storage: add watch ID to identify watchings

One watcher includes multiple watchings, and their events are
sent out through one channel. For the received event, user would like to
know which watching it belongs to.

Introduce a watch ID. When watching on some key, user will get a watch
ID. The watch ID is attached to all events that is observed by this
watch.

storage/kv_test.go

@@ -735,7 +735,7 @@ func TestWatchableKVWatch(t *testing.T) {
 
 	w := s.NewWatcher()
 
-	cancel := w.Watch([]byte("foo"), true, 0)
+	wid, cancel := w.Watch([]byte("foo"), true, 0)
 	defer cancel()
 
 	s.Put([]byte("foo"), []byte("bar"))
@@ -750,6 +750,7 @@ func TestWatchableKVWatch(t *testing.T) {
 				ModRevision:    1,
 				Version:        1,
 			},
+			WatchID: wid,
 		}
 		if !reflect.DeepEqual(ev, wev) {
 			t.Errorf("watched event = %+v, want %+v", ev, wev)
@@ -770,6 +771,7 @@ func TestWatchableKVWatch(t *testing.T) {
 				ModRevision:    2,
 				Version:        1,
 			},
+			WatchID: wid,
 		}
 		if !reflect.DeepEqual(ev, wev) {
 			t.Errorf("watched event = %+v, want %+v", ev, wev)
@@ -778,7 +780,10 @@ func TestWatchableKVWatch(t *testing.T) {
 		t.Fatalf("failed to watch the event")
 	}
 
-	cancel = w.Watch([]byte("foo1"), false, 1)
+	w.Close()
+
+	w = s.NewWatcher()
+	wid, cancel = w.Watch([]byte("foo1"), false, 1)
 	defer cancel()
 
 	select {
@@ -792,6 +797,7 @@ func TestWatchableKVWatch(t *testing.T) {
 				ModRevision:    2,
 				Version:        1,
 			},
+			WatchID: wid,
 		}
 		if !reflect.DeepEqual(ev, wev) {
 			t.Errorf("watched event = %+v, want %+v", ev, wev)
@@ -812,6 +818,7 @@ func TestWatchableKVWatch(t *testing.T) {
 				ModRevision:    3,
 				Version:        2,
 			},
+			WatchID: wid,
 		}
 		if !reflect.DeepEqual(ev, wev) {
 			t.Errorf("watched event = %+v, want %+v", ev, wev)

storage/storagepb/kv.proto

@@ -32,5 +32,6 @@ message Event {
   // a delete/expire event contains the previous
   // key-value
   KeyValue kv = 2;
+  // watchID is the ID of watching this event is sent to.
+  int64 watchID = 3;
 }
-

storage/watchable_store.go

@@ -32,7 +32,7 @@ const (
 )
 
 type watchable interface {
-	watch(key []byte, prefix bool, startRev int64, ch chan<- storagepb.Event) (*watching, CancelFunc)
+	watch(key []byte, prefix bool, startRev, id int64, ch chan<- storagepb.Event) (*watching, CancelFunc)
 }
 
 type watchableStore struct {
@@ -173,14 +173,15 @@ func (s *watchableStore) NewWatcher() Watcher {
 	}
 }
 
-func (s *watchableStore) watch(key []byte, prefix bool, startRev int64, ch chan<- storagepb.Event) (*watching, CancelFunc) {
+func (s *watchableStore) watch(key []byte, prefix bool, startRev, id int64, ch chan<- storagepb.Event) (*watching, CancelFunc) {
 	s.mu.Lock()
 	defer s.mu.Unlock()
 
 	wa := &watching{
 		key:    key,
 		prefix: prefix,
 		cur:    startRev,
+		id:     id,
 		ch:     ch,
 	}
 
@@ -273,8 +274,9 @@ func (s *watchableStore) syncWatchings() {
 			}
 
 			w.ch <- storagepb.Event{
-				Type: evt,
-				Kv:   &kv,
+				Type:    evt,
+				Kv:      &kv,
+				WatchID: w.id,
 			}
 			pendingEventsGauge.Inc()
 		}
@@ -311,6 +313,7 @@ func (s *watchableStore) notify(rev int64, ev storagepb.Event) {
 				if !w.prefix && i != len(ev.Kv.Key) {
 					continue
 				}
+				ev.WatchID = w.id
 				select {
 				case w.ch <- ev:
 					pendingEventsGauge.Inc()
@@ -362,6 +365,7 @@ type watching struct {
 	// If cur is behind the current revision of the KV,
 	// watching is unsynced and needs to catch up.
 	cur int64
+	id  int64
 
 	// a chan to send out the watched events.
 	// The chan might be shared with other watchings.

storage/watchable_store_bench_test.go

@@ -65,7 +65,7 @@ func BenchmarkWatchableStoreUnsyncedCancel(b *testing.B) {
 	cancels := make([]CancelFunc, watcherSize)
 	for i := 0; i < watcherSize; i++ {
 		// non-0 value to keep watchers in unsynced
-		cancel := w.Watch(testKey, true, 1)
+		_, cancel := w.Watch(testKey, true, 1)
 		cancels[i] = cancel
 	}
 
@@ -102,7 +102,7 @@ func BenchmarkWatchableStoreSyncedCancel(b *testing.B) {
 	cancels := make([]CancelFunc, watcherSize)
 	for i := 0; i < watcherSize; i++ {
 		// 0 for startRev to keep watchers in synced
-		cancel := w.Watch(testKey, true, 0)
+		_, cancel := w.Watch(testKey, true, 0)
 		cancels[i] = cancel
 	}
 

storage/watchable_store_test.go

@@ -49,7 +49,7 @@ func TestNewWatcherCancel(t *testing.T) {
 	s.Put(testKey, testValue)
 
 	w := s.NewWatcher()
-	cancel := w.Watch(testKey, true, 0)
+	_, cancel := w.Watch(testKey, true, 0)
 
 	cancel()
 

storage/watcher.go

@@ -26,7 +26,9 @@ type Watcher interface {
 	// The whole event history can be watched unless compacted.
 	// If `prefix` is true, watch observes all events whose key prefix could be the given `key`.
 	// If `startRev` <=0, watch observes events after currentRev.
-	Watch(key []byte, prefix bool, startRev int64) CancelFunc
+	// The returned `id` is the ID of this watching. It appears as WatchID
+	// in events that are sent to this watching.
+	Watch(key []byte, prefix bool, startRev int64) (id int64, cancel CancelFunc)
 
 	// Chan returns a chan. All watched events will be sent to the returned chan.
 	Chan() <-chan storagepb.Event
@@ -42,21 +44,27 @@ type watcher struct {
 	ch        chan storagepb.Event
 
 	mu      sync.Mutex // guards fields below it
+	nextID  int64      // nextID is the ID allocated for next new watching
 	closed  bool
 	cancels []CancelFunc
 }
 
 // TODO: return error if ws is closed?
-func (ws *watcher) Watch(key []byte, prefix bool, startRev int64) CancelFunc {
-	_, c := ws.watchable.watch(key, prefix, startRev, ws.ch)
+func (ws *watcher) Watch(key []byte, prefix bool, startRev int64) (id int64, cancel CancelFunc) {
 	ws.mu.Lock()
 	defer ws.mu.Unlock()
 	if ws.closed {
-		return nil
+		return -1, nil
 	}
+
+	id = ws.nextID
+	ws.nextID++
+
+	_, c := ws.watchable.watch(key, prefix, startRev, id, ws.ch)
+
 	// TODO: cancelFunc needs to be removed from the cancels when it is called.
 	ws.cancels = append(ws.cancels, c)
-	return c
+	return id, c
 }
 
 func (ws *watcher) Chan() <-chan storagepb.Event {

storage/watcher_test.go

@@ -0,0 +1,64 @@
+// Copyright 2015 CoreOS, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package storage
+
+import "testing"
+
+// TestWatcherWatchID tests that each watcher provides unique watch ID,
+// and the watched event attaches the correct watch ID.
+func TestWatcherWatchID(t *testing.T) {
+	s := WatchableKV(newWatchableStore(tmpPath))
+	defer cleanup(s, tmpPath)
+
+	w := s.NewWatcher()
+	defer w.Close()
+
+	idm := make(map[int64]struct{})
+
+	// synced watchings
+	for i := 0; i < 10; i++ {
+		id, cancel := w.Watch([]byte("foo"), false, 0)
+		if _, ok := idm[id]; ok {
+			t.Errorf("#%d: id %d exists", i, id)
+		}
+		idm[id] = struct{}{}
+
+		s.Put([]byte("foo"), []byte("bar"))
+
+		ev := <-w.Chan()
+		if ev.WatchID != id {
+			t.Errorf("#%d: watch id in event = %d, want %d", i, ev.WatchID, id)
+		}
+
+		cancel()
+	}
+
+	s.Put([]byte("foo2"), []byte("bar"))
+	// unsynced watchings
+	for i := 10; i < 20; i++ {
+		id, cancel := w.Watch([]byte("foo2"), false, 1)
+		if _, ok := idm[id]; ok {
+			t.Errorf("#%d: id %d exists", i, id)
+		}
+		idm[id] = struct{}{}
+
+		ev := <-w.Chan()
+		if ev.WatchID != id {
+			t.Errorf("#%d: watch id in event = %d, want %d", i, ev.WatchID, id)
+		}
+
+		cancel()
+	}
+}