@
+package server
+
+import (
+	"encoding/json"
+	"fmt"
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/gorilla/websocket"
+	"github.com/nspcc-dev/neo-go/pkg/core"
+	"github.com/nspcc-dev/neo-go/pkg/core/transaction"
+	"github.com/nspcc-dev/neo-go/pkg/rpc/response"
+	"github.com/stretchr/testify/require"
+	"go.uber.org/atomic"
+)
+
+func wsReader(t *testing.T, ws *websocket.Conn, msgCh chan<- []byte, isFinished *atomic.Bool) {
+	for {
+		ws.SetReadDeadline(time.Now().Add(time.Second))
+		_, body, err := ws.ReadMessage()
+		if isFinished.Load() {
+			require.Error(t, err)
+			break
+		}
+		require.NoError(t, err)
+		msgCh <- body
+	}
+}
+
+func callWSGetRaw(t *testing.T, ws *websocket.Conn, msg string, respCh <-chan []byte) *response.Raw {
+	var resp = new(response.Raw)
+
+	ws.SetWriteDeadline(time.Now().Add(time.Second))
+	require.NoError(t, ws.WriteMessage(websocket.TextMessage, []byte(msg)))
+
+	body := <-respCh
+	require.NoError(t, json.Unmarshal(body, resp))
+	return resp
+}
+
+func getNotification(t *testing.T, respCh <-chan []byte) *response.Notification {
+	var resp = new(response.Notification)
+	body := <-respCh
+	require.NoError(t, json.Unmarshal(body, resp))
+	return resp
+}
+
+func initCleanServerAndWSClient(t *testing.T) (*core.Blockchain, *Server, *websocket.Conn, chan []byte, *atomic.Bool) {
+	chain, rpcSrv, httpSrv := initClearServerWithInMemoryChain(t)
+
+	dialer := websocket.Dialer{HandshakeTimeout: time.Second}
+	url := "ws" + strings.TrimPrefix(httpSrv.URL, "http") + "/ws"
+	ws, _, err := dialer.Dial(url, nil)
+	require.NoError(t, err)
+
+	// Use buffered channel to read server's messages and then read expected
+	// responses from it.
+	respMsgs := make(chan []byte, 16)
+	finishedFlag := atomic.NewBool(false)
+	go wsReader(t, ws, respMsgs, finishedFlag)
+	return chain, rpcSrv, ws, respMsgs, finishedFlag
+}
+
+func TestSubscriptions(t *testing.T) {
+	var subIDs = make([]string, 0)
+	var subFeeds = []string{"block_added", "transaction_added", "notification_from_execution", "transaction_executed"}
+
+	chain, rpcSrv, c, respMsgs, finishedFlag := initCleanServerAndWSClient(t)
+
+	defer chain.Close()
+	defer rpcSrv.Shutdown()
+
+	for _, feed := range subFeeds {
+		var s string
+		resp := callWSGetRaw(t, c, fmt.Sprintf(`{
+  "jsonrpc": "2.0",
+  "method": "subscribe",
+  "params": ["%s"],
+  "id": 1
+}`, feed), respMsgs)
+		require.Nil(t, resp.Error)
+		require.NotNil(t, resp.Result)
+		require.NoError(t, json.Unmarshal(resp.Result, &s))
+		subIDs = append(subIDs, s)
+	}
+
+	for _, b := range getTestBlocks(t) {
+		require.NoError(t, chain.AddBlock(b))
+		for _, tx := range b.Transactions {
+			var mayNotify bool
+
+			if tx.Type == transaction.InvocationType {
+				resp := getNotification(t, respMsgs)
+				require.Equal(t, response.ExecutionEventID, resp.Event)
+				mayNotify = true
+			}
+			for {
+				resp := getNotification(t, respMsgs)
+				if mayNotify && resp.Event == response.NotificationEventID {
+					continue
+				}
+				require.Equal(t, response.TransactionEventID, resp.Event)
+				break
+			}
+		}
+		resp := getNotification(t, respMsgs)
+		require.Equal(t, response.BlockEventID, resp.Event)
+	}
+
+	for _, id := range subIDs {
+		var b bool
+
+		resp := callWSGetRaw(t, c, fmt.Sprintf(`{
+  "jsonrpc": "2.0",
+  "method": "unsubscribe",
+  "params": ["%s"],
+  "id": 1
+}`, id), respMsgs)
+		require.Nil(t, resp.Error)
+		require.NotNil(t, resp.Result)
+		require.NoError(t, json.Unmarshal(resp.Result, &b))
+		require.Equal(t, true, b)
+	}
+	finishedFlag.CAS(false, true)
+	c.Close()
+}
+
+func TestMaxSubscriptions(t *testing.T) {
+	var subIDs = make([]string, 0)
+	chain, rpcSrv, c, respMsgs, finishedFlag := initCleanServerAndWSClient(t)
+
+	defer chain.Close()
+	defer rpcSrv.Shutdown()
+
+	for i := 0; i < maxFeeds+1; i++ {
+		var s string
+		resp := callWSGetRaw(t, c, `{"jsonrpc": "2.0", "method": "subscribe", "params": ["block_added"], "id": 1}`, respMsgs)
+		if i < maxFeeds {
+			require.Nil(t, resp.Error)
+			require.NotNil(t, resp.Result)
+			require.NoError(t, json.Unmarshal(resp.Result, &s))
+			// Each ID must be unique.
+			for _, id := range subIDs {
+				require.NotEqual(t, id, s)
+			}
+			subIDs = append(subIDs, s)
+		} else {
+			require.NotNil(t, resp.Error)
+			require.Nil(t, resp.Result)
+		}
+	}
+
+	finishedFlag.CAS(false, true)
+	c.Close()
+}
+
+func TestBadSubUnsub(t *testing.T) {
+	var subCases = map[string]string{
+		"no params":              `{"jsonrpc": "2.0", "method": "subscribe", "params": [], "id": 1}`,
+		"bad (non-string) event": `{"jsonrpc": "2.0", "method": "subscribe", "params": [1], "id": 1}`,
+		"bad (wrong) event":      `{"jsonrpc": "2.0", "method": "subscribe", "params": ["block_removed"], "id": 1}`,
+	}
+	var unsubCases = map[string]string{
+		"no params":         `{"jsonrpc": "2.0", "method": "unsubscribe", "params": [], "id": 1}`,
+		"bad id":            `{"jsonrpc": "2.0", "method": "unsubscribe", "params": ["vasiliy"], "id": 1}`,
+		"not subscribed id": `{"jsonrpc": "2.0", "method": "unsubscribe", "params": ["7"], "id": 1}`,
+	}
+	chain, rpcSrv, c, respMsgs, finishedFlag := initCleanServerAndWSClient(t)
+
+	defer chain.Close()
+	defer rpcSrv.Shutdown()
+
+	testF := func(t *testing.T, cases map[string]string) func(t *testing.T) {
+		return func(t *testing.T) {
+			for n, s := range cases {
+				t.Run(n, func(t *testing.T) {
+					resp := callWSGetRaw(t, c, s, respMsgs)
+					require.NotNil(t, resp.Error)
+					require.Nil(t, resp.Result)
+				})
+			}
+		}
+	}
+	t.Run("subscribe", testF(t, subCases))
+	t.Run("unsubscribe", testF(t, unsubCases))
+
+	finishedFlag.CAS(false, true)
+	c.Close()
+}
+
+func doSomeWSRequest(t *testing.T, ws *websocket.Conn) {
+	ws.SetWriteDeadline(time.Now().Add(time.Second))
+	// It could be just about anything including invalid request,
+	// we only care about server handling being active.
+	require.NoError(t, ws.WriteMessage(websocket.TextMessage, []byte(`{"jsonrpc": "2.0", "method": "getversion", "params": [], "id": 1}`)))
+	ws.SetReadDeadline(time.Now().Add(time.Second))
+	_, _, err := ws.ReadMessage()
+	require.NoError(t, err)
+}
+
+func TestWSClientsLimit(t *testing.T) {
+	chain, rpcSrv, httpSrv := initClearServerWithInMemoryChain(t)
+	defer chain.Close()
+	defer rpcSrv.Shutdown()
+
+	dialer := websocket.Dialer{HandshakeTimeout: time.Second}
+	url := "ws" + strings.TrimPrefix(httpSrv.URL, "http") + "/ws"
+	wss := make([]*websocket.Conn, maxSubscribers)
+
+	for i := 0; i < len(wss)+1; i++ {
+		ws, _, err := dialer.Dial(url, nil)
+		if i < maxSubscribers {
+			require.NoError(t, err)
+			wss[i] = ws
+			// Check that it's completely ready.
+			doSomeWSRequest(t, ws)
+		} else {
+			require.Error(t, err)
+		}
+	}
+	// Check connections are still alive (it actually is necessary to add
+	// some use of wss to keep connections alive).
+	for i := 0; i < len(wss); i++ {
+		doSomeWSRequest(t, wss[i])
+	}
+}